BMS Protection Functions

Low/High Voltage Protection: battery cells provide electrical current at a given voltage. What that means is if you connect the positive and negative terminals of the cells current will low at a specific voltage associated with that cell. But that voltage isn't always the same. Cells follow what is called a voltage curve (XXX), which means that the voltage a cell will output will depend on how much energy that cell is storing. When cells are fully charged, they will output voltage at the top of their voltage curve, and as they discharge that output voltage will drop. When they are near fully discharged they will output at the bottom of this voltage curve. This means that we can use the voltage the battery is outputting to estimate how charged a battery is, which is how it is done in actuality. But every battery cell must stay in within this voltage range, otherwise bad things happen (battery destroyed, explosion, etc.). Practically speaking this requires that you must not over charge your battery, or over discharge your battery.

Because the voltage tells us how much charge a battery is holding, we can always use the voltage to know when to stop charging or discharging the battery. So when a battery is being charged and the voltage of the battery reaches the top of its voltage curve, we stop charging it (and now can discharge it). When we are discharging a battery and the voltage gets to the bottom of the charge curve, we stop discharging it (and now charge it). The BMS does this monitoring of the voltage automatically for us.

Over Current Protection: battery cells are also limited in how much current they can charge or discharge at any given time. Too much current in or out of a battery at a time can easily result in damage to the battery cells or something much worse (again, explosion). Importantly though, battery cells themselves will try to absorb as much current is provided or provide as much current as is requested. It is the job of the BMS to monitor the flow of current into or out of the battery, and ensure that the current is within the batteries parameters. If it is not, then the BMS will stop current flowing to the battery (some BMS will stop either stop charge or discharge, depending on which of the two is over the batteries limit, and allow the other).

Charging and discharging rates are measured in terms of C, with 1 C being the total amp hour capacity of the battery cell. For example, a Lifepo4 cell with 10 ah that had a 1 C max charge and discharge could charge or at a maximum rate of 10 amps (and consequently could be fully charged or discharged in one hour). If the max charge/discharge of that same battery was .5 C, then the maximum amperage rate you could apply to it would be 5 amps, and instead if the max charge/discharge rate was 2 C then you could apply up to 20 amps to the battery (the max charge and discharge currents are often the same, do not have to be). For Lifepo4 batteries, maximum charge and discharge rates can vary, but are often around 1 C (although they can be higher and lower and you should always confirm with the datasheet of the battery).

Optional BMS Functions

Temperature Disconnect: battery cells operate in certain temperature ranges. Charging or discharging them out of that range can damage them (and also, again, lead to explosions). Some BMSs will monitor the temperature of the battery through a temperature probe and disconnect charging or discharging of the battery if the temperature leaves this range. Not all BMSs will do this, so you'll need to confirm if your particular BMS has this or not. See this page for info on the optimal temperature for Lifepo4 cells. Any system that has any risk of being in outdoor temperatures should have BMS with a temperature disconnect (this is also called low temperature disconnect because that is the more salient risk for Lifepo4).

Data Logging: batteries produce all sorts of data, the most important of which is how much power your battery is currently storing. Many BMS will keep track of the amount of power going and out of your battery, and using that data can tell you much power there currently is. This can be very useful for obvious reasons, but can also be really important for debugging your system if you are having problems.

Bluetooth (or other comms): some, but by no means all, BMSs have a way to provide information or be programmable. Most do this through bluetooth, which, when used with an app, often tells you much power is in your battery, its current status (for example how much power is currently flowing and which way), and allows you to program/customize the settings of the BMS. While inexpensive BMS often lack this function, any larger system should try to get a programmable BMS, which will allow you to make sure all the parameters your BMS uses to protect your cells are too your specifications.