The voltage window is a hard limit
Every lithium cell has a permissible voltage window outside of which it takes chemical damage. For LiFePO4 it lies roughly at 2.5 V to 3.65 V per cell, for Li-NMC at about 3.0 V to 4.2 V. If the upper limit is exceeded, the electrolyte decomposes and metallic lithium can be deposited. Below the lower limit the copper current collector of the anode dissolves — irreversibly, and a later short-circuit nucleus.
The BMS therefore measures each cell voltage individually, not just the pack voltage. Because 48 V at the pack says nothing about whether one cell is already at 3.6 V and another only at 3.1 V. Only single-cell measurement detects the spread.
Balancing, protection, current limiting
In balancing the BMS pulls the cells back together. Passively this is done via a resistor that deliberately discharges the highest cell until the others catch up — typical balancing currents are in the range of a few tens to a few hundred milliamperes. Active balancing shifts charge from full to empty; more elaborate, but with lower losses at a large spread.
The protection circuit monitors over- and undervoltage, over- and undertemperature as well as overcurrent and short circuit, and disconnects the circuit before a limit is reached. Decisive is the charge-current limiting as a function of temperature: charging below 0 °C forces lithium plating, so the BMS blocks or throttles the charge current in the cold. Likewise it limits the C-rate in the upper charge range, because high currents at a high state of charge stress the cell the most.
SoC (state of charge) is usually calculated by the BMS through coulomb counting — integrating the current over time — corrected at open-circuit-voltage points. This is demanding with LFP, because the discharge curve is flat over wide ranges and a few millivolts mean large capacity differences. SoH (state of health) is derived from the measured remaining capacity and the rising internal resistance compared with the new state — the basis for maintenance planning and remaining-life forecasting.
Why the machine controller has to have a say
A BMS that only switches off is not enough in mobile machines. A vehicle that disconnects the battery without warning during operation ends up stranded somewhere in the plant under full load. The machine controller has to know the state before a limit is reached.
That is why the BMS should be connected to the controller via a fieldbus — in industry usually CAN bus — and deliver SoC, SoH, single-cell voltages, temperatures and fault codes in real time. The controller sees the remaining capacity and plans the route to the charging station before the lower limit comes within reach. If a cell reports a temperature anomaly, the controller throttles in an orderly manner instead of disconnecting hard. Especially in continuous operation this data flow between BMS and controller is as important as the pack itself.
Checklist for the BMS design
- ✓Voltage limits set to match the chemistry (LFP 2.5–3.65 V, NMC 3.0–4.2 V per cell)?
- ✓Single-cell measurement across all series members, not just pack voltage?
- ✓Balancing sized for the expected cell spread over the service life?
- ✓Charge current temperature-dependently limited, charge lock-out below 0 °C without heating?
- ✓Communication protocol defined (CAN bus, J1939, SMBus, RS485) and matched to the real controller?
- ✓Behaviour in the limit case clarified: orderly throttling via the controller instead of a hard disconnect?
- ✓SoC method matched to the chemistry — with the flat LFP curve, pure coulomb counting with open-circuit-voltage correction?
The first four points protect the pack. The fifth and sixth decide whether the machine works reliably with it. How the BMS is connected to your controller and which values run over the bus, we are happy to clarify against your specification.