Battery Management Systems, BMS

- overview of the basics and technology behind battery management systems, BMS used for managing and monitoring rechargeable battery systems.

Battery management systems, BMS are in widespread use. With many items of electrical equipment requiring battery supply, the use of rechargeable cells as risen dramatically. To ensure that the batteries are charged correctly and their life and use is optimised, battery management or fuel gauge systems have grown in parallel.

Battery management systems are now well established. These battery management systems not only track the amount of charge left in a cell, but also manage the charging, preventing the cells from being damaged by overcharging, as well as providing a number of other functions. As such these battery management systems are an essential element in many rechargeable battery packs today.

Battery management basics

Battery management systems vary in the functionality they provide and accordingly in their flexibility. However it is possible to break down their functions into a number of areas. Each particular system may provide some or all of the different areas of functionality depending upon the requirements.

Some areas of the battery management functionality include:

  • Monitoring:   In order that the battery management system is able to understand the state of the battery, it needs to monitor various aspects of the battery.
  • Computation:   The battery management system may deduce certain aspects of the state of the battery from the results obtained by monitoring the battery.
  • Environment management:   The battery management system uses the results it obtains from monitoring the battery to control the environment for the battery.
  • Communication:   In certain applications the battery management system may need to communicate aspects of the battery system to other elements within the electronics apparatus, or even externally.

The different elements of a battery management system may be present and may also include different levels of functionality according to the requirements of the overall system.

Battery management monitoring

One of the key elements of a battery management system is to monitor the state of the battery.

The battery monitoring provides an indication of the level of charge within the battery.

By knowing the level of charge in the battery, it is possible to detect the length of time that the equipment can operate before it is re-charged. It is also possible to stage an orderly shut-down should the need require.

Using the battery monitoring system it also possible to manage the charging cycle, providing the right level of charge at various states of charge and also terminating the charge before the battery becomes over-charged.

To gain access to this information a variety of aspects of the operation of the battery are monitored as described later.

Battery management computation

In order that the readings taken by the battery management can be assessed, the system needs to undertake a degree of computation. These enable the readings to provide more useful information and provide a higher level of accuracy.

By using various algorithms the battery management system is able to determine figures including the Sate of Charge or Depth of Discharge as well as the State of Health of the battery.

The state of health of a battery is important because as a rechargeable battery or cell is used, its capacity reduces, and this needs to be computed by the battery management system to enable it to know the full charge level and capacity.

Environment battery management

One of the key functions of the battery management system is to manage the battery system ensuring it does not become overcharged, or its charge does not fall to a stage where the equipment it is running has to shut down as the power runs out without warning. The battery management system should provide warning to enable the equipment to be shut down in an orderly fashion.

The environmental management section of a battery management system will decide when the battery or cell is to be charged, and also how much charge it is to receive. As many batteries such as lithium ion batteries and cells have a life which is largely determined by the number of charge / discharge cycles, many battery management systems will only charge the battery when needed. They will assess the amount of charge in the battery, and for applications such as laptop and other portable computers, they will only charge the battery when the charge is low, even if the laptop is connected to an external mains or line power source. By doing this the battery life is maximised.

The battery management system will also track the amount of charge within the cell and give an indication of the level of charge left. To achieve this the battery management system must have a knowledge of the overall capacity of the battery or cell. As this changes over the life of the battery, the management system must update its figures for this. Normally this is done by allowing the battery to be fully charged and then noting the amount of current, and hence charge, it takes before the battery nears the end of its charge and the voltage begins to fall. By constantly updating figures, the management system will be able to have a reasonably accurate figure for this.

Battery management communication

In some instances it may be necessary to communicate the information to the battery management system elsewhere. One example of this could be to enable a system to shut down or to turn on to alternative power as the battery charge runs low.

In order to achieve a good level of communications, a digital interface may be required to send the data in a standard format. Many battery management systems incorporate a digital interface and as such are able to communicate information about the battery to a variety of points.

By Ian Poole

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