Battery Management System

Source: www.evexpert.eu

The battery management system (BMS) is commonly referred to as a battery nanny or a battery housekeeper, which is mainly for the intelligent management and maintenance of each battery (cell), preventing the battery from overcharging, over-discharging and overcurrent, and prolonging the use of the battery Life, monitor battery status (voltage, current, temperature, etc.), functions include thermal management, balance control, fault diagnosis, charge management, participation in energy recovery, etc. Basically, electronic parameters are used to identify changes in physical and chemical reactions. Therefore, it is very important to equip the battery pack with a BMS protection system.

BMS Construction

The design of the BMS corresponds to a large extent to the design of the battery. It consists of a battery monitoring integrated circuit (BMIC), a cell management controller (CMC) and a battery management controller (BMC).

The BMIC monitors the individual battery cells and must be able to quickly - in a matter of microseconds - inform the CMC of the situation so that the CMC or BMC is able to react and, if necessary, correct an unfavourable situation. In order to prevent temperature leaks, the BMIC needs to notice immediately the battery cell that is overheating and send this information further on as quickly as possible. The BMC must decide how serious the situation is, how to proceed, and the overheated cell must be shut down if necessary. And all of this must happen in the blink of an eye.

The accuracy of the measurement and the ability to respond to adverse conditions depend primarily on the frequency of communication from the BMIC to the CMC and BMC, the more often they communicate, the greater the chance that they will successfully resolve a situation that could be risky. However, it is not at all easy to design an effective communication network in an electric car due to electrical noise.

Of course, before correcting the BMS tries to prevent any unfavorable situation, so apart from the BMIC, CMC and BMC modules it also consists of circuits that balance the energy loads of different cells, in this way all cells work more or less the same and do not tend to cause problems.

Depending on how complicated the electric car is, several intelligent microcontrollers may be added to monitor and control various specific tasks. Each BMS must be able to monitor not only the battery but also itself and it must be able to determine if alarms or prompts are real and not fake.

Basic functions of BMS

Discharge and charging control
Determination of the current state of charge (State of Charge)
Determination of State of Health
Charge balancing
Recording and communication

BMS development

In order for each BMS system to be able to perform its functions correctly, several puzzles need to be overcome for its development. The first is the Battery State Estimation. This is a value that is important both for the driver and for the system itself and its proper management.

The second is Battery Cell Charge Balancing. There are passive and active methods, as well as various active balancing topologies that need to be properly designed to optimize BMS properties.

Battery State Estimation

The state of charge of the battery is absolutely essential information for the driver. Moreover, the use of the battery could be extended if it was possible to obtain an accurate value.

Battery cell charge balancing

Battery cells are differences can be in internal resistance, level of degradation, capacity, ambient temperature, and many more. These inevitable differences between cells can cause many problems, and overall capacity can be dramatically reduced. This imbalance then causes the individual cells to be over-discharged or over-charged, which is dangerous for the battery pack as a whole, and thus charge balancing between the individual cells is of paramount importance to maintain performance and extend battery lifespan.

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