Battery Technology and Battery Management Systems
What this domain covers
Questions focus on lithium-ion cells, pack construction, the meaning of SoC and SoH, and the BMS functions that keep the pack safe and long-lived.
Core rules and concepts
- Cell to pack: cells form modules, modules form the pack. Series raises voltage; parallel raises Ah.
- Chemistries: LFP nominal about 3.2 V per cell, stable and long-cycling; NMC about 3.6–3.7 V, higher energy density.
- Energy: kWh = nominal volts × Ah ÷ 1000.
- C-rate: 1C discharges the Ah rating in one hour; 2C in half an hour.
- BMS jobs: cell voltage and temperature monitoring, over/under-voltage protection, over-current and short protection, cell balancing, SoC/SoH estimation, contactor control, isolation monitoring and CAN communication.
- Charging: constant current then constant voltage; charging tapers near full.
- Thermal runaway: triggered by overcharge, internal short, overheating or damage; produces heat, smoke and toxic gas.
Worked example
A pack uses 16 LFP cells in series, each 3.2 V and 100 Ah. Pack voltage = 16 × 3.2 = 51.2 V. Energy = 51.2 × 100 ÷ 1000 = 5.12 kWh. If two such strings are paralleled, voltage stays 51.2 V and capacity becomes 200 Ah, giving 10.24 kWh.
Traps that cost marks
Do not use 3.7 V for an LFP cell. Do not confuse Ah with kWh. Do not say the BMS cools the pack - it monitors and may command the thermal system. Do not claim passive balancing instantly equalises all cells; it bleeds small amounts of energy over time. Do not ignore a swollen or leaking pack.