BMS: What is it? Why is it important?

What is a BMS?

The Battery Management System (BMS) is an essential electronic component of the Li-ion battery. It ensures a permanent monitoring and control of the whole battery and in particular of the cells. It balances them when necessary and allows charging and discharging according to the voltage limits in order to optimise the life of the battery.

What is its role in a battery?

The BMS is the brain of the Li-ion battery, adding protection against short circuits, over and under voltage, overcharge and temperature rise.

Its functions can be as simple as stopping charging or discharging according to voltage, or as complex as ensuring communication between several cells, and managing cell balancing to optimise battery use in real time and ensure permanent intelligent control.

Active or passive balancing, what are the differences?

The differences in the internal resistances of each cell lead to an imbalance within the module during the charge/discharge cycles. This will result in a drop in the useful capacity of the battery if no balancing is carried out.

2 strategies exist:

• Passive balancing, which consists of discharging the cell containing the most energy in order to bring it to the same level as the group of cells.
  Advantage: Easy to implement and more economical.
  Disadvantage: This strategy generates a loss of capacity since the discharged energy is dissipated as heat in a resistor.
• Active balancing: each cell contributes a part of its energy to the most discharged cell in order to homogenise the state of charge.
  Advantage: This is a form of inter-cell balancing, with no loss of energy.
  Disadvantage: The profitability is more noticeable in large systems.

What topologies are there among BMSs?

BMSs are nowadays mainly centralised or distributed.

The centralised BMS, generally for small systems, consists of a single management card, connected to each cell or module. It centralises all the information sent by the cells (current, voltage, temperature, etc.), analyses it and then sends them orders (start or stop loads/discharges).

For systems with several modules, the BMS can be distributed (master/slave): in this case there is one "master" BMS and several "slave" BMSs. Each module is connected to an electronic board (slave BMS), these slave BMSs send information to the motherboard, the master, which takes decisions for optimal use.

What types of communication to interact with related equipment?

The centralised BMS or Master will also have the role of communicating and controlling external equipment (charger, inverter, motor, cooling circuit, etc.). This communication can be wired (CAN, Ethernet, UART, USB...) or wireless for remote control (Wifi, Bluetooth, 3G/4G...).