Battery

The traction battery is the heart and the most expensive part of an EV. Learn lithium-ion cell chemistry (NMC, LFP, solid-state), cell formats, specific energy and power, series/parallel pack design, C-rate, internal resistance and heat, thermal management, degradation and safety.

An EV battery stores 40–120 kWh in thousands of lithium-ion cells, delivers hundreds of kilowatts, survives a decade of fast charging and vibration, and must never catch fire. Its chemistry and design set range, performance, cost and lifetime.

Engineers juggle energy density, power, cost, safety, longevity and temperature behaviour.

An analogy

A battery pack is like a water tank made of thousands of small bottles: series connections stack the height (voltage), parallel connections widen the tank (capacity).

How a Li-ion cell works

Lithium ions shuttle between graphite anode and metal-oxide cathode through an electrolyte; electrons flow through the external circuit. Charging pushes ions into the anode; discharging releases them. A separator prevents short circuits.

Chemistries and formats

NMC/NCA: high energy (≈ 250–300 Wh/kg cell), used for long range. LFP: cheaper, safer, longer cycle life, lower energy (≈ 160–200 Wh/kg), flat voltage curve. Emerging: sodium-ion, silicon anodes, solid-state. Formats: cylindrical (2170, 4680), prismatic, pouch.

Pack design

Cells in series set voltage (400 V or 800 V class); parallel sets capacity. Modules or cell-to-pack designs, busbars, contactors, fuses, cooling plates, crash-proof enclosure. Pack-level energy density is ≈ 60–80 % of cell level.

Degradation, thermal management and safety

Capacity fades through SEI growth, lithium plating (fast charging in the cold), cathode cracking; high temperature and high SOC accelerate it. Liquid cooling keeps cells ≈ 20–40 °C. Thermal runaway (cells above ≈ 150–200 °C) is prevented by BMS limits, cell spacing, fire barriers and venting design; UN 38.3, ECE R100 and GB 38031 tests govern safety.

Formulas

• Pack voltage and capacity: V_pack = n_s × V_cell, Q_pack = n_p × Q_cell
• Energy: E = V_pack × Q_pack
• C-rate: C = I / Q = P / E
• Voltage sag and heat: ΔV = I R, P_loss = I² R
• Specific energy: Wh/kg = V × Ah / mass

Worked example: Designing a 400 V LFP pack

Given: LFP cell 3.2 V; Target ≈ 400 V, 60 kWh

Find: Series count, pack capacity, cells with 100 Ah prismatic cells.

• n_s = 400/3.2 = 125 cells
• Q = 60 000/400 = 150 Ah
• With 150 Ah cells → 125s1p; with 100 Ah cells → 125s2p (80 kWh) or accept 40 kWh at 1p

Answer: 125 cells in series, 150 Ah

Worked example: Power, C-rate and heat

Given: 75 kWh pack delivering 150 kW; Pack resistance 0.1 Ω, current 300 A

Find: C-rate, voltage sag, heat.

• C = 150/75 = 2C
• ΔV = 300 × 0.1 = 30 V
• P_loss = 300² × 0.1 = 9 kW of heat

Answer: 2C, 30 V sag, 9 kW heat — cooling is essential

Common mistakes

• Wrong: Quoting cell energy density as pack density. — Right: Pack density is lower (housing, cooling, electronics).
• Wrong: Fast charging a cold battery at full power. — Right: Pre-heat — cold fast charging causes lithium plating.

Applications

• Passenger EVs: 40–120 kWh packs; LFP in standard range, NMC in long range.
• Commercial EVs: Buses and trucks with 300–1000 kWh packs.
• Second life: Retired EV packs reused for stationary storage.

Key takeaways

• V_pack = n_s V_cell; E = V × Ah.
• C = P/E; heat = I²R.
• NMC for energy, LFP for cost/life.
• Thermal management and BMS protect life and safety.

Going further

• Cell-to-pack / cell-to-body: Eliminating modules raises pack energy density and simplifies structure.
• Electrochemical models: Doyle–Fuller–Newman models predict voltage, heat and ageing.
• Battery passport: EU regulation tracking carbon footprint, materials and SOH.