A Battery Management System (BMS) is circuitry located inside or on the case of a battery or battery pack. It’s main purpose is to store data that will be useful to the user.

A battery that contains a BMS is often referred to as a Smart Battery, depending on the complexity of the BMS.

The data falls into two categories.

Static Data

• Data on the voltage and capacity (Ah or mAh) of the battery (in case the label is damaged).
• Who manufactured the battery.
• The date the battery was manufactured.
• How many times it can be discharged and charged (cycled) before it will reach the end of its service life.

Dynamic Data

• How many times it has been charged and discharged.
• To what level it has been charged and discharged each time.
• How fast the battery has been, and is currently being, discharged at.
• The internal temperature of the battery.

There are some further features which are covered in the section ‘The BMS Gray Area’ further down this article.

Note that not all BMSs store all of this data. Some may only store the static data.

The BMS can then use this data to provide useful information to the battery user.

Immediately useful information

Apart from the basic, but very useful, ability to report how much charge remains in the battery even the simplest data can be valuable to the user.

Predicting how long a battery will take to recharge has always been difficult and although some chargers in the past did display an estimate time this was no more than a rough guess.

A BMS knows much more about the battery and so can provide far more accurate estimates about how long it will take to recharge and other factors such as how much energy it will consume to achieve this.

On a large scale this can then help the user choose, for example, charging times when electricity prices are lower.

Optimizing the charging process

The charging process inside a battery creates heat.

Charging a battery too fast can lead to overheating, in extreme conditions this can cause an explosion (as seen in the image below) or a fire.

On the other hand a battery that is cool could be charging faster. Some BMSs can monitor the temperature of a battery and, when connected to a compatible charger, increase or reduce the charging current.

This optimization can significantly reduce charging times.

Predicting it’s own replacement

For example, we know batteries degrade gradually over time and that this deterioration is accelerated the more the battery is used. But without expert knowledge and the right formulas we don’t know how much longer the service life of any particular battery will be.

But the BMS knows when it was manufactured and how many discharge and recharge cycles it can achieve before it can no longer be charged. If it also knows how many discharge and recharge cycles it has already gone through it can make an accurate prediction on how much longer it will last.

Battery service life is also further reduced the more the battery is deep cycled (discharged to almost empty before being recharged). The BMS may keep a record of how many times it has been deep cycled and then use a formula to estimate the end of its service life taking this into account.

It can then report this to the battery user who will now know when they will need to purchase a replacement battery and avoid any disruption.

If the BMS is advanced it will also be able to see over what period of time it has been used and how intensively it has been used. With this knowledge it will be able to calculate an approximate date when it will need to be replaced.

BMS User Interfaces

Many batteries with a BMS are two small or located in devices where it is impracticable to mount a screen which can relay the information that the BMS has to offer.

As such the BMS often includes a Bluetooth Connection which can be accessed via an app (computer or Smartphone based) where the data can be presented in an easy to read way.

Some user interfaces are one way. You can only see certain bits of information. Others, like the one in the image above, allow user to interact with the BMS and take actions such as halting or delaying a charging process.

The BMS Gray Area

A BMS is one of 3 modules that can be found on some batteries. The other two modules are:

• A Protective Circuit Module (PCM) to both prevent the battery from overheating due to an internal malfunction and to protect the battery if it is connected to an unsuitable charger.
• A Cell Balancing Module which balances the charging process across all the cells in a multi-cell battery.

Some BMSs will include one or both of the above modules and the manufacturer will just say they are part of the BMS.

Other batteries will have these installed as two distinctively separate modules do it is possible to remove the BMS without disturbing the functionality of the Protective Circuit Module or the Cell Balancing module.

The takeaway is that a BMS can be a very basic piece of kit that just stores a few pieces of basic data to help manage basic processes like charging or discharging or it can be a highly complex module including layered defense against both internal malfunctions and external abuse.

To read more about Battery Management Systems including their evolution see our article on What is a Smart Battery.