A battery management system (BMS) is any electronic system that helps to manage a rechargeable battery (cell or battery pack), like by protecting the battery from operating outside its area of safe operating, monitoring its state, calculating the secondary data and reporting that data, controlling its environment, authenticating it and / or balancing it.
A battery pack built together using a battery management system with an external communication data bus is a smart battery pack. A smart battery pack needs to be charged by a smart battery charger.
A BMS can monitor the state of the battery as represented by various items, like:
- Voltage: Total voltage, voltages of individual cells, minimum as well as maximum cell voltage or voltage of periodic taps
- Temperature: Average temperature, coolant intake temperature, coolant output temperature, or individual cells’ temperatures
- State of charge (SOC) or depth of discharge (DOD), for indicating the charge level of the battery
- State of health (SOH), a variously-defined measurement of the remaining capacity of the battery as a percentage of the original capacity
- State of power (SOP), the amount of power which is available for a defined time interval given the current power usage, temperature and other conditions
- Coolant flow: for air or for fluid cooled batteries
- Current: Current in or out of the Car battery
Challenges in battery management for Vehicles
The battery management system (BMS) defines in how well a battery stack meets each of these design challenges. The heart of the BMS consists of a battery monitoring integrated circuit (IC). This IC calculates individual cell voltages which are used to determine state of charge and battery stack health.
The most critical characteristics of a battery monitor IC include accuracy, data robustness, as well as fault detectability to ensure safety. The accuracy of the monitor IC can directly affect system cost, battery pack reliability and lifetime. Each cell has a limited capacity which must be managed carefully. Overcharging can cause safety as well as reliability issues, while over-discharging can affect the lifetime of the cell. Using a less accurate monitor IC needs that the system designer use larger “guard bands” for protecting against overvoltage and under voltage, therefore limiting the amount of total available capacity for the vehicle. A higher accuracy monitor IC can make use of more or each cell’s total capacity while reducing the total cost of the battery stack system.