The voltage window is a hard limit
Every lithium cell has a permissible voltage window outside which it takes chemical damage. For LiFePO4 it is roughly 2.5 V to 3.65 V per cell, for Li-NMC about 3.0 V to 4.2 V. If the upper limit is exceeded, the electrolyte decomposes and metallic lithium can plate out. Below the lower limit the copper current collector of the anode dissolves — irreversibly, and a later short-circuit seed.
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 per-cell measurement detects the spread.
Balancing, protection, current limiting
During balancing the BMS pulls the cells back together. Passively this happens via a resistor that deliberately discharges the highest cell until the others catch up — typical balancing currents are a few tens to a few hundred milliamps. Active balancing shifts charge from full to empty; more complex, but with lower losses at 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 temperature-dependent charge current limiting: charging below 0 °C forces lithium plating, so the BMS blocks or throttles the charge current in the cold. It likewise limits the C rate in the upper charge range, because high currents at a high state of charge stress the cell most.
SoC (State of Charge) is usually calculated by the BMS via coulomb counting — integrating the current over time — corrected at open-circuit voltage points. This is demanding for 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 versus the new state — the basis for maintenance planning and remaining-life forecasting.
Why the machine controller must be involved
A battery management system (BMS) that only switches off is not enough in mobile machines. An automated guided vehicle (AGV) that disconnects the battery in operation without warning is left standing under full load somewhere in the plant. The machine controller must know the state before a limit is reached.
The battery management system (BMS) should therefore be connected to the controller via a fieldbus — in industry, depending on the platform, CAN bus or Profinet — and deliver SoC, SoH, individual 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 way instead of disconnecting hard. Especially in continuous operation this data flow between BMS and controller is as important as the pack itself.
Checklist for BMS design
- ✓Voltage limits matched to the chemistry set (LFP 2.5–3.65 V, NMC 3.0–4.2 V per cell)?
- ✓Per-cell measurement across all series elements, not just pack voltage?
- ✓Balancing sized for the expected cell spread over the service life?
- ✓Charge current temperature-dependent limited, charge block below 0 °C without heating?
- ✓Communication protocol defined (CAN bus, J1939, SMBus, RS485) and aligned with the real controller?
- ✓Behaviour at the limit clarified: orderly throttling via the controller instead of a hard disconnect?
- ✓SoC method matched to the chemistry — for 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 should run over the bus, we are glad to clarify based on your specification.