The Battery Management System (BMS) is an electronic system that underpins the performance, safety and longevity of a rechargeable battery. The BMS will typically control parameters of voltage, current (in or out), temperature, and the battery cooling system (if applicable). Whilst the voltage of the complete battery is monitored, the voltage and state of health of individual cells in the battery pack may also be checked. The BMS is also able to monitor the current the battery is providing. It can protect the battery against overcurrent by opening electronic switches internal to the battery pack.
Generally, the BMS has two modes - charging the battery and driving the application, or energy in and energy out. 'Energy in' covers charging and possible recuperation (charging by braking with motor), while 'energy out' covers the provision of electrical energy, to drive a motor for example.
A rechargeable lithium-ion battery contains a stack of single li-ion cells which can become damaged when they are running below the minimum voltage. A lithium-ion battery pack can have a voltage level above its minimum voltage, even though it has an individual cell voltage below its minimum level. The BMS is able to identify this condition and interrupt the battery operation.
The BMS system is an essential piece of equipment for monitoring and reporting on the health and status of the battery, while also protecting it, guaranteeing a safe operation.
The BMS system contains a central controller which communicates internally with its hardware (cell level), or externally with higher level hardware.
The drive system controller serves as the control room for the operation of the application concerned. It collects information and data from other controllers, sensors and the drive system, to then decide which mode of operation to use and what the top priorities are.
During communication with the drive system controller, the BMS obtains information about the state of charge, state of health, temperature of the battery's cells and more. The drive system can then adapt the mode of operation considering the status of the battery. It may limit the power available to the drive system, adjust an automated duty cycle, prioritise regeneration, etc. By doing this, the drive system helps to extend the life of the battery and ultimately the life of the product.
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Charging and discharging current levels can be very different. It is common to see applications where the discharging current is 10 times bigger than the charging current. This can be a limitation of the battery cell, or just a way to maximise the battery's longevity. The drive system and the battery charger control the current level to guarantee it is operating under the limit at all times.
Batteries also have minimum and maximum temperatures for charging and discharging. The BMS, drive system and battery charger limit the operation of the battery inside the optimum temperature range. For lithium-ion batteries operating between 45°C and 60°C, it is possible to use the product, but it is not possible to charge its battery.