Battery Management System (BMS)¶
The BMS is the brain of every modern battery system. It protects the battery from conditions that would damage it or create safety hazards.
BMS Core Functions¶
| Function | What It Does |
|---|---|
| Cell voltage monitoring | Measures each cell's voltage in real time |
| SOC estimation | Calculates and reports state of charge |
| Temperature monitoring | Tracks cell and ambient temperature |
| Overcharge protection | Cuts off charge if any cell exceeds max voltage |
| Over-discharge protection | Cuts off discharge if any cell drops below minimum voltage |
| Overcurrent protection | Limits charge/discharge current to safe rates |
| Thermal protection | Shuts down if temperature exceeds safe limits |
| Cell balancing | Redistributes charge between cells to keep them matched |
| Communication | Reports status to inverter and monitoring system |
Cell Balancing¶
Over time, individual cells within a battery pack drift apart in capacity. Without balancing, some cells will be overcharged while others are under-discharged, reducing overall capacity and risking cell damage.
Passive balancing: Dissipates excess energy as heat from higher-capacity cells. Simple but slightly wasteful.
Active balancing: Transfers energy from stronger cells to weaker ones. More efficient.
BMS Communication to Inverter¶
The BMS tells the inverter: - Current SOC - Battery temperature - Allowed charge current (at this moment) - Allowed discharge current (at this moment) - Fault/alarm status
If the BMS communication fails, the inverter typically falls back to a default charge profile or stops charging/discharging for safety.
This is why BMS-inverter compatibility matters. Different manufacturers use different protocols (CAN bus, RS485) and communication standards. Mixing incompatible hardware can result in no BMS communication — a safety risk.
BMS Faults in the Field¶
| Fault | Likely Cause | Action |
|---|---|---|
| Over-temperature | Ambient too hot, fan failure, thermal runaway risk | Evacuate if runaway signs; otherwise check cooling |
| Cell overvoltage | Inverter charging above BMS limit | Check inverter charge settings |
| Cell undervoltage | Over-discharged | Restore charge; check inverter DoD settings |
| Communication lost | Wiring, protocol mismatch | Check communication cable and settings |
| Balancing fault | Cell divergence too large | May require service from battery manufacturer |