What is a lithium-ion battery?

A lithium-ion (Li-ion) battery is a rechargeable (secondary) battery that stores and releases electrical energy by moving lithium ions back and forth between two electrodes through a liquid or gel electrolyte. It does not use metallic lithium in normal operation; the lithium stays in ionic form inside the electrode materials.

How it works

Each cell has a cathode (positive electrode, for example NMC, LFP or NCA), an anode (negative electrode, usually graphite), an electrolyte and a separator. During charging, lithium ions travel from the cathode into the anode and are stored between graphite layers (this storage is called intercalation). During discharge they travel back to the cathode, and the electrons that cannot pass through the electrolyte flow through the external circuit and power the load.

Why it became the default choice

• High energy density: roughly 150-270 Wh/kg at cell level, several times more than lead-acid (about 30-40 Wh/kg).
• High cell voltage: about 3.6-3.7 V nominal for NMC/NCA, so fewer cells are needed for a given pack voltage.
• Long life: hundreds to thousands of cycles depending on chemistry.
• Low self-discharge: typically only a few percent per month.
• No memory effect and high charge efficiency (above 95% in most cases).

Limitations

Li-ion cells need a Battery Management System (BMS) because overcharge, deep discharge, high temperature or physical damage can cause degradation or even fire. They also cost more per kWh than lead-acid at purchase, although the cost per cycle is usually lower.

Real-world example

The 48-72 V packs in Indian electric two-wheelers and three-wheelers, mobile phone batteries, laptops, power tools and grid-scale energy storage all use Li-ion technology.

What are the four main components of a Li-ion cell?

The four main components of a Li-ion cell are the cathode (positive electrode), the anode (negative electrode), the electrolyte and the separator. Together with metal current collectors and the outer casing, they form a complete working cell.

What each part does

• Cathode: a lithium metal-oxide or phosphate coated on aluminium foil, such as NMC (nickel manganese cobalt oxide), NCA, LCO, LMO or LFP. It largely decides the cell voltage, capacity, cost and safety.
• Anode: usually graphite coated on copper foil. Some newer cells add silicon to raise capacity, and LTO (lithium titanate) is used where very fast charging and long life matter.
• Electrolyte: a lithium salt (commonly LiPF₆) dissolved in organic carbonate solvents. It carries lithium ions between the electrodes but does not conduct electrons. It is flammable, which is one reason safety design is important.
• Separator: a thin porous polymer film (typically polyethylene or polypropylene, often ceramic-coated) that keeps the electrodes apart so they cannot short, yet lets ions pass through its pores.

Supporting parts

Current collectors are aluminium foil on the cathode side and copper foil on the anode side; they carry electrons to the cell terminals. The casing may be a steel or aluminium can (cylindrical, prismatic) or a soft aluminium-polymer pouch, usually with a vent or safety feature to release gas if pressure builds up.

Why aluminium and copper?

Aluminium is stable at the high cathode potential, while copper is stable at the low anode potential. Using the wrong foil on either side would corrode or alloy with lithium.

What happens inside a Li-ion cell during discharge?

During discharge, lithium ions move from the anode (graphite) through the electrolyte and separator to the cathode, while electrons flow from the anode through the external circuit to the cathode, delivering power to the load. The chemical energy stored in the lithiated graphite is converted into electrical energy.

Step by step

1. The anode is full of lithium (as LiC₆) at the start. The load is connected across the terminals.
2. At the anode, lithium releases an electron and becomes a lithium ion: LiC6 -> C6 + Li+ + e-.
3. The electron cannot cross the electrolyte, so it travels through the external circuit (motor, lamp, controller). This current is what the user gets.
4. The Li⁺ ion crosses the electrolyte and separator and enters the cathode crystal structure.
5. At the cathode, the ion and the arriving electron combine, for example Li+ + e- + CoO2 -> LiCoO2 in an LCO cell.

What the user sees

As lithium leaves the anode and fills the cathode, the cell voltage slowly falls, for example from about 4.2 V when full to roughly 3.0 V when nearly empty for an NMC cell. The battery is considered discharged at the manufacturer's cut-off voltage, not at zero volts.

Key point

The positive terminal is the cathode during discharge, and conventional current flows from the positive terminal through the load. Ions and electrons always travel in the opposite sense to charging.

What happens inside a Li-ion cell during charging?

During charging, an external charger forces current into the cell so that lithium ions leave the cathode, travel through the electrolyte and separator, and are stored in the graphite anode, while electrons are pushed through the external circuit to the anode. The cell is converting electrical energy into stored chemical energy.

Step by step

1. The charger applies a voltage higher than the cell voltage, for example up to 4.2 V per NMC cell.
2. At the cathode, lithium is oxidised and released as ions: LiCoO2 -> CoO2 + Li+ + e-.
3. Electrons travel through the charger circuit to the anode.
4. Li⁺ ions move through the electrolyte and enter the graphite layers (intercalation): C6 + Li+ + e- -> LiC6.

Why charging must be controlled

• If the voltage is too high, the cathode can become unstable and the electrolyte can break down.
• If charging is too fast or too cold, lithium may deposit as metal on the anode surface (lithium plating) instead of entering the graphite. Plating reduces capacity and can create dendrites, which may pierce the separator and cause an internal short.
• Heat is produced, so temperature must be monitored.

Typical practice

Li-ion cells are charged with the CC-CV method: constant current until the cell reaches its upper voltage limit (4.2 V for NMC, 3.65 V for LFP), then constant voltage while the current tapers down to a small cut-off value, often around C/20 to C/50.

What is the nominal voltage of a standard Li-ion cell?

The nominal voltage of a standard Li-ion cell is about 3.6-3.7 V for the common NMC, NCA, LCO and LMO chemistries. LFP cells are different, with a nominal voltage of about 3.2 V, and LTO cells are lower still at roughly 2.3-2.4 V.

What 'nominal' means

Nominal voltage is an average, representative voltage during discharge, used for labelling and for calculating energy. The real voltage changes with state of charge, load and temperature. It is not a fixed value.

Typical voltage window

Why it matters

• Pack design: a 48 V NMC pack typically uses 13 cells in series (13 x 3.6 V = 46.8 V nominal, 54.6 V full). A 48 V LFP pack often uses 15 or 16 cells in series (16 x 3.2 V = 51.2 V).
• Energy calculation: Energy (Wh) = nominal voltage x capacity (Ah). A 3.6 V, 5 Ah cell holds about 18 Wh.
• Charger selection: charger voltage must match the pack's full-charge voltage.

Compared with about 1.2-1.5 V for NiMH or alkaline cells, one Li-ion cell can replace three cells in series.

What is "capacity" of a battery, and in what unit is it measured?

Capacity is the amount of electric charge a battery can deliver from full charge down to its cut-off voltage under specified conditions. It is measured in ampere-hours (Ah), or milliampere-hours (mAh) for small cells (1 Ah = 1000 mAh).

Understanding the unit

A 5 Ah cell can theoretically supply 5 A for 1 hour, or 1 A for 5 hours. Capacity in Ah describes charge; to know the actual energy you must multiply by voltage.

For example, a 48 V, 30 Ah pack stores about 1440 Wh = 1.44 kWh. This is the figure that decides the range of an electric scooter, not Ah alone.

Rated vs. actual capacity

• Rated (nominal) capacity is measured by the manufacturer at a specified C-rate (often 0.2C or 0.5C), temperature (about 25 degrees C) and cut-off voltage.
• Actual capacity is lower at high discharge currents, at low temperature and as the cell ages.
• State of Health (SoH) is often defined as present capacity divided by original capacity.

How it is measured

The cell is fully charged, rested, and then discharged at a constant current until it reaches cut-off voltage. Capacity = current x time. Battery analysers and cyclers record this automatically.

Common confusion

Ah is not energy. Two packs of 30 Ah can have very different energy if one is 48 V and the other is 72 V. Chargers and energy meters therefore also quote Wh or kWh.

What is "C-rate" in battery terminology?

C-rate expresses charge or discharge current relative to the battery's rated capacity. Current (A) = C-rate x Capacity (Ah). A rate of 1C means the current that would fully discharge the cell in one hour, 2C in half an hour, and 0.5C in two hours.

Worked example

For a 20 Ah pack:

• 1C = 20 A (discharges in about 1 hour)
• 0.5C = 10 A (about 2 hours)
• 2C = 40 A (about 30 minutes)
• 0.1C = 2 A (about 10 hours)

The ideal time in hours is the reciprocal of the C-rate: Time (h) = 1 / C-rate. In practice the real time is a little shorter at high C-rates because capacity falls when current is high.

Why C-rate matters

• Datasheet ratings: maximum continuous charge and discharge currents are given in C-rates or amperes.
• Heat generation: heat is proportional to I2R, so doubling C-rate gives roughly four times the heating.
• Ageing and safety: very high charge C-rates can cause lithium plating and faster degradation.
• Application matching: energy cells for e-scooters usually run at about 0.5-1C; power cells for tools or drones may support 10C or more; a fast-charging bus might charge at 1-3C.

Typical standard charging of an EV pack is around 0.3-0.5C, so a full charge takes a few hours.

What is "energy density" and why is it important for Li-ion batteries?

Energy density is the amount of energy a battery stores per unit mass or volume. Gravimetric energy density is in Wh/kg (also called specific energy), and volumetric energy density is in Wh/L. It tells you how much energy you can carry for a given weight or space.

Typical numbers

Pack-level values are lower than cell-level because of casing, busbars, BMS, cooling and structure. A typical EV pack is often around 120-180 Wh/kg depending on design.

Why it is important

• Range: higher energy density gives more km for the same battery weight. This is critical for cars and e-2Ws.
• Weight and space: portable electronics, drones and scooters have strict size and weight limits.
• Efficiency: a lighter vehicle uses less energy per km.
• Cost per kWh: better utilisation of materials can reduce cost per unit of energy.

Worked example

A 3 kWh scooter battery at 120 Wh/kg pack-level weighs about 25 kg. The same energy in lead-acid at 35 Wh/kg would weigh nearly 86 kg.

Trade-off

Higher density chemistries like NMC/NCA usually have lower thermal stability and cycle life than LFP, so the best choice depends on the application.

Quick check

When comparing datasheets, confirm whether the quoted Wh/kg is for the bare cell or the complete pack, since the two can differ by 30-40% or more.

What is meant by a battery's "cycle life"?

Cycle life is the number of complete charge-discharge cycles a battery can undergo before its capacity falls below a defined end-of-life limit, usually 80% of the original rated capacity. It is a measure of how long the battery will last in service.

How cycles are counted

One full cycle means discharging the equivalent of the full capacity, not necessarily in one go. Using 50% of the capacity on two separate days is counted as about one equivalent full cycle. Cycle life is always quoted with test conditions: depth of discharge (DoD), C-rate and temperature.

Typical values

• NMC/NCA: roughly 500-2000 cycles, depending on the cell and usage.
• LFP: often 2000-5000 or more cycles.
• LTO: can exceed 10000 cycles.
• Lead-acid: typically a few hundred cycles at deep discharge.

What reduces cycle life

• Deep discharges and always charging to 100%
• High charge and discharge C-rates
• High temperatures (and charging in cold conditions)
• Poor cell balancing or a faulty BMS

Real-world meaning

If an e-scooter pack with 1000 usable cycles delivers 60 km per cycle, it can in theory serve about 60,000 km before reaching 80% capacity. Daily use of one cycle gives roughly 2.7 years. Note that calendar life (ageing with time even when idle) is separate from cycle life, and the real life is whichever limit is reached first.

What is the difference between "primary" and "secondary" batteries?

A primary battery is designed for one-time use: once its chemical energy is consumed it is discarded because the reaction cannot be reversed efficiently. A secondary battery is rechargeable: the chemical reaction can be reversed by passing current backwards, so the battery can be used for many cycles.

Comparison

Important clarification

Not every lithium battery is lithium-ion. A lithium primary cell (for example a CR2032 coin cell) contains lithium metal and is not rechargeable, while a Li-ion cell is a secondary battery. Trying to charge a primary cell can cause leakage, venting or fire.

Choosing between them

Use primary cells where the device draws very little current for years and recharging is impractical. Use secondary cells where the device is used often and needs hundreds of cycles, as in vehicles and power tools.

Quick memory aid

Think of a primary battery as a disposable lighter and a secondary battery as a refillable one: both give energy, but only the secondary can be restored by charging. In interviews, always add that secondary cells need a matching charger and, for Li-ion, a protection circuit.

What are the common Li-ion cell form factors?

The three common Li-ion cell form factors are cylindrical, prismatic and pouch. A fourth, the small coin/button format, is used mostly for low-power rechargeable devices. The form factor affects mechanical strength, packing efficiency, cooling and cost.

Comparison

Where each is used

• Cylindrical: laptops, power tools, many e-scooter packs, Tesla vehicles and several e-2Ws.
• Prismatic: a large share of EV cars, e-buses and energy storage systems, especially LFP prismatic cells.
• Pouch: smartphones, drones, and some EV makers such as those using pouch NMC modules.

How to choose

For high volume and modular packs, cylindrical cells are practical. For large packs where fewer parts and simpler assembly matter, prismatic is preferred. For thin or light designs, pouch is attractive, but the designer must handle swelling and mechanical protection.

What do the numbers in a cell name like "18650" or "21700" mean?

In a cylindrical cell code such as 18650, the first two digits are the diameter in millimetres, the next two digits are the length in millimetres, and the final 0 means the cell is cylindrical. So an 18650 cell is 18 mm in diameter and 65 mm long, and a 21700 cell is 21 mm in diameter and 70 mm long.

Decoding the common sizes

The 4680 cell used in newer EVs follows the same idea but with a different digit split: 46 mm diameter and 80 mm length.

Why 21700 replaced 18650 in many designs

• A 21700 stores roughly 40-50% more energy per cell than an 18650.
• Fewer cells are needed for the same pack energy, which means fewer welds and connections.
• It keeps good heat dissipation because the can is still small.

Worked example

A 2 kWh pack needs about 2000 Wh / (3.6 V x 3.5 Ah = 12.6 Wh per 18650) = roughly 160 cells. With 21700 cells of about 18 Wh each (3.6 V x 5 Ah), the same pack needs about 110 cells.

The code gives only the size, not the chemistry or capacity, so these must be read from the datasheet.

What are the main types of Li-ion battery chemistries?

The main Li-ion chemistries are named after the cathode material: LCO (lithium cobalt oxide), LMO (lithium manganese oxide), NMC (lithium nickel manganese cobalt oxide), NCA (lithium nickel cobalt aluminium oxide), LFP (lithium iron phosphate) and LTO (lithium titanate, which is an anode material). Each offers a different balance of energy, power, safety, life and cost.

Comparison (typical, varies by manufacturer)

Practical guidance

• Choose NMC or NCA when range and weight are the priority.
• Choose LFP when safety, long life and low cost per cycle matter, as in buses, storage and heavy use in hot conditions.
• Choose LTO for very high cycling and fast charging where the extra cost is justified.

In India, both NMC and LFP are widely used in e-2Ws, while e-buses and stationary storage lean towards LFP.

Why is LFP (Lithium Iron Phosphate) considered safer than NMC?

LFP is considered safer than NMC because its olivine crystal structure with strong phosphorus-oxygen bonds holds oxygen firmly. Even at high temperature or when overcharged, LFP releases far less oxygen and heat, so thermal runaway is much less likely to start and is less violent when it does.

Technical reasons

• Stronger bonds: NMC's layered oxide structure begins to break down and release oxygen at roughly 200 degrees C, while LFP stays stable to much higher temperatures (commonly quoted above 250-300 degrees C).
• Less oxygen means less fuel: released oxygen feeds combustion of the flammable electrolyte. With LFP there is little oxygen, so the reaction is slower.
• Lower voltage: LFP operates at about 3.2 V (3.65 V full), which puts less stress on the electrolyte than NMC at 4.2 V.
• Tolerance of abuse: LFP copes better with overcharge, short circuit and nail penetration in tests, although it is not immune to fire.

Other benefits

• Long cycle life, often 2000-5000 cycles or more
• No cobalt or nickel, giving more stable cost and supply
• Flat voltage curve during discharge

Trade-offs

LFP has lower energy density (roughly 100-170 Wh/kg at cell level against 200-270 Wh/kg for high-nickel NMC), so a pack of the same energy is heavier and bigger. Cold-weather performance is weaker, and the flat voltage curve makes State of Charge estimation harder.

This is why LFP is popular in e-buses, e-3Ws and stationary storage, while NMC is chosen when range per kg matters most. Safety still depends on good BMS and pack design.

What is the typical charging method used for Li-ion batteries (CC-CV)?

CC-CV (constant current - constant voltage) is the standard way to charge Li-ion cells. The charger first supplies a steady current until the cell voltage reaches its upper limit, then holds that voltage constant while the current gradually falls, and stops when the current drops to a small cut-off value.

The stages

1. Pre-charge (if deeply discharged): a small current, around 0.05-0.1C, until the cell reaches a safe minimum voltage, often about 2.8-3.0 V for NMC.
2. Constant current (CC): typically 0.5C-1C for standard cells. The voltage rises steadily; about 70-80% of the charge is delivered here.
3. Constant voltage (CV): at 4.2 V per NMC cell (3.65 V for LFP) the charger holds the voltage and the current tapers down.
4. Termination: charging stops when current falls to about 0.02-0.05C, and the charger does not trickle charge.

Worked example

For a 13S NMC pack of 20 Ah, the CC stage might be 10 A (0.5C) until the pack reaches 54.6 V (13 x 4.2 V). The charger then holds 54.6 V and stops once current falls to roughly 1 A.

Why this method

• The CC stage allows quick charging without exceeding the safe current.
• The CV stage prevents overvoltage, which can damage the cell or cause plating and gas generation.
• The tapering current lets the cell become fully charged without stress.

Chargers also check temperature, and the BMS can cut off charging if any cell exceeds its limit. Lead-acid chargers, by contrast, use float charging, which is unsuitable for Li-ion.

Why must Li-ion batteries never be overcharged?

Li-ion batteries must never be overcharged because charging beyond the maximum voltage (4.2 V for NMC, 3.65 V for LFP) causes irreversible chemical damage, excessive heat, gas formation and potentially thermal runaway. The cell has no natural way to absorb extra charge safely, unlike lead-acid or NiMH cells that can partly dissipate it as gas or heat.

What happens inside

• Cathode instability: too much lithium is removed from the cathode, the structure becomes unstable and can release oxygen at higher temperature.
• Lithium plating: on the anode, excess lithium cannot fit in graphite and deposits as metal. This can form dendrites that may pierce the separator and cause an internal short.
• Electrolyte decomposition: at high voltage the electrolyte oxidises, producing gas (swelling) and heat.
• Capacity loss: even mild overcharge shortens life by thickening the SEI layer and consuming active lithium.

Typical protection

1. The charger limits voltage to 4.2 V per cell (within about +/- 1%).
2. The BMS monitors each cell and opens a MOSFET or contactor if a cell exceeds its overvoltage threshold, often about 4.25-4.30 V.
3. Cells also have safety features such as vents and PTC or CID devices in cylindrical designs.

Real-world example

A faulty or non-matching charger on an e-scooter pack, or a failed balancing circuit that lets one cell run ahead of the others, can push a cell above 4.2 V even if the pack voltage looks normal. This is one of the common causes of pack fires reported in practice.

Why is over-discharging a Li-ion battery harmful?

Over-discharging is harmful because taking a Li-ion cell below its minimum voltage (about 2.5-3.0 V for NMC, about 2.5 V for LFP) damages the electrodes permanently, reduces capacity, raises internal resistance and can make the cell unsafe to recharge.

What happens inside

• Copper dissolution: at very low voltage the anode potential rises and the copper current collector can dissolve into the electrolyte. When recharged, this copper may deposit as metallic dendrites and cause an internal short circuit.
• SEI breakdown: the protective layer on the anode can be damaged, causing extra reactions and loss of capacity.
• Structural damage: the electrodes can suffer cracking or loss of active material.
• Gas generation and swelling may follow.

Typical limits and protection

• The BMS undervoltage cut-off is commonly set around 2.8-3.0 V per NMC cell, with a safe recovery margin above it.
• Many cells below roughly 2.0 V should not be recharged normally; manufacturers often advise discarding them.
• Most chargers include a gentle pre-charge for mildly deep-discharged cells but refuse very low-voltage cells.

Common field causes

A battery left connected to a load or a BMS standby current for months can self-drain to a dangerous level. For example, a 10 Ah pack with a 5 mA standby draw loses 10 Ah / 0.005 A = 2000 hours (about 83 days) of charge, so a pack stored at low charge can reach over-discharge within a few months. This is why packs for storage should be kept at partial charge, not empty.

What role does the separator play in preventing internal short circuits?

The separator is a thin, porous, electrically insulating membrane between the anode and cathode. It physically keeps the electrodes apart so they cannot touch and short-circuit, while letting lithium ions pass through its tiny pores filled with electrolyte.

Key features

• Material: usually polyethylene (PE), polypropylene (PP) or a PE/PP multilayer film, often with a ceramic (alumina) coating for higher heat resistance.
• Thickness: typically around 10-25 micrometres (thinner than a human hair), with pore size on the order of a tenth of a micrometre or less.
• Ionic conductivity: porosity of roughly 40-50% lets ions move freely when soaked in electrolyte.

The shutdown function

Many separators have a built-in thermal shutdown. If the cell heats to around 130 degrees C (the melting point of PE), the polymer softens and its pores close. Ion flow stops, current drops and the reaction is halted. Multilayer PP/PE/PP films keep structural strength after shutdown, because PP melts at a higher temperature (about 165 degrees C).

What can still defeat it

• Metallic lithium or copper dendrites growing through the film
• Manufacturing defects such as metal particles or burrs
• Mechanical damage, crush or nail penetration
• Temperatures beyond the separator's melting point, which cause shrinkage and a large short circuit

For this reason, separator quality, coating and cell manufacturing cleanliness are major factors in cell safety, and many cell failures are traced to internal shorts through a damaged separator.

What is the function of a BMS (Battery Management System) for a Li-ion pack?

A Battery Management System (BMS) is the electronic controller of a Li-ion pack. It monitors every cell or cell group, protects the pack from unsafe conditions, balances the cells, estimates the battery's state and communicates with the charger and vehicle or device. In practice it is the safety and intelligence layer of the battery.

Main functions

• Protection: overvoltage, undervoltage, overcurrent, short circuit and over- or under-temperature. It disconnects using MOSFETs or contactors when limits are crossed.
• Monitoring: measures cell voltages, pack current and temperatures at several points.
• Cell balancing: equalises cell voltages so that the weakest cell does not limit the pack (passive or active).
• State estimation: State of Charge (SoC, the 'fuel gauge'), State of Health (SoH) and sometimes State of Power.
• Thermal management control: switches fans, heaters or liquid cooling.
• Communication: sends data and fault codes over CAN bus, UART or RS485, and reports to the charger and vehicle controller.
• Data logging and diagnostics for service engineers.

Why it is essential

Individual Li-ion cells have a narrow safe window (for example 2.5-4.2 V for NMC). A pack of 13 cells in series has 13 chances to go out of limits, and the pack voltage alone cannot show that. The BMS watches each cell, so a problem is caught before it becomes a fire or a costly failure.

Indian context

For electric vehicles in India, AIS-156 and AIS-038 require safety features such as protection against overcharge, over-discharge, overcurrent and thermal events, which the BMS usually implements.

What is a "protection circuit module" (PCM/PCB) on a small Li-ion battery pack?

A Protection Circuit Module (PCM), also called a Protection Circuit Board (PCB), is a small electronic board fitted to a small Li-ion cell or pack that cuts off the current if voltage, current or temperature goes outside safe limits. It is essentially a simplified, low-cost BMS used in phones, power banks, Bluetooth speakers and small packs.

What it contains

• A protection IC that senses cell voltage and current.
• Two back-to-back MOSFETs acting as switches, one for charge and one for discharge.
• A sense resistor to measure current, and often an NTC thermistor for temperature.

Typical protection thresholds (single NMC cell, varies by design)

• Overcharge cut-off: about 4.25-4.35 V
• Over-discharge cut-off: about 2.3-3.0 V
• Overcurrent and short-circuit cut-off: set per cell rating, with a very fast response for short circuit (microseconds to milliseconds)

PCM vs full BMS

Practical note

A bare cell with no PCM can be dangerous in consumer products. A protected 18650 cell has a PCM at the bottom, which makes it slightly longer than 65 mm and sometimes unable to fit tight holders. Because the board is cheap, quality varies, so cells and packs from reliable sources should be used.

What is "cell balancing" and why is it needed in a multi-cell pack?

Cell balancing is the process of equalising the voltage (and therefore state of charge) of all the cells in a series string so that no cell is overcharged or over-discharged while the others are not. It is needed because no two cells are perfectly identical, and small differences grow over time.

Why cells drift apart

• Slight differences in capacity and internal resistance from manufacturing
• Different self-discharge rates
• Temperature differences within the pack (cells in the hotter area age faster)
• Ageing, which widens the gap further with every cycle

Why it matters in a series string

In series, the same current flows through every cell, so the weakest (lowest-capacity) cell decides the usable pack capacity. During charging, that cell reaches 4.2 V first and the BMS must stop charging, even though the other cells are not full. During discharge, the same cell reaches its minimum voltage first and the pack must stop, even though other cells still have energy.

Worked example

In a 13S pack, if twelve cells have 5.0 Ah but one has 4.5 Ah, the pack is limited to about 4.5 Ah per parallel group, a loss of roughly 10% of capacity. Without balancing the gap grows, so the usable range shrinks cycle by cycle and the weak cell is stressed.

Result of good balancing

• Higher usable capacity and range
• Longer pack life
• Reduced risk of overvoltage on a single cell, which improves safety

Balancing normally happens near the top of charge (and in some designs during discharge or rest). It is a key function of the BMS and is especially important in large packs with many cells in series, such as 16S LFP storage packs or 100S+ EV packs.

What is the difference between passive and active cell balancing?

Passive balancing removes excess energy from the higher-voltage cells and burns it as heat in a resistor, while active balancing moves energy from the higher cells to the lower cells (or to the whole pack) using capacitors, inductors or transformers, so little energy is wasted.

How each works

• Passive (resistive or dissipative): when a cell's voltage is above the others, the BMS turns on a MOSFET that connects a bleed resistor across that cell. The cell discharges slowly until the others catch up. Typical balancing currents are small, around 30-100 mA, though some BMS designs go higher.
• Active (redistributive): energy is transferred between cells by switched capacitors, inductor-based converters or isolated transformers, with balancing currents from several hundred mA up to a few amperes.

Comparison

Worked example

If a cell must shed 0.1 Ah with a 50 mA passive bleed, it takes about 2 hours: Time = 0.1 Ah / 0.05 A. This is why passive balancing can be slow for large capacity differences and needs time at the top of charge.

Most Indian e-2W and e-3W packs use passive balancing because of cost, while active balancing is considered for large LFP storage and mismatched second-life packs.

What is meant by "internal resistance" of a Li-ion cell?

Internal resistance (IR) is the opposition to current flow inside a cell, made up of the resistance of the electrodes, electrolyte, separator, current collectors and the contact and interface effects. It is usually in the range of a few milliohms for a good cell and it causes a voltage drop and heat whenever current flows.

Key formulas

• Voltage drop: V drop = I x R. A cell with 20 milliohm resistance carrying 10 A loses 0.2 V.
• Heat generated: P loss = I2 x R. At 10 A and 20 milliohm, that is 2 W per cell.

Typical values

A healthy 18650 cell might have roughly 15-40 milliohm DC resistance, a large 100 Ah prismatic LFP cell under 1 milliohm, and a full 48 V pack typically tens of milliohm. Values depend on the chemistry, size, temperature and state of charge.

Why it matters

• Power capability: lower IR lets the cell deliver high current with less voltage sag.
• Efficiency and heating: higher IR wastes more energy as heat.
• Ageing indicator: IR rises as the cell ages; a rise of about 50-100% over its original value is often treated as end of life in EV applications.
• Cold performance: IR increases sharply at low temperature, which is why voltage sags in winter.
• Matching: cells with very different IR share current unequally and age at different rates.

How it is measured

Two common methods are DCIR (apply a load current step and measure the voltage change, R = delta V / delta I) and AC impedance at 1 kHz using a battery tester. The two give different values, so they should not be compared directly.

What factors cause a Li-ion battery to degrade over time?

Li-ion batteries degrade through slow, mostly irreversible chemical and mechanical changes inside the cell. The main causes are growth of the SEI layer, lithium plating, electrode cracking and electrolyte decomposition, driven by temperature, charge level, current rate and time. The result is loss of capacity and rise in internal resistance.

Main degradation mechanisms

• SEI growth: the Solid Electrolyte Interphase is a thin protective film on the anode. It keeps growing slowly, consuming active lithium and increasing resistance.
• Lithium plating: occurs with fast or cold charging, and removes lithium from circulation.
• Particle cracking and structural change in electrodes due to repeated expansion and contraction.
• Electrolyte decomposition at high voltage or temperature.
• Loss of contact in the electrode layers and current collector corrosion.

Operating factors that accelerate ageing

• High temperature: a common rule of thumb is that reaction rates roughly double for every 10 degrees C rise, so heat strongly shortens life.
• High state of charge: storing at 100% (about 4.2 V) ages cells faster than at 40-60%.
• Deep cycling: larger depth of discharge means fewer total cycles.
• High C-rates for charge or discharge.
• Cold charging (below about 0 degrees C) causing plating.
• Poor balancing and overcharge or over-discharge events.
• Time (calendar ageing) even when unused.

How to extend life

Keep the pack cool, avoid sitting at 100% or 0% for long periods, use moderate charging rates, and use a good BMS with proper balancing. For daily-use vehicles, charging to about 80-90% when full range is not needed can add noticeably to the cycle life.

What is the recommended storage state-of-charge for a Li-ion battery kept unused for a long time?

For long-term storage, a Li-ion battery should be kept at a partial state of charge of roughly 40-60%, which corresponds to about 3.7-3.8 V per NMC cell (and about 3.2-3.3 V per LFP cell), in a cool, dry place. Many manufacturers specify about 50%, with some ranges quoted from 30% to 60%.

Why not full or empty?

• Stored at 100%: the high cell voltage accelerates electrolyte oxidation and SEI growth, causing permanent capacity loss, particularly in warm conditions.
• Stored at 0% or near empty: self-discharge and the BMS standby current slowly drain the cell below its minimum voltage, risking copper dissolution and permanent damage. A deeply discharged cell may also be unsafe to recharge.

Storage best practice

1. Charge or discharge to about 50% before storing.
2. Store at roughly 10-25 degrees C; avoid direct sun, vehicles in summer heat and damp locations.
3. Disconnect the pack from the load and, if possible, switch off the BMS or use a sleep mode.
4. Check the voltage every 2-3 months and top up to about 50% if it has dropped noticeably.
5. Keep it on a non-flammable surface away from combustible material.

Typical numbers

Li-ion self-discharge is low, often about 1-3% per month at room temperature, so a pack stored at 50% will usually remain in a safe range for many months. At high temperature the loss is much faster.

Example

A 13S NMC pack stored at 50% would read about 13 x 3.7 V = 48 V; one at 100% reads about 54.6 V. Service engineers often use this simple voltage check to confirm the storage state before putting stock in the warehouse.

What is "thermal runaway" in a Li-ion battery?

Thermal runaway is an uncontrolled, self-accelerating rise in a cell's temperature, in which heat from internal chemical reactions raises the temperature further and triggers more reactions. It can end in venting of hot gas, fire and, in a pack, spread to neighbouring cells (thermal propagation).

How it develops

1. A trigger (see the next question) raises the local temperature or causes an internal short.
2. At roughly 80-120 degrees C the protective SEI layer on the anode begins to decompose, releasing heat.
3. Around 130-150 degrees C the separator may melt and cause a larger internal short.
4. Above roughly 150-200 degrees C the cathode (in NMC/NCA) can break down and release oxygen.
5. The electrolyte vaporises and burns, gas pressure builds and the cell vents. Temperatures can reach several hundred degrees C, and the process may finish within seconds to minutes.

Why it is dangerous

• It generates its own oxygen in some chemistries, so it cannot be smothered easily.
• The vented gas is flammable and toxic (for example hydrogen, carbon monoxide, hydrocarbons, and hydrogen fluoride).
• Heat from one failing cell can set off its neighbours and spread through the pack.

How designers reduce the risk

• Choose stable chemistry such as LFP where possible
• BMS protection and temperature sensing
• Spacing, insulation or barriers between cells, and heat-absorbing materials
• Venting paths and cooling systems
• Cell-level safety devices such as CID and PTC

In India, the AIS-156 standard for L-category electric vehicles includes safety provisions related to thermal events and thermal propagation, which is why pack designers pay special attention to cell spacing and early warning.

What common triggers can cause thermal runaway in a Li-ion battery?

Thermal runaway is typically triggered by an internal short circuit, electrical abuse, thermal abuse, mechanical abuse or a manufacturing defect. In most field cases, the final trigger is an internal short that releases heat faster than the cell can lose it.

Common triggers

Why overcharge and plating are so dangerous

Overcharge destabilises the cathode and electrolyte, while lithium plating creates dendrites that can pierce the separator over many cycles. Such internal shorts may develop slowly and fail later, sometimes while the vehicle is parked and charging.

Practical examples

• An e-scooter charged overnight with a non-original charger that has no voltage cut-off.
• A battery pack that has been submerged in water during flooding and then charged.
• A pack with a loose connection that heats up under load and damages nearby cells.

Prevention

Use certified cells and chargers, a quality BMS, proper pack insulation and sealing (IP rating), correct installation torque on connections, and avoid charging damaged batteries.

Why can a Li-ion battery fire reignite even after appearing to be extinguished?

A Li-ion fire can reignite because cells inside the pack may still be hot or still in thermal runaway, and they still contain stored electrical and chemical energy. Extinguishing the visible flames does not remove the heat or the energy, so neighbouring cells can continue to heat and ignite minutes, hours or even days later.

Reasons for reignition

• Self-contained oxygen: in NMC and NCA cells the cathode itself releases oxygen when hot, so the reaction can continue without outside air.
• Remaining stored energy: undamaged or partly damaged cells still hold charge, and a short circuit can release it as heat again.
• Cell-to-cell propagation: a pack contains many cells; the fire may have consumed some while others remain at high temperature but are not yet venting.
• Shielded cells: the casing can hide hot cells from water or extinguishing agent.
• Flammable electrolyte and gas: residual electrolyte and vented gases can reignite from a spark or hot surface.

What responders do

1. Cool the pack with large amounts of water or other suitable cooling agent to take heat away (the aim is cooling, not only putting out flames).
2. Keep monitoring the temperature, often with a thermal camera.
3. Isolate the damaged battery outdoors, in a safe area, away from combustible material, and consider a sand or water tank for fully submerging in some cases.
4. Observe it for a long period, commonly 24 hours or more, with attention to smoke, hissing and heat.

Real-world example

Electric vehicle fire services report that wrecked EVs have reignited in storage yards after being towed, which is why such vehicles are kept apart from other vehicles and buildings for a monitoring period.

What are common warning signs that a Li-ion battery may be failing or unsafe?

Common warning signs that a Li-ion battery is failing or unsafe include swelling, unusual heat, a strange smell, leakage, smoke or hissing, rapid capacity loss, voltage problems and physical damage. Any of these signs means the battery should be taken out of service and inspected.

Physical signs

• Swelling or bulging of a pouch, prismatic or pack casing, caused by gas generated inside the cell.
• Leakage of liquid, crystalline residue or a sweet or chemical smell (the electrolyte solvents).
• Dents, cracks or corrosion on the case or terminals, or discoloured or melted connectors.
• Water ingress marks or rust.

Thermal and sound signs

• The battery becomes hot to touch during normal charging or at rest, or far hotter than usual under load.
• Hissing, popping or crackling sounds, or visible smoke or vapour.

Electrical and performance signs

• Sudden or rapid capacity loss, much shorter run time or range than before.
• Large voltage sag under load or a quick drop in voltage after charging.
• Large voltage difference between cells (for example more than about 50-100 mV at full charge, depending on BMS limits), which suggests a weak cell or balancing failure.
• Frequent BMS faults, unexplained shutdowns, or the pack refusing to charge.
• Very long charge times, or charging that never reaches full.
• Rising internal resistance noticed during diagnostics.

Service tip

Log voltages, temperature and fault codes through the BMS interface when possible. A gradual trend, for example one cell always lower than the rest, helps predict failure before it becomes dangerous.

What should you do if you notice a Li-ion battery swelling or overheating?

If you notice a Li-ion battery swelling or overheating, stop using and charging it immediately, disconnect it from the load and charger if it is safe to do so, move it away from people and flammable materials, and keep it under observation in a safe, open or non-combustible area. Never ignore, puncture or press a swollen cell.

Immediate steps

1. Stay safe: do not hold or carry a hot battery with bare hands; if it is smoking or hissing, keep your distance and ventilate the area.
2. Disconnect: unplug the charger and turn off the device, but only if this can be done without risk.
3. Isolate: move it, using insulated gloves or tools, to a non-combustible surface such as concrete or sand, outdoors or near an exit. Keep it away from cars, paper, cloth and fuel.
4. Observe: keep watching for smoke, heat and noise for at least several hours; thermal events can be delayed.
5. If it ignites or smokes heavily: evacuate, call the fire emergency number (101 in India, or 112 national emergency), and inform them it is a lithium-ion battery.

What not to do

• Do not puncture, press or try to 'release the gas' from a swollen cell.
• Do not continue charging or using it 'one more time'.
• Do not put it in a closed drawer, bag, vehicle or bedroom.
• Do not dispose of it in household waste.

After the incident

Record the date, charger, usage and symptoms for the warranty claim or root-cause analysis. The battery must be sent through an authorised service or recycler, with proper packaging for damaged and defective batteries, since transporting such a pack is regulated. A service engineer should also check the charger and the remaining packs from the same lot.

What type of fire extinguisher or method is recommended for a lithium-ion battery fire?

For a Li-ion battery fire, the recommended method is to apply large quantities of water to cool the battery and stop heat spreading, together with evacuation and a call to the fire service. Li-ion cells contain very little metallic lithium, so the fire is mainly a fire of hot, burning electrolyte and electrode materials, and the key problem is heat.

Choosing the method by situation

Important points

• Cooling is the goal. CO₂ or dry powder may knock down flames but do not cool the cells, so the fire can reignite.
• Class D (metal fire) extinguishers are meant for metallic lithium and other metals, and are generally not the right choice for Li-ion cells.
• Some specialised agents, such as aqueous vermiculite or wet-chemical-based cooling agents, are designed for lithium-ion fires; availability varies.
• Leave large or high-voltage packs to trained firefighters, who follow set procedures because of the electric shock risk.

Safety message

Personal safety comes first. Smoke from Li-ion fires contains toxic gases, including hydrogen fluoride, so do not fight a large pack fire without training, and keep monitoring the battery for a long time afterwards since reignition can occur.

Why should damaged or defective Li-ion batteries never be thrown in regular household trash?

Damaged or defective Li-ion batteries should never be thrown in regular household trash because they can short-circuit, overheat and start fires in bins, garbage trucks and waste facilities, and because they contain hazardous and valuable materials that must be handled and recycled properly. In India, used batteries fall under the Battery Waste Management Rules, 2022.

Safety hazards

• Compaction and crushing: garbage trucks and shredders can puncture or crush cells, causing internal shorts and fire.
• Residual charge: a discarded battery is rarely fully empty, so touching metal objects in the bin can short the terminals.
• Delayed fires: a damaged cell may fail hours later, igniting paper and plastic waste.
• Waste facility fires: fires in recycling and landfill sites are well documented worldwide and put workers at risk.

Environmental and health hazards

• Batteries contain electrolyte with fluorinated lithium salts, and metals such as cobalt, nickel and manganese, which can contaminate soil and groundwater in landfills.
• Burning or heating releases toxic gases, including hydrogen fluoride.

Resource loss

Cobalt, nickel, lithium, copper and aluminium can be recovered and reused. India imports most of its battery raw materials, so recovery is valuable for supply security.

What to do instead

1. Tape the terminals with insulating tape, to prevent short circuits.
2. Place the battery in a non-conductive, non-flammable container, ideally separate from other items, with sand or vermiculite for damaged cells.
3. Hand it over to the manufacturer's take-back system, an authorised collection point or a registered recycler.

Under the extended producer responsibility (EPR) principle in the rules, producers and importers are responsible for collecting and recycling the batteries they place on the market.

What is "battery recycling" and why is it important for Li-ion batteries?

Battery recycling is the process of collecting end-of-life batteries and recovering their materials, such as lithium, cobalt, nickel, manganese, copper, aluminium and graphite, so that they can be reused in new products rather than dumped. It matters because it reduces fire and pollution risk, saves scarce raw materials and lowers the cost and carbon footprint of new batteries.

Main stages

1. Collection and transport under safety rules for used and damaged batteries.
2. Discharging and dismantling of packs into modules and cells.
3. Mechanical processing: shredding, usually in an inert atmosphere, then separation of casing, foils and 'black mass' (a powder rich in electrode materials).
4. Material recovery by one or more routes.

Recycling routes

• Pyrometallurgy: high-temperature smelting, which recovers metals such as cobalt, nickel and copper as an alloy, but lithium and aluminium are often lost to slag.
• Hydrometallurgy: dissolving black mass in acids and recovering metals chemically, with higher recovery including lithium and lower energy use.
• Direct recycling: recovering and rejuvenating cathode material without breaking it down fully; still emerging.

Why it is important

• Resource security: India imports most of the lithium, cobalt and nickel it uses, so recovering them from used batteries reduces dependence.
• Environment: reduces mining impact and prevents toxic waste.
• Safety: prevents unsafe disposal and landfill fires.
• Economics: recovered metals offset part of the cost of new cells.

Indian context

The Battery Waste Management Rules, 2022 introduce extended producer responsibility, requiring producers to ensure collection and recycling of used batteries and to use a minimum share of recycled material in new ones as set by the rules. Recycling is also linked to second-life reuse, which usually comes first when a pack still has useful capacity.

What is "voltage cutoff" and why does a device stop working before the battery is truly at 0V?

A voltage cutoff is the minimum voltage at which a battery or its BMS stops discharge to protect the cell. A device stops working before the battery reaches 0 V because discharging a Li-ion cell below its safe lower limit causes permanent damage and safety risk, and because the remaining energy below that point is very small anyway.

Typical cutoff values

• NMC/NCA/LCO: about 2.5-3.0 V per cell (many devices cut at about 3.0 V).
• LFP: about 2.5 V per cell, sometimes up to 2.8-3.0 V in conservative designs.
• LTO: about 1.5-1.8 V per cell.

Why not run it down to 0 V?

• Copper dissolution and SEI damage at very low voltage can lead to internal shorts when the cell is recharged.
• Little extra energy: the discharge curve falls steeply near the end, so only a few percent of capacity lies between about 3.0 V and 2.5 V.
• Voltage sag: under load, the terminal voltage is lower than the open-circuit voltage; a cutoff stops the load before the cell is stressed.
• Device needs: electronics need a minimum input voltage to run; a phone may shut down at 3.3 V because its regulator can no longer operate.

Layers of cutoff

1. The device or vehicle controller reduces power (limp mode) and shows 0% SoC at a set voltage.
2. The BMS undervoltage protection opens the discharge switch slightly lower, often about 2.8-3.0 V for NMC.
3. The cell's own design limit remains the absolute lower bound.

Example

In a 13S NMC pack, a 3.0 V cutoff per cell means the pack stops at about 13 x 3.0 = 39 V, even though the pack was 54.6 V when full. A pack that is stopped at 39 V is 'empty' but still safe.

What is the difference between "series" and "parallel" connection of Li-ion cells in a pack?

In a series connection, the positive terminal of one cell is connected to the negative terminal of the next, so the voltages add while the capacity (Ah) stays the same. In a parallel connection, all positive terminals are joined together and all negative terminals are joined together, so the capacity (Ah) and current capability add while the voltage stays the same.

Comparison

Worked example

Three 3.6 V, 2.5 Ah cells:

• In series (3S): 10.8 V, 2.5 Ah, 27 Wh.
• In parallel (3P): 3.6 V, 7.5 Ah, 27 Wh.

The energy is the same, only the split between voltage and capacity differs.

Practical notes

• Real packs combine both: a 13S4P pack has four cells in parallel to form a 'group', and 13 such groups in series.
• Cells in parallel should have matched voltage before connecting; a big difference can cause a high equalising current.
• In parallel groups, individual cell fuses or wire bonds protect against a shorted cell.

What does a battery pack specification like "13S4P" mean?

A pack specification such as 13S4P means the pack contains 13 cells (or cell groups) connected in series, and each group is made of 4 cells connected in parallel. The total number of cells is 13 x 4 = 52 cells. The letter S stands for series and P for parallel, and the number in front of each letter tells how many.

How to read it

• 13S: the pack voltage is 13 times the single-cell voltage.
• 4P: the pack capacity is 4 times the single-cell capacity.

Worked example

Using NMC cells rated 3.6 V nominal, 4.2 V full, 5 Ah, 18 Wh:

• Nominal voltage = 13 x 3.6 = 46.8 V (commonly called a '48 V' pack)
• Full-charge voltage = 13 x 4.2 = 54.6 V
• Cut-off voltage (3.0 V per cell) = 13 x 3.0 = 39 V
• Capacity = 4 x 5 Ah = 20 Ah
• Energy = 46.8 V x 20 Ah = 936 Wh (equal to 52 cells x 18 Wh)

Why it matters

• BMS selection: a 13S pack needs a 13-series BMS, with 13 voltage-sense channels, and a current rating that suits the whole pack.
• Charger selection: the charger must produce 54.6 V full-charge voltage for 13S NMC.
• Design and service: engineers know exactly how many cell groups are in series when troubleshooting a low group voltage.

Notation note

Some manufacturers write it as 4P13S or give cell groups in the opposite order; always read the numbers next to the letters, and ask whether 'P' groups are wired before series connection, which is the normal approach.

How would you calculate the total voltage of a battery pack with cells connected in series?

The total voltage of cells connected in series is the sum of the individual cell voltages. For identical cells, Pack voltage = number of cells in series x voltage of one cell. The capacity in Ah remains equal to that of a single cell (or a single parallel group).

Voltage values to use

Always calculate three useful voltages, not just the nominal:

• Nominal voltage (for energy): N x 3.6 V for NMC, N x 3.2 V for LFP.
• Maximum (full charge) voltage (for the charger setting and BMS overvoltage): N x 4.2 V for NMC, N x 3.65 V for LFP.
• Minimum (cut-off) voltage (for the controller or inverter lower limit): N x about 3.0 V for NMC, N x about 2.5 V for LFP.

Worked examples

How to check on a real pack

Measure each cell group voltage with a multimeter or through the BMS, and add them up. The sum should be close to the pack terminal voltage. A large mismatch indicates a wiring, contact or measurement issue.

Common mistakes

• Using the nominal voltage for charger settings; the charger needs the full-charge value.
• Forgetting that cells in parallel do not add voltage.
• Ignoring voltage sag under load, which makes the on-load voltage lower than the open-circuit voltage.

The same approach gives the voltage of a module in a larger pack: module voltages in series add up to the pack voltage, which can reach 300-800 V in electric cars and buses.

How would you calculate the total capacity of a battery pack with cell groups connected in parallel?

The total capacity of cell groups connected in parallel is the sum of the capacities of the parallel cells, while the voltage stays the same. For identical cells, Pack capacity (Ah) = number of cells in parallel x capacity of one cell. In a series-parallel pack, only the parallel count (the P number) multiplies the Ah.

Worked examples

• 4 cells of 5 Ah in parallel (4P): 4 x 5 = 20 Ah, at the same 3.6 V.
• A 13S4P pack of 5 Ah cells: 20 Ah (the 13 series groups do not add Ah). At 46.8 V nominal, energy is 936 Wh.
• A 16S2P pack of 100 Ah LFP cells: 200 Ah at 51.2 V, which is 10.24 kWh.

Related current ratings

The parallel connection also adds current capability. If each cell supports 10 A continuous discharge, 4 cells in parallel support about 40 A. The pack current in amperes is the sum of the currents in each parallel branch.

Why parallel matching matters

• Cells in parallel share the voltage, so if their internal resistance differs, the lower-resistance cell carries more current and ages faster.
• If one cell in a parallel group fails open, the group capacity falls by that share, e.g. from 20 Ah to 15 Ah.
• If a cell shorts, the others can push current into it, so cell-level fuses or wire bonds are used to isolate it.

Using capacity for runtime

Runtime estimate: Runtime (h) = capacity (Ah) / load current (A). A 20 Ah pack feeding a 10 A load runs about 2 hours in the ideal case; in practice usable time is a little less because of voltage cut-off, temperature and ageing.

What is the significance of the "maximum continuous discharge current" rating on a Li-ion cell/pack?

The maximum continuous discharge current is the highest steady current that a cell or pack can supply for a sustained period without exceeding its safe temperature limit or damaging itself. It is the main rating used to decide whether a cell or pack can power a given load, such as an e-scooter motor controller or a power tool.

Why it matters

• Heat limit: heat is I2R, so current beyond the rating overheats the cell, speeds up ageing and may trigger protection or thermal runaway.
• Voltage sag: at high current the voltage drops more, so the system may hit cut-off early.
• Protection settings: the BMS overcurrent threshold must be set in line with the rating.
• Sizing: it tells you how many cells must be in parallel to support the load.

Related ratings

• Peak or pulse discharge current: a much higher current allowed for a short time (for example 10-30 seconds); used for acceleration.
• Continuous charge current: usually lower than discharge.

Sizing example

An e-scooter controller draws 40 A continuous from a 48 V pack. If each 21700 cell is rated for 10 A continuous, the pack needs at least 40 / 10 = 4 cells in parallel. A safety margin of 20-30% is normal, so a 5P design is a better choice. If the pack is 20 Ah (4P x 5 Ah), 40 A is 2C, which is demanding for energy cells.

Typical ranges

High-energy cells commonly allow about 0.5-2C continuously; high-power cells can allow 5-20C or more. The rating is usually quoted at a given temperature and a surface temperature limit (often about 60 degrees C), so datasheet conditions must be read carefully.

Why does a Li-ion battery's usable capacity drop noticeably in cold temperatures?

A Li-ion battery's usable capacity drops in cold temperatures because the chemical and ionic processes inside the cell slow down: the electrolyte becomes more viscous, lithium ions move more slowly, and the internal resistance rises sharply. The cell voltage therefore sags under load and reaches the cut-off voltage earlier, so less of the stored charge can be delivered.

Mechanisms

• Slower ion transport: lower electrolyte conductivity and slower diffusion in the graphite and cathode particles.
• Higher internal resistance: the voltage drop I x R is larger, and the voltage hits the cut-off earlier.
• Slower reaction at the electrode surfaces (higher charge-transfer resistance).
• The energy is not permanently lost; much of the capacity returns when the cell warms up.

Typical effect

Around 0 degrees C, usable capacity might be roughly 80-90% of that at 25 degrees C, and at about -20 degrees C it may fall to about 50-70% depending on chemistry, rate and design. LFP typically loses more at low temperature than NMC.

Charging in the cold is more dangerous

Charging below about 0 degrees C can cause lithium plating on the anode, which permanently reduces capacity and can create dendrites. Most BMS units block or limit charging below a set temperature (often 0-5 degrees C) and some include heaters.

Practical solutions

• Pre-heat the battery using a heater or by using the vehicle before charging.
• Insulate the pack, and park indoors in cold regions.
• Use cells designed for low temperature, such as certain NMC or LTO cells.
• Charge at lower current at low temperatures.

In India, this matters for Himalayan and northern winter operations, while heat is the larger concern in most other regions.

What is a "fuel gauge IC" or "gas gauge" in a battery pack?

A fuel gauge IC (also called a gas gauge) is a dedicated chip in a battery pack that measures voltage, current and temperature and calculates the State of Charge (SoC), remaining capacity and often State of Health (SoH), the battery equivalent of a car's fuel gauge. The name comes from the 'gas gauge' in a petrol vehicle.

Why a simple voltage reading is not enough

Li-ion voltage depends on load, temperature and age, and for some chemistries the discharge curve is very flat. LFP, for example, stays near 3.2-3.3 V across most of its range, so voltage alone gives a poor SoC estimate.

How it estimates SoC

• Coulomb counting: a small sense resistor measures current, and the chip integrates it over time. SoC = SoC(initial) - (integral of I dt) / capacity. It is accurate in the short term but drifts over time.
• Voltage-based (open-circuit voltage lookup): uses the voltage after rest, with temperature compensation, to correct the drift.
• Model-based algorithms: some ICs use impedance tracking or Kalman filters to combine current, voltage, temperature and a battery model for better accuracy.

Typical outputs

• SoC (%) and remaining capacity (mAh or Ah)
• Full-charge capacity and State of Health
• Time to empty and time to full
• Cycle count and temperature
• Communication to the host over I2C, SMBus, UART or CAN

Where it is used

Gauge ICs are found in laptops, phones, tablets and medical devices, from vendors such as Texas Instruments and Analog Devices. In bigger EV packs, the same function is a software algorithm inside the BMS. Accuracy of a good gauge is often within a few percent, but it can worsen as the cell ages unless it learns the new full capacity.

What is "coulombic efficiency" of a Li-ion cell?

Coulombic efficiency (CE), also called Faradaic efficiency, is the ratio of the charge removed from a cell during discharge to the charge put in during the preceding charge: CE (%) = (discharge capacity in Ah / charge capacity in Ah) x 100. It shows how much of the charge is actually recovered rather than lost to side reactions.

Typical values

A healthy Li-ion cell has a coulombic efficiency of roughly 99% or higher in steady cycling, and research cells are often discussed at 99.9%. The first cycle is lower, often around 85-95%, because lithium is consumed to build the SEI layer on the anode.

Worked example

If a cell takes 5.00 Ah in during charge and delivers 4.97 Ah during discharge, CE = 4.97 / 5.00 = 99.4%. The missing 0.03 Ah has gone into side reactions such as SEI growth and electrolyte decomposition, and each cycle small losses build up.

Why it matters

• Indicator of ageing: low or falling CE means active lithium is being lost, and capacity will fade faster.
• Cell quality check: manufacturers and researchers use precise CE measurement to compare cell designs and electrolytes quickly.
• Cycle-life prediction: a CE of 99.9% implies roughly 0.1% lithium loss per cycle, whereas 99.0% would lose about 1% per cycle.

Do not confuse with energy efficiency

Energy (round-trip) efficiency compares watt-hours out to watt-hours in, and is lower than coulombic efficiency (typically around 90-95% for Li-ion) because the charging voltage is higher than the discharging voltage and internal resistance causes heat. Coulombic efficiency counts charge only, in ampere-hours.

What safety standards/certifications are commonly associated with Li-ion batteries?

Li-ion batteries are associated with several safety, transport and performance standards, which differ by application: UN 38.3 for transport, IEC 62133-2 (and its Indian equivalent IS 16046 Part 2) for portable cells and batteries, IEC 62619 for industrial and stationary batteries, UL 1642 / UL 2054 for cells and packs, and AIS-156 and AIS-038 for electric vehicles in India.

Common standards by purpose

Why standards matter

• Standards set tests such as overcharge, external short circuit, crush, thermal abuse, vibration and propagation.
• Certification is often required before the product can be marketed, imported, insured or used on the road.
• They give users confidence that the product was tested and help the manufacturer to avoid recalls.

Practical check

Always check the latest revision and the scope before claiming compliance, since standard numbers and amendments change. A certificate covers only the tested model and configuration, so ask the manufacturer for test reports from accredited labs.

Why must Li-ion batteries pass special testing (like UN 38.3) before being transported by air or sea?

Li-ion batteries must pass UN 38.3 before air or sea transport because they are classified as dangerous goods (Class 9, miscellaneous). The test series proves that the cell or battery can survive the stresses of shipping, such as pressure change, vibration, shock, heat and short circuit, without leaking, venting, catching fire or exploding.

Why the testing is needed

• A fire in the cargo hold of an aircraft or in a ship container can be very hard to control, and lithium batteries have been linked to several cargo fires.
• Shipping exposes batteries to low pressure at altitude, temperature swings, vibration and rough handling.
• Regulators (UN, IATA, ICAO, IMDG) require evidence of safety before a battery is accepted for carriage.

The eight tests (T1-T8)

1. T1 Altitude simulation: low pressure for at least 6 hours.
2. T2 Thermal test: repeated cycling between extreme hot and cold temperatures.
3. T3 Vibration: simulates transport vibration.
4. T4 Shock: half-sine shocks.
5. T5 External short circuit.
6. T6 Impact or crush (for cells).
7. T7 Overcharge (for batteries).
8. T8 Forced discharge (for cells).

The pass criteria include no mass loss, no leakage, no venting, no disassembly, no rupture and no fire, with the open-circuit voltage staying above 90% of its pre-test value for most tests.

Related transport rules

• UN numbers: UN3480 for Li-ion batteries shipped alone and UN3481 for those packed with or contained in equipment.
• Batteries shipped alone by air are normally limited to about 30% state of charge.
• Damaged or defective batteries are subject to special packaging and often need prior approval.

A test summary must be available from the manufacturer, and a supplier who cannot provide one may not be able to ship the product legally.

What are the main applications of Li-ion batteries today?

Li-ion batteries today power a wide range of products: consumer electronics, electric vehicles, energy storage systems, power tools and industrial equipment, medical devices and telecom backup. Their high energy density, long life and falling cost have made them the dominant rechargeable battery technology.

Main application areas

Indian market highlights

• Electric two- and three-wheelers are the largest EV segments by volume, along with growing e-bus and e-cargo fleets.
• Battery energy storage is growing alongside solar and wind, and for telecom and UPS replacement of lead-acid.
• The government's PLI scheme for Advanced Chemistry Cell (ACC) battery storage aims to build domestic cell manufacturing capacity.

Why the spread

Li-ion's usefulness comes from its flexibility: the same technology can be built as a 3 Wh phone cell or a multi-MWh storage container, by choosing the chemistry, format and pack design to fit the application.

What test equipment is commonly used to evaluate/diagnose a Li-ion battery pack?

A Li-ion pack is evaluated with a combination of electrical, thermal and data tools: a digital multimeter, a battery analyser or cycler, an internal resistance tester, a BMS diagnostic interface, a thermal camera, and an electronic load and programmable charger. Together they verify voltage, capacity, resistance, balance, temperature and protection behaviour.

Typical equipment and what it checks

Typical diagnostic flow

1. Visual inspection for swelling, damage and corrosion.
2. Measure open-circuit voltage and compare cell-group voltages.
3. Read BMS data and fault history.
4. Check internal resistance.
5. Do a controlled discharge and charge to measure the real capacity against the rated value.
6. Observe temperature during load for hot spots.

Safety gear, such as insulated gloves, tools and a fire-safe test area, is part of the setup.

What precautions should be taken while measuring voltage on a multi-cell Li-ion pack?

When measuring voltage on a multi-cell Li-ion pack, the main precautions are to treat the pack as live and energised, use insulated tools and a correctly rated meter, avoid shorting any two points, and measure with the pack isolated and in a safe condition. Even a small pack can deliver very high short-circuit current, and a high-voltage pack can be lethal.

Before measuring

• Inspect first: do not handle a swollen, leaking or damaged pack without protective measures.
• Use the right PPE: insulated gloves, safety glasses and, for high-voltage packs, rated gloves and non-conductive footwear.
• Remove metal jewellery and watches, and keep metal tools away from the terminals.
• Check the meter: use a DMM with the correct CAT rating and voltage range (a 54.6 V pack needs a meter rated well above that), in good condition with undamaged leads.
• Isolate the pack from load and charger, and for EV packs follow the manufacturer's service disconnect procedure.

While measuring

1. Set the meter to DC voltage and the right range, and keep the probes on the intended points only, with probe tips shrouded if possible.
2. Use fine probes or sense leads on a balance connector; do not let the probes touch neighbouring pins, because that creates a short circuit.
3. Measure the total pack voltage, then each cell-group voltage, and verify that the sum matches the total.
4. Do not use the meter in the current (A) mode across the terminals, as this effectively shorts the pack.
5. Work with one hand where possible, and avoid contact with two different potentials at once.

After measuring

Record readings with date and temperature, note the cell-voltage spread (for example more than about 50-100 mV difference at rest suggests imbalance), and reconnect only after checking that nothing is left shorted. Measure at rest, because the pack voltage under load or just after charging is not stable.

Why should mismatched or different-age cells generally not be mixed in the same pack?

Mismatched or different-age cells should not be mixed in a pack because differences in capacity, internal resistance and voltage cause unequal stress. In series, the weakest cell limits the whole pack and is overstressed; in parallel, the cells share current unevenly and age at different rates. The result is lower capacity, shorter life and a higher safety risk.

What goes wrong

• In series: the same current passes through all cells, so the lowest-capacity cell is fully charged and discharged first. It reaches the overvoltage or undervoltage limit before the others, so usable pack capacity equals that weak cell's capacity, and the BMS balancing struggles to keep up.
• In parallel: cells with lower internal resistance carry more current and heat up more, so they age faster. A big voltage difference when joining cells drives a large equalising current.
• Different chemistries or voltages have different charge limits and cannot be charged safely by the same charger and BMS settings.
• Different ages mean different capacity fade and resistance rise; the older cells are weaker and the pack behaves as if it were all as old as the worst cell.

Worked example

If new 5 Ah cells are mixed with old cells that have faded to 3.5 Ah, the series pack delivers only about 3.5 Ah, so 30% of the new cells' capacity is wasted. The old cell is also cycled at 100% depth of discharge while the new ones are not, so it fails sooner and may swell or heat.

Good practice

• Use cells of the same make, model, batch, chemistry and similar age.
• Match capacity and internal resistance within a tight tolerance (often within a few percent) before building a pack.
• Sort and grade cells in second-life packs, and use a BMS suited to the remaining mismatch, perhaps with active balancing.

Replacing one weak cell group in an old pack is therefore rarely a good fix.

What key points would you check before reusing or repurposing old EV/Li-ion battery packs (second-life batteries)?

Before reusing or repurposing old EV or Li-ion packs (second-life use), you must verify their remaining health, safety and history, grade and sort them, and confirm that the pack can be made safe and economical for the new application. A used EV pack is usually retired at roughly 70-80% of original capacity, which still suits many stationary applications.

Key points to check

1. History and data: vehicle usage, BMS logs, cycle count, fault codes, any accident, flood, overheating or deep-discharge events. Verify the chemistry and cell model.
2. Visual and mechanical condition: swelling, leakage, corrosion, damaged casing, connector or busbar problems and signs of water ingress.
3. State of Health: do a full charge-discharge capacity test to measure actual capacity against rated.
4. Internal resistance: measure DCIR or AC resistance; high or widely scattered values mean poor power capability.
5. Cell voltage spread and self-discharge: rest the pack and look for cells that drift, which suggest internal micro-shorts.
6. Insulation resistance and high-voltage safety: check isolation, contactors, fuses and the pack's sealing.
7. BMS and communication: confirm the original BMS can be read and used, or plan a replacement with suitable settings.
8. Thermal system: check cooling and heating, sensors and fans.
9. Matching and grouping: sort modules or cells by capacity and resistance, and avoid mixing very different ones.

Economic and compliance points

• Check ownership, warranty and the regulatory requirements such as the Battery Waste Management Rules, 2022 in India.
• Estimate cost of testing and rework against a new pack and expected remaining life.
• Choose low-power, low-stress applications, for example home backup, telecom backup or solar storage.

Remaining life is also uncertain, so any warranty or performance guarantee for a second-life pack should be conservative.

What overall checklist would you use to select the right Li-ion battery/pack for a given application?

To select the right Li-ion battery or pack for an application, first define the load requirement (voltage, energy, power and duty cycle), then choose the chemistry, form factor, configuration, BMS and safety features that meet those needs within the cost, size and environmental limits. A structured checklist avoids both under-sizing and over-spending.

Selection checklist

1. Voltage: required system voltage window (controller or inverter limits) decides the series count, for example 13S NMC for about 48 V.
2. Energy and runtime: Energy (Wh) = load power (W) x runtime (h), divided by usable depth of discharge. A 500 W load for 4 hours needs 2000 Wh, or about 2.5 kWh when usable DoD is 80%.
3. Power and current: peak and continuous current set the C-rate and parallel count; check the cell's continuous discharge rating.
4. Chemistry: NMC or NCA for high energy density, LFP for safety, long life and heavy cycling, LTO for extreme cycling and fast charge.
5. Cycle life and warranty: required years or cycles, depth of discharge, and cost per cycle rather than only price per kWh.
6. Operating environment: temperature range, humidity, dust, vibration, IP rating, altitude and need for cooling or heating.
7. Charging: available charger, charge time, fast-charge needs and charging interface.
8. Size and weight: space, mounting and weight limits.
9. BMS and communication: protections, balancing type, SoC accuracy, and CAN or RS485 compatibility with the vehicle or inverter.
10. Safety and certification: relevant standards such as UN 38.3, IS 16046 or IEC 62133-2, IEC 62619, AIS-156 or AIS-038 as applicable; fusing and thermal propagation design.
11. Cost and total cost of ownership: purchase price, lifetime, efficiency, maintenance, and end-of-life recycling under the Battery Waste Management Rules.
12. Supplier support: cell origin, quality records, service network, spare parts and local support.

Example

For an e-3W with a 48 V controller needing 5 kWh and heavy daily use in hot weather, a 16S LFP pack of about 100 Ah (51.2 V) is usually a sensible choice for its safety and cycle life.