High-Voltage Battery Systems Service and Diagnosis
What is inside the pack
A pack is cells grouped into modules, wired series for voltage and parallel for capacity. Lithium-ion cells are about 3.6–3.7 V nominal, ~4.1–4.2 V at full charge, and 2.5–3.0 V at cutoff; NiMH cells are about 1.2 V nominal. Energy in kWh equals nominal pack voltage times Ah divided by 1000. A 96-series lithium pack at 3.7 V per cell is roughly 355 V nominal.
Controls and sensing
The BMS watches every cell group through sense leads, temperatures through thermistors, and pack current through a Hall sensor or shunt. It commands the main positive and negative contactors, closing them through a precharge resistor first and opening them on fault. Passive balancing bleeds energy from high cells; active balancing moves charge between them. SOC is estimated by coulomb counting corrected against open-circuit voltage, while SOH compares present capacity to rated capacity.
Traps
Assuming the dash SOC is true state of charge; measuring module voltage with the interconnect bars still in place, which reads the whole string; replacing one module without recalibration; poking at a swollen or impact-damaged pack; storing a damaged pack indoors. Another common trap: a single weak cell group pulling the entire pack down and being misdiagnosed as a BMS fault.
Rule of thumb
At rest and at equal temperature, cell groups should sit within roughly 30–50 mV of each other. A wider spread, or one group that dives under load, points to a weak module.
Worked example: 96 groups read 3.85 V except group 42 at 3.61 V - a 240 mV spread. The BMS will cut power to protect that group; this needs module-level service, not a software update.