How to create your own Battery Management System (BMS)

What is BMS (Battery Management System) ?

A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack) such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.

BMS is taking number of inputs like Voltage, Temperature, Current and running number of algorithms (Fuel gauge) and estimations to generate outputs like Safe Operating Area, State of Charge, State of Health and Safe Operating Envelope. BMS is to keep the battery pack operating Safely and keeping it operating Reliably.

Functions of a BMS -  Measurement, Management, Evaluation, Communications, Logging.

Functions of BMS

The fastest way to make your own BMS, with a minimum amount of risk, is to start from an existing design. Various BMS designs are available to implement, adapt, and expand according to particular needs. Some are open source projects using standard ICs, while others are complete BMS chip sets.

If you want to create your own BMS, you need to know about Battery Electronics :

·  Protectors

· Monitor

· Gauge

Battery Electronics

The type of battery electronics needed varies depending on the type of battery pack. For simple packs, a simple protector maybe all that is needed. This can range from a basic over-voltage protector to a more advanced protector that responds to under-voltage, temperature faults, or current faults. Many advanced battery packs used in higher cell count batteries may require a battery monitor. A battery monitor measures individual cell voltages, battery current, and temperature, and reports these values to a controller. This information can be used by the system to adjust performance accordingly.

For example, the system could use this information to reduce the operating current if the temperature is too high. Battery monitors may also provide a cell balancing feature that can extend battery run time and also extend the lifetime of the battery. Monitors also include protections available that protect our ICs, but with much higher configurability.

For other advanced battery packs, a gauge IC may be used, which includes the features of the battery monitor, integrated with a controller that provides advanced gauging algorithms. Gauge ICs report the remaining capacity, runtime, and state of charge of the battery. Protections may be enhanced even further with software-based protections.

       

So which electronics should be used for a given system? Protectors offer the lowest complexity for simple pack designs. Monitors offer the highest flexibility. The pack designer is able to write code specific to their system needs, and this is often important where the system needs are unique. Gauge ICs offer the highest level of integration. They offer high accuracy state of charge information and faster development time since firmware is included, but they might limit flexibility.

Here is an example solution using the BQ769x0 battery monitor. This monitor family has devices for 5 cell, 10 cell, and 15 cell batteries. The monitor continuously measures the battery cell voltages, the temperature, and the current through the sense resistor, and reports this information to the microcontroller. It also provides multiple configurable hardware protections and will open charge and discharge FETs as needed to respond to fault conditions.

The microcontroller can make decisions based on the information provided by the monitor. It can also enable and disable the FETs, control the cell balancing feature, and can even do some basic gas gauging based on the voltage, current, and temperature information.

Simple BMS Model

Here's a second example of a slightly more advanced battery pack. Here we see the same monitor family, working with the BQ78350 companion controller.

The BQ78350 comes equipped with firmware designed to work directly with the BQ769x0 digital monitor, helping to accelerate product development. It also has fuel gauging and state of health reporting, as well as many of the other advanced features commonly included in TI fuel gauges, such as lifetime data logging and black box recording.

Many systems require the redundancy of a secondary protector for over-voltage. This example features the BQ7718 stackable over-voltage protector that can directly open the fuse if the primary protector was to fail. Some systems may require the use of high side FETs. High-side FETs allow for continuous communication to the pack, regardless of whether the FETs are on

or off. This allows the system to read critical pat parameters, despite safety faults, enabling the system to assess pack conditions before allowing operations to resume. The BQ76200 high-side in FET driver can be used with the BQ769x0 monitor and systems where high-side FETs are needed.                            

                   Advanced BMS model

Choose Battery Protectors, Monitors , Fuel gauge IC’s and Controllers according to your requirement and start designing your own BMS.