Battery Cells, Modules & Packs
From a single lithium-ion cell to a 400-volt traction pack — the hierarchy, the voltage and energy arithmetic, and the cell chemistries that define an EV.
An EV's 'battery' is not one cell — it's thousands, welded into modules, bolted into a pack, managed by a computer that watches every one. The engineering is in how they're arranged and controlled.
A pack is thousands of cells
A single lithium-ion cell has a nominal voltage of ~3.2–3.7 V and stores maybe 5–100 Wh — far too little for a car. An EV pack strings many cells together in a hierarchy. Cells (the smallest unit) are welded in parallel into modules (parallel adds capacity/energy, voltage stays the same), and modules are connected in series into the pack (series adds voltage). A typical 400 V pack might use ~100–200 cells in series (each ~3.6 V) to reach ~400 V, with several in parallel per group for capacity — thousands of cells in total. Higher-voltage 800 V architectures halve the current for the same power, cutting copper mass and charging heat, which is why premium EVs are moving to 800 V.
Chemistry: the energy-safety-cost triangle
The dominant cell chemistries trade energy density, safety, cost, and cycle life. NMC (nickel-manganese-cobalt) and NCA (nickel-cobalt-aluminium) offer the highest energy density — more range per kilogram — so they dominate long-range and performance EVs, but they're costlier and less thermally stable. LFP (lithium iron phosphate) sacrifices some energy density for lower cost, excellent cycle life, and better thermal safety (it's harder to push into thermal runaway), making it popular for standard-range and commercial EVs. The choice is a product-level trade-off: long-range performance favours NMC/NCA; value-and-durability favours LFP. All are 'lithium-ion' — the cathode chemistry is the variable.
Lithium-ion cells are unforgiving: over-charge, over-discharge, or operating outside their temperature window degrades them fast and can trigger thermal runaway (a self-heating, hard-to-stop fire). With thousands of cells, no two age identically, so the battery management system (BMS) watches every cell's voltage and temperature, balances charge between them (bleeding the high ones so the low ones can catch up), enforces safe voltage and current limits, and estimates state-of-charge and state-of-health. The BMS is what makes a large lithium-ion pack safe and long-lived — without it, the pack would destroy itself within months. It is as central to an EV as the ECU is to a combustion engine.
- V_pack = N_s × V_cell = 108 × 3.7 = 399.6 V ≈ 400 V
- E_pack = V_pack × Q = 399.6 V × 100 Ah = 39,960 Wh ≈ 40.0 kWh
- N_s = V_pack / V_cell = 800 / 4.0 = 200 cells in series
Check your understanding
- A traction pack is cells (parallel, for capacity) → modules → pack (series, for voltage); thousands of cells total
- Pack voltage = N_s × V_cell; energy = V_pack × capacity (Ah)
- NMC/NCA maximise energy density (range/performance); LFP trades density for cost, life, and thermal safety
- The BMS monitors/balances every cell and enforces safe limits — essential for pack safety and longevity