BMS: What a Battery Management System Actually Does
A BMS is an electronic system that monitors and controls a battery pack. It measures current, voltage, and temperature across cells and uses that data to protect, optimise, and communicate battery state.
A Battery Management System (BMS) is an electronic system embedded in a battery pack. Its job is to monitor the physical and chemical state of the battery, enforce safe operating limits, and report battery state to the rest of the system.
What It Measures
The BMS continuously measures three physical quantities across all cells:
- Current: individual cell currents i₁, i₂, … iₙ from the energy storage module
- Voltage: per-cell voltage
- Temperature: per-cell and pack-level temperature
These three measurements feed every decision the BMS makes.
Roles
Battery protection and optimisation: prevents cells from operating outside safe limits for voltage, current, and temperature.
Cell monitoring: tracks the charging and discharging state of each cell individually. The primary metric used here is State of Charge (SOC).
Battery lifespan management: tracks degradation over time. The primary metric used here is State of Health (SOH).
Power management: controls how much current each cell is permitted to accept during charging, and ensures discharge voltage does not drop too low.
Safety: in electric vehicles, voltage, current, and temperature must be monitored continuously. A failure in any of these can be catastrophic.
Diagnostics: the BMS logs fault conditions and communicates them to other systems.
The Control Unit
The control unit runs algorithms to compute derived quantities from the raw measurements:
- State of Charge (SOC): current charge level of the battery
- State of Health (SOH): current health relative to a new battery
- State of Power (SoP): available power at any given moment
- Depth of Discharge (DoD): how deeply the battery has been discharged
- Thermal state: temperature distribution across the pack
- Power optimisation: balancing output across cells
Communication
The BMS communicates with other Electronic Control Units (ECUs) in the vehicle and sends data to the display. Other systems depend on BMS outputs. The powertrain, charging system, and driver display all consume SOC and SOH readings.