Battery Protection PCBs or Battery Management Systems (BMS) are used with Lithium Ion batteries for safety and to protect the battery when a fault is detected.
So what is a Battery Management System? and what does it do?
A Battery Management System (BMS) is a small printed circuit board assembly that is provided with lithium ion batteries to protect the battery and/or the application/user in the event of a fault. It also provides some house-keeping functions for the battery pack itself to prevent things from going wrong in normal use.
There are many different types of BMS ranging from simple protection boards to hideously complicated circuits with gauges and microprocessors. Let’s have a look at the main functions of these.
Under-voltage protection: During normal use, a li-ion battery voltage will decrease as it is used. At the point where the battery is almost fully discharged, the voltages decreases more rapidly. If it is allowed to discharge too far, then the cells will be damaged so the BMS turns of the load current when either the pack voltage or an individual cell voltage gets too low.
Over-voltage protection: During charge, the battery voltage increases to its normal maximum charge level and then it is held at this voltage until the charge current tapers off to a low value (usually C/10). If an incorrect charger is used or the charger voltage is set too high, then the battery will try to over-charge. For instance, if the nominal charge voltage is 4.2V/cell the BMS will turn off the charge current to the battery if it detects a charge voltage of 4.35V/cell, to prevent this over-charge.
Over-current protection: A BMS is chosen for a battery so that it will deliver the desired load current and a little bit more. It should prevent a large over-current as this is likely to be a fault. This can happen on either charge (if an incorrect charger is used) or discharge (maybe a short circuit on the load). In either case, the charge or load current is interrupted if such a condition is detected.
Over- / under-temperature: If you read the specification for a battery or cell you will find that the manufacture will quote the min/max temperatures for normal use (often -20 to +60C for discharge and 0-40C for discharge). A BMS that supports this will detect any temperature outside these parameters as a fault and will turn off the charge or load current appropriately.
Fuel gauge: Sometimes a BMS will incorporate a facility to display the state of charge of the battery with 3, 4, or 5 LEDs. Sometimes these are simple voltage measurement devices or, for the more expensive varieties, can monitor the charge and discharge current, temperature usage statistics and performance over-time to evaluate a more accurate state of charge.
Cell balance: Cell voltages have a tendency to drift during discharge as some will be weaker than others (even if from the same manufacturing batch) and this is particularly true as the battery ages. A BMS can detect this drift and correct it by robbing some of the current from a higher voltage cell to allow others to catch up during the charge process and thereby maintaining the health of the battery and extending its normal life.
Communication: Some applications need to understand the battery condition in detail and need to interrogate the battery to see what is happening. The BMS can provide this level of communication and pass the battery parameters to the host computer via a serial communications bus.
Programmability: Most BMSs provide some or all of the above features and are fixed in terms of the values used to define the limits of the parameters being monitored. Some have the facility to connect to a host computer to change these values. Generally it is a must to know what you’re doing before changing anything as it is possible to make some expensive mistakes.