Placing multiple battery banks or cells in parallel increases capacity. There are several reasons to do this. For example, because you want to increase the capacity of an existing battery, or perhaps because the desired cell capacity is not available in one battery package.
To increase capacity, multiple cells can be connected in parallel or you can place multiple battery banks in parallel. Each situation has advantages and disadvantages and, of course, things to look out for.
Multiple cells parallel #
The big advantage of cells parallel is that the cells keep each other balanced. The voltage on each cell is always the same. This means that you do not have to monitor each cell in parallel, but only the voltage of the whole parallel branch. This saves on BMS costs.
The capacity of a string of cells parallel is the sum of the indivual capacities. For example: when you want a 12V battery with a capacity of 200Ah and you want to build this from 100Ah cells (3.3V), then configure the battery as 2P4S. This means that you need 8 cells in total: each 2 cells are parallel, then this parallel branch in series with the next parallel branch.
Multiple banks in parallel #
Even though placing cells in parallel cuts in BMS cost, sometimes multiple banks in parallel are required. For example, when redundancy is needed. In this case, each battery bank needs it’s own BMS.
Protecting each bank #
When multiple banks are parallel, the voltage of each cell in a bank is not the same as the voltage of the same cell number in a different bank. In example: in you have two banks parallel, it can be that the voltage of cell 1 in bank 1 is quite different from the voltage of cell 1 in bank 2, even though the bank voltage is the same. That is why you need a BMS per bank.
Besides a BMS, each bank also needs a power relay to fully protect the bank.
Let’s take as example two banks with each 4 cells in series. Under normal conditions, the bank voltage is about 3.3V x 4 cells = 13.2V.
If cell 2 in bank 1 would ever go bad and drop to a very low voltage (0.5V), the total bank voltage would drop. However because a second bank (bank 2) is in parallel, this bank will directly start to charge bank 1 to keep it to the same voltage as it’s own bank. This means that the 13.2V of bank 2 is distributed over the 3 working cells in bank 1, charging each healthy cell to over 4.2V!
That is why you need to give each bank it’s own power relay: to give the BMS the option to disconnect itself from the other banks, preventing overcharging of the healthy cells.
The power relay per bank is also needed for redundancy. This ensures that if one BMS shuts down the bank, for example because of an overvoltage, the other bank can continue to operate.
For safety reasons, always add a power relay to each bank in parallel. This automatically adds redundancy.
Installation tip #
Before connecting cells or banks parallel, always make sure the voltage of each cell of bank is about the same as the others. Otherwise when connecting in parallel, very high inrush currents will flow, leading to possible sparks or even damage.