What is a BMS, and why is it essential?

A lithium pack is not just one big cell: it's an assembly of dozens, even hundreds, of cells wired in series and parallel. The BMS (Battery Management System) is the electronics that monitors and controls that assembly. Without it, a lithium pack would be dangerous: a single cell in overvoltage, short-circuit or overheating can be enough to trigger thermal runaway.

In practice, the BMS continuously measures the voltage of each cell group, the overall current and temperature at several points, then acts accordingly.

A BMS's key functions

  • Voltage protection: cut-off if a cell exceeds its maximum voltage (overcharge) or falls below its minimum (deep discharge) — both damage the cell and create risk.
  • Current protection: limiting or cutting off on excessive current (charging or discharging too fast, short-circuit).
  • Thermal protection: cut-off or derating if temperature leaves the safe range, when charging or discharging.
  • Cell balancing: without intervention, cells drift and become unbalanced over time. The BMS realigns them — the most critical point in second life (see below).
  • State estimation: the BMS computes SOC (state of charge, instantaneous level) and tracks SOH (state of health, ageing) to inform the user and adapt management.
  • Communication: it sends this data to the inverter or a supervisor, usually over a CAN or RS485 bus.

Passive or active balancing: what's the difference?

Passive balancing dissipates the excess from the most-charged cells as heat, through resistors, until everyone is aligned to the weakest cell. Simple and cheap, but it wastes a little energy and doesn't "lift" weak cells.

Active balancing actually transfers energy from strong cells to weak ones. More complex, it makes better use of the available capacity — a major asset when cells are heterogeneous, which is exactly the case in second life.

Why a second-life BMS is a special case

An off-the-shelf BMS is designed for new, identical cells of the same age and history. It assumes a uniformity that doesn't exist in a reuse pack: our packs combine cells of different origins and ages, selected for their SOH but never perfectly identical. A standard BMS would handle these gaps poorly and throttle the whole pack to its weakest cell.

That's why cirBATT develops its own BMS, designed to:

  • manage heterogeneous cells and compensate for their gaps in capacity and internal resistance;
  • finely detect a cell that is dropping off, before it becomes a risk;
  • secure the pack accordingly, with thresholds suited to reused cells.

A connected BMS for monitoring and diagnostics

Our BMS units are also connected: they continuously report the pack's vital data (voltage, temperature, SOC, SOH, cycles) and allow a quick diagnosis from a smartphone, or remotely. This is useful for technical monitoring in operation as well as for customer support, and it extends the battery's traceability throughout its second life.

This development builds on our team's experience in power electronics and protection for the automotive sector — demanding know-how, applied here to reuse.