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Example: Battery Management System (BMS)

Mobile devices often rely on rechargeable lithium batteries. They have great advantages in the power density and weight, but have also a sensitive operational behaviour. Stress during discharge or during recharge affects the reliabiliy (and the useful lifetime) and can cause damages, even fire. An effective measure is to place a supervising circuit between battery and load. This circuit cares for the safe operation of then battery and is called "Battery Management System" (BMS).

The task of the BMS is: to protect the battery, not the load (the essential mobile device).

A BMS consists of several sub-units for different functions. They depend on the various operational scenarios of then battery:
  1. Operation: The battery has to function with high reliability even on a high power level (as usual).
  2. Charging: After the operation the battery needs recharging. The charging process must stay close to the manufacture's specifcation. Especially lithium batteries require a considerable effort: cut-of voltage level, cell balancing.
  3. Storage: The reliability of batteries heavily depends on the storage conditions between "Charging" and "Operation". Some manufacturers recommend to discharge the batteries after several days, when non-operational.

A BMS has to collect data (voltages, currents, temperatures), but also control processes. There are several protective measures connected to power switches which are operated by the BMS (similar to the classic one-off fuse, but with a more sophisticated operation).

Typical functions or sub-processes for a BMS are:
  1. Function during operation
    - protection against under-voltage (e.g. on heavy discharge)
    - protection against over-voltage (e.g. with regenerative loads, as motors)
    - protection against over-current or short-circuit (e.g. on overload or damaged load)
    - collecting operation data
  2. Functions during charging
    - monitoring the charging current and voltage with cut-off
    - cell balancing: a dedicated control circuit detects the voltage of each single cell and levels out differences.
    - monitoring the cell temperature
    - collecting charging data
  3. Functions during storage
    - Even during off-state it is necessary to monitor the battery. After several days non operational it is recommended to partially discharge the battery.
Depending on the circumstances some of these functions can be left out or other functions need to be added. Mostly the mission profile of the mobile device dictats which basic functions are required. But also space and complexity are important. Then we can make the decision where to place the specific function: close to the battery (e.g. protective measures), in the mobile device (e.g. collecting data) or in an external service device (e.g. a charging unit).