BMS fundamentals
- A BMS keeps every cell inside its safe voltage, current and temperature window. Lithium cells can fail dangerously when abused, so protection must work even if firmware crashes.
- State of charge is an estimate, not a measurement. Coulomb counting with voltage-based correction, and sometimes Kalman filtering, gives usable accuracy.
- Balancing matters in long series strings. Passive balancing is simple; active balancing helps large or unmatched packs.
- Standards apply. Indian cell and EV battery requirements are covered by standards such as IS 16046 and AIS-156, depending on product.
Chemistry and limits
| Chemistry | Typical cell voltage window | Notes |
|---|---|---|
| Li-ion NMC/NCA | About 3.0-4.2 V per cell | High energy density; needs tight charge limits and thermal control |
| LiFePO4 (LFP) | About 2.5-3.65 V per cell | Safer thermal behaviour; flat voltage curve makes SoC harder |
Always use the limits in the specific cell datasheet; the figures above are typical values only.
BMS functions
- Per-cell voltage and pack temperature monitoring with dedicated analog front-end ICs.
- Overvoltage, undervoltage, overcurrent and short-circuit protection, with a hardware path independent of firmware.
- Charge control including constant-current and constant-voltage phases and temperature limits.
- Cell balancing and pack-level logging.
- SoC and state-of-health estimation.
- Communication via CAN, RS-485 or UART to a charger, motor controller or display.
Design considerations
Sensing accuracy is limited by layout: shunt resistor routing, Kelvin connections and thermistor placement all matter. Pre-charge circuits prevent contactor welding and inrush damage. Isolation and creepage requirements rise with pack voltage. We cover these in hardware design and test with fault-injection before any pack is built.
Applications
- Electric two-wheelers and light vehicles.
- Solar storage and UPS packs.
- Portable medical and industrial equipment.
- Robots and AGVs; see robotics engineering.
Sources and further reading
- Bureau of Indian Standards — Standards such as IS 16046 for secondary lithium cells and batteries.
- ARAI — Automotive testing and AIS standards for vehicle batteries.
- IPC — PCB standards relevant to high-current layouts.
Last reviewed: October 2026. Regulations and product availability change; confirm current rules with the issuing authority before you commit to a design.