What is a Battery Management System (BMS)?

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This white paper is based on a recent AVID Learning podcast episode we released where we answered several listeners’ requests to talk more in-depth on Battery Management Systems or BMS to explain what they do and why they are needed.

Battery Packs

The key enabling component in electric and hybrid vehicles is the battery pack. Higher power density and lower cost lithium battery systems have transformed the way that we can store and use power on board a vehicle.battery pack in a tesla

 

Going back to early electric vehicles, heavy lead-acid batteries were used but they simply did not have enough storage capacity per unit of mass to be effective except in some special smaller applications like forklift trucks and milk floats.

After these, we then had more advanced energy storage systems such as NiCad batteries and then NiMH batteries. There were also attempts at using molten sodium salt batteries but these had many issues as the electrolyte needs to be maintained at a very high temperature to maintain its conductivity.

The step change was brought about with the invention of commercially viable lithium-ion battery chemistries that offered significantly improved energy storage densities and power factors that made them very suited for mobile applications. We have talked about lithium battery cells in previous white papers and podcast episodes so please check these out for more information. Lithium battery cells have some significant challenges that require a sophisticated electronic control system known as a Battery Management System or BMS.

One of AVID Technology's Battery Management Systems
One of AVID Technology’s Battery Management Systems

 

Battery Management Systems

Lithium batteries are relatively difficult to manage because they have a very non-linear charge/discharge curve. With a lead acid battery, it is quite easy to tell how charged it is by simply measuring its voltage and there is a pretty straightforward way of working out a state of charge from that information. However, with a lithium-ion battery, the state of charge relative to voltage is very flat through around 60% of the charge/discharge curve with sharp inflections at both ends. This means it is very hard to tell what the state of charge is of a cell or a pack.

With a lithium-ion battery cell, it is also relatively easy to cause permanent damage by both over and undercharging the cell, so it is far more important to make sure that the cell is not being damaged. There are also safety considerations, as it is possible for damaged cells to overheat and get into a thermal runaway condition.

The BMS, therefore, plays a very important role. It makes sure we can accurately report the state of charge of the battery pack and also monitors the cells to make sure there are no issues that might lead to a problem, in order to ensure that the pack can be safely shut down first.

The BMS typically consists of several separate circuit boards. First of all, there are the module control units or MCUs, these are sometimes called slave BMS modules. These units will monitor the voltage of each cell, or a small group of cells in the battery very accurately. The MCU will also typically include some temperature measurements of the inside of the battery pack and possibly the cells themselves. The MCU will communicate up to a master BMS module. The master BMS will aggregate data from all of the slave MCUs and it will also measure bulk current flow in and out of the battery pack. From this, it can work out what the battery state of charge is. And report this so that it can be communicated to the driver. Battery state of charge would also impact things such as how much regenerative braking you can use particularly if the vehicle is not equipped with a braking resistor system. If the pack is already near to the maximum safe level the BMS will prevent the vehicle from dumping more power into the pack under regen braking. This is the reason why there is normally a little bit of useful pack overhead in order to try and keep a consistent braking feel for the driver this feature can usually be turned off with an accompanying warning about braking feel.

The capacity of the pack is then limited by the ability to charge each cell up to its maximum safe voltage and discharge it down to its minimum safe voltage. Due to tolerance build up in the manufacturing processes of the cells they have small variations in how quickly they will charge and discharge. In order to make sure that the whole pack is not constrained to the capacity of the cell that gets “full or empty” the quickest, the BMS also serves to balance the cells. It does this by using balancing resistors that switch on and off to discharge off small amounts of power from individual cells or small groups of cells and make sure that the overall pack capacity can be maximised and not constrained to the capacity of the weakest cell. This is also the reason why very fast charging never goes to 100% of the pack capacity because the balancing resistors need some time to do their job when getting very close to the upper limits of the pack.

Researchers have been looking into ways to move away from the balancing resistor as these have an impact on the charging efficiency of the pack, whilst in theory, this is possible the practical implementation is currently complex and costly. Also as production methods are improving for lithium cells the need for balancing is reducing to the point where its impact on overall system efficiency is limited.

In some smaller battery packs with a low number of cells, the MCU and master may be consolidated into a single device. This would be a typical configuration for a low capacity 48V battery pack for automotive use and also for small batteries for applications such as robotics and other industrial equipment.

The BMS needs to be able to communicate with the other powertrain subsystems in order to communicate charge level and send any inhibit messages if there is a problem with the battery pack and will typically broadcast the status of the pack via the CANbus, which is a serial communication protocol widely used in industry. Depending on the vehicle architecture the Master BMS module may directly control things like main DC link pre-charge and power contactors directly or via the vehicle CANbus and a separate vehicle body control unit. The BMS will also interface with both onboard AC/DC chargers and off-board direct DC fast chargers to tell the charger what mode to be charging the battery in. Because of its safety-critical nature both in maintaining the battery in a safe condition and also maintaining vehicle motion i.e. you do not want the BMS instructing the main DC link capacitors to open thinking there is an issue unless it is really a very important issue when the vehicle or machine is in operation.

In summary, it can be seen that the battery management system (BMS) is a critical part of the EV powertrain. The BMS is needed to measure the state of charge and condition of the Lithium battery due to their charge/discharge characteristics and safety considerations. It performs several key functions to ensure that the pack capacity can be maximised such as cell balancing and the control of chargers. The BMS will need to be tailored to each different battery pack and powertrain configuration.

If you need help with a battery system or BMS development, please do not hesitate to contact AVID Technology directly.

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