Battery management unit

Example

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Battery control unit

In layered battery systems, the controller between the module-level BMUs and the vehicle or site controller: it aggregates several strings or racks, decides which may connect, and presents one battery to the outside. Stationary storage usually calls the levels cell, module, rack and system.

Battery management system

The electronics and software that keep a battery inside its safe window: measuring every cell voltage and many temperatures, estimating SOC and SOH, balancing cells, computing current limits, controlling the contactors and reporting over the vehicle network.

BMS power modes

The states a BMS moves between to save energy. In active mode it measures, estimates and drives the contactors; asleep it draws microamperes to a few milliamperes from the auxiliary battery and wakes on CAN traffic, ignition, a charger plug or a timer to check cells and balance. Sleep current decides how long a parked vehicle can stand.

CAN bus

The serial network over which a BMS talks to the vehicle or inverter: a two-wire differential bus, defined in ISO 11898, carrying short prioritised messages at up to 1 Mbit/s, or faster with CAN FD. Every node hears every message, and a dominant bit wins arbitration without any collision.

Cell supervision circuit

The board on each module that measures its cell voltages and temperatures and switches its balancing resistors, reporting to the central BMS controller. It is usually built around a dedicated monitoring chip and linked by isoSPI or CAN.

Contactor

A heavy-duty electrically operated switch that connects the pack to the vehicle or inverter and opens it on fault or shutdown. Packs use at least two, one per pole, plus a pre-charge path to avoid welding them on the first connection.

References & further reading

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

1. 3rd generation of the CAN data link layer

   CAN in Automation, CAN Newsletter · 2020

   Topic background · Classical CAN, CAN FD and CAN XL data-link concepts; publication describes the 2020 state.

2. Getting Started with the BQ76925

   Texas Instruments · 2019

   Topic background · A concrete BMS analog front-end example: cell voltage, current, temperature measurement and MCU integration.