A lithium phosphate battery cell is 3.2V nominal (between 2.65 – 3.65V). This is the limit of the cell’s chemistry, so if we want more than 3.2V, we need to put the cells in series. (Translation for people with a social life – connect the positive terminal of one cell to the negative terminal of the next)
2 cells gives you 3.2 + 3.2 = 6.4V, 4 cells gives you 12.8V, 8 cells gives you 25.6V etc.
So if you want a 12V battery (nominal), you put 4 cells in series and charge them up. Simple, right?
That’s how it works with many batteries, but lithium cells tend to become unbalanced. If you put 4 in series and charge the whole pack to 14.2V (3.55V / cell, at which point the battery is fully charged although the cells can technically go a little higher), the charge becomes unevenly distributed. After a few cycles, one cell will reach 3.8V or higher and die.
Enter the BMS, or ‘Battery Management System’. The purpose of a BMS is to balance the cells, and turn off the battery if one cell gets too high or low, so the whole battery doesn’t fail. All BMS systems have cell monitoring and protection, and most have cell balancing incorporated, although some require separate balancing. (note – some BMS’s on the market incorporate a charging system as well. We will be focusing solely on the protection / balancing aspect)
A BMS must monitor the voltage of each cell and be able to shuffle power around in order to keep them at the same level of charge. To do this, a lead is run to each cell. In a 12V system, you will have one wire going to the negative point of the bank, and a wire going to the positive of each cell. The BMS monitors each cell relative to that first negative, so it expects to see 3.2V from the first to the second lead, then 6.4V from the first to the third, etc. For that reason it is very important to get your leads in the correct order (if you don’t your BMS will go pop) – Hint: Use a multimeter to check the voltage at the pins of the plug before you plug in to the BMS.
A BMS must also provide protection. Most BMS boards have the protection incorporated – they will have a copper plate where the output cable can be soldered or bolted to the device itself. One side goes to the battery, the other to your load. If any one cell gets to high or too low, the BMS cuts out to protect the battery bank. Some BMS systems rely on a system of relays in stead, but we will cover the types of BMS more in depth later.
Whilst most BMS systems have cell balancing incorporated, it is generally passive or resistive balancing. It only works in the top 15 – 20% of the charging cycle, and they tend to get hot. With larger cells, or with a battery bank that is working hard, passive balancing may not keep up, causing the BMS to trip more often or the bank to operate at a lower capacity. Active balancing that uses capacitors and operates through the entire charging cycle produces less heat is advisable – by keeping the cells at an even voltage, it ensures each cell gets a full charge with every cycle which improves both the efficiency and the cycle life of the battery.
