BMS

What does a BMS monitor?

A BMS measures every cell voltage, the charge and discharge current and the cell temperature. If a value leaves the permitted range, it disconnects the battery: the charge path on overvoltage, the discharge path on undervoltage, both on overtemperature or short circuit. At the same time it balances the cells so that no cell is full or empty before the others.

Internal or external BMS?

In most lithium batteries in a standard case, the BMS sits inside the case and switches by itself. Larger systems often use an external BMS that controls relays, chargers and loads. The difference shows when something goes wrong: an internal BMS cuts the battery off the system abruptly, an external one can switch off charging sources and loads first.

What matters when planning?

• The currents of inverter, anchor windlass and bow thruster must stay below the continuous and peak current the BMS allows.
• Charging sources must tolerate a disconnect by the BMS, above all the alternator.
• A BMS with a data interface shows cell voltages and faults on the monitor instead of switching off silently. BullTron batteries, for example, pass these values to the philippi PCU via the P-BUS adapter.