Lithium-ion cells, battery packs and thermal management

Why it matters

The battery is the most expensive, heaviest and most safety-critical part of an electric vehicle, and the choice of cell chemistry, pack layout and cooling decides range, charging speed, life and fire risk. Battery fires in Indian two-wheelers led to tighter battery-safety standards, and pack design now dominates EV engineering jobs. You must be able to go from a cell datasheet to pack voltage, energy, losses and cooling requirement.

Key ideas

How a lithium-ion cell works. Both electrodes are intercalation hosts: lithium ions slot into their crystal structures. On discharge, lithium leaves the negative electrode (anode, usually graphite, sometimes with silicon), travels through the electrolyte and separator, and enters the positive electrode (cathode, a lithium metal oxide or phosphate); electrons flow through the external circuit. Charging drives the ions back. On the first charges a thin solid-electrolyte interphase (SEI) forms on the graphite; it protects the anode but slowly grows and consumes lithium, which is one cause of ageing.

• Electrolyte: a lithium salt (usually LiPF₆) in organic carbonate solvents — flammable, which is why thermal runaway is dangerous.
• Separator: a porous polyethylene/polypropylene film that keeps the electrodes apart but passes ions; many melt at about 130 °C and close their pores ("shutdown").

Cathode chemistries.

• NMC / NCA (nickel-manganese-cobalt / nickel-cobalt-aluminium): nominal about 3.6–3.7 V, voltage window about 2.5–4.2 V, highest specific energy — common in cars.
• LFP (lithium iron phosphate): nominal about 3.2 V, window about 2.5–3.65 V, lower specific energy but longer cycle life, better thermal stability and no cobalt — widely used in Indian two-, three-wheelers, buses and many cars. Its flat voltage curve makes state of charge harder to estimate.
• LTO anodes (lithium titanate) give very long life and fast charging but low voltage (about 2.3–2.4 V) and energy. Exact values come from the cell datasheet.

Cell formats. Cylindrical (18650, 21700, 4680), prismatic (hard aluminium case) and pouch (laminated foil, needs external support and allows swelling). Large prismatic and pouch cells reduce the number of connections; cylindrical cells are cheap and robust but need many interconnections.

Ratings. Capacity in A·h; C-rate = current ÷ capacity (1C empties the cell in about one hour); specific energy (Wh/kg) and energy density (Wh/L), which fall from cell to pack level because of housings, busbars, cooling and electronics; cycle life (often quoted to 80 % of initial capacity); internal resistance, which sets power capability and heat.

From cell to pack. Cells are joined in parallel groups (to add capacity) and the groups in series (to add voltage), described as xSyP. Groups are packaged as modules or placed directly in the pack (cell-to-pack). The pack also contains the BMS (voltage and temperature of every series group), main and pre-charge contactors, fuses, a service disconnect, current sensor, cooling plates and a sealed, crash-resistant enclosure. Indian battery-safety requirements are set by AIS standards (AIS-156 for L-category two- and three-wheelers, AIS-038 Rev. 2 for cars and commercial vehicles); take test details from the current standard.

Thermal management.

• Li-ion works best around 15–35 °C. Ageing accelerates sharply above about 40–45 °C; below about 0 °C fast charging causes lithium plating on the anode, which loses capacity and can form internal shorts.
• Heat comes mainly from I²R losses (plus a reversible entropic term). Cells in the middle of a pack run hotter, and the spread between cells must be kept small (typically a few kelvin) so they age evenly.
• Methods: passive or forced air (cheap, used in many two-wheelers), liquid cold plates with water–glycol (most cars), refrigerant (direct) cooling, immersion in dielectric fluid, and phase-change materials. Heating uses PTC heaters or the heat pump.
• Thermal runaway: above a chemistry-dependent onset temperature (often around 150–200 °C for NMC, higher for LFP), exothermic reactions heat the cell faster than it can cool, venting flammable gas and possibly fire. Packs are designed to stop propagation from one cell to the next.

Formulas

V_pack = N_s × V_cell; Q_pack = N_p × Q_cell

• N_s: cells in series, N_p: cells in parallel, V (V), Q (A·h).

E_pack = V_pack × Q_pack = N_s × N_p × V_cell × Q_cell

• E (Wh); 1 Wh = 3600 J.

I = C_rate × Q

• I (A), C_rate (1/h), Q (A·h).

R_pack = N_s × R_cell / N_p

• R_cell: cell internal resistance (Ω). Ignores busbar and contact resistance.

P_loss = I² × R_pack, V_t = V_oc − I × R_pack

• Heat generated (W) and terminal voltage under load (V).

ṁ = P_loss / (c_p × ΔT)

• Coolant mass flow (kg/s) to remove P_loss with a coolant temperature rise ΔT (K); c_p: coolant specific heat (J/(kg·K), about 3400–3600 for 50 % water–glycol).

ΔT_cells = P_loss × t / (m × c_p,cell)

• Adiabatic temperature rise (K) of cells of mass m (kg) over time t (s); c_p,cell about 900–1100 J/(kg·K) — take from cell data.

Worked examples

Example 1 (standard). A car pack uses NMC cylindrical cells of 3.65 V nominal, 5.0 A·h and 70 g in a 96S16P arrangement. Find the pack voltage, capacity, energy, number of cells, cell mass and cell-level specific energy.

1. V_pack = 96 × 3.65 = 350.4 V.
2. Q_pack = 16 × 5.0 = 80 A·h.
3. E = 350.4 × 80 = 28,032 Wh ≈ 28.0 kWh.
4. Cells = 96 × 16 = 1536; mass = 1536 × 0.070 = 107.5 kg.
5. Specific energy = 28,032 / 107.5 = 261 Wh/kg (cells only; the pack figure is lower).

Answer: 350.4 V, 80 A·h, about 28.0 kWh, 1536 cells, 107.5 kg, about 261 Wh/kg.

Example 2 (GATE level). Each cell in Example 1 has an internal resistance of 20 mΩ. The pack is discharged at 2C. (a) Find the current, pack resistance, voltage drop and heat generated. (b) What water–glycol flow (c_p = 3500 J/(kg·K)) removes this heat with a 5 K coolant rise? (c) If cooling fails, by how much would the cells heat up in 10 minutes (take c_p,cell = 1000 J/(kg·K))?

1. I = 2 × 80 = 160 A.
2. R_pack = 96 × 0.020 / 16 = 0.12 Ω.
3. Drop = 160 × 0.12 = 19.2 V; P_loss = 160² × 0.12 = 3072 W.
4. ṁ = 3072 / (3500 × 5) = 0.176 kg/s.
5. ΔT = 3072 × 600 / (107.5 × 1000) = 17.1 K.

Answer: 160 A, 0.12 Ω, 19.2 V drop, about 3.07 kW of heat; about 0.18 kg/s of coolant; about 17 K rise in 10 min.

Common mistakes

• Adding capacities of cells in series, or voltages of cells in parallel. Series adds voltage; parallel adds capacity.
• Treating A·h as energy. Energy needs voltage: Wh = V × A·h.
• Using nominal voltage of one chemistry for another (3.2 V LFP vs about 3.6–3.7 V NMC).
• Comparing cell-level and pack-level specific energy as if they were the same.
• Forgetting that heat grows with I², so doubling the C-rate quadruples the loss.
• Charging fast in the cold: the risk is lithium plating, not just slower charging.
• Calling separator shutdown a cure for thermal runaway; it helps only in the early stage.

For GATE ME

Battery-pack questions combine simple electrical relations with heat transfer: series–parallel arrangements, energy in Wh and J, I²R heating, ṁ c_p ΔT for coolant flow, and lumped (adiabatic) heating. Practise unit conversion (mΩ, A·h, kWh, MJ) and energy balances.

Quick check

1. A pack is 14S2P with 3.6 V, 2.5 A·h cells. What are its voltage and capacity?
2. What does 0.5C mean for a 100 A·h pack?
3. Why is LFP popular for Indian two- and three-wheelers?
4. Why is fast charging restricted below about 0 °C?
5. A 3.6 V, 5 A·h cell stores how much energy?

Answers: 1. 50.4 V and 5 A·h. 2. A current of 50 A. 3. Better thermal stability and cycle life, lower cost and no cobalt. 4. To avoid lithium plating on the graphite anode. 5. 18 Wh.