What is a BMS Parallel Module?

The main purpose of a BMS Parallel Module is to regulate the current flow between 2 or more banks of batteries connected in parallel.

When 2 or more banks of batteries are connected in parallel, the State of Charge (SOC) of the different banks will equalise.  That means, they will transfer current from the highest charged bank to the lowest charged bank until they have the same SOC, and therefore have the same voltage.  With LiFePO4 Cells, this transfer of energy can be significant, and the high current flow will most likely be too great for BMS, while also causing damage the LiFePO4 cells.

The charge profile for LiFePO4 cells is relatively flat, that means there can be a large variance in the SOC between 2 or more cells, with a small variance in their comparative voltages.  For example, you may have only 0.01V difference in voltage between 2 cells, however there will be a big variance in the SOC between these cells.

By using a parallel module, if 2 or more banks are connected in parallel, it will restrict the amount of current transferred between the banks, therefore protecting the BMS and the cells from any potential damage.

Some might say that if you have 2 more bank always connected in parallel then they will always be equalised… Although this might be true in principle, practically there are a couple of scenarios which may see 2 banks attempt to equalise, creating a situation of high current transfer, such as:

  1. Before initially connecting banks in parallel, you should need to top balance ALL cells which will ensure they have the same SOC otherwise they will equalise when connected in parallel.
  2. If you have 2 banks connected in parallel, 1 bank maybe disconnected by the BMS due to one of the protection parameters (like cell voltage variance) being triggered. If this happens, then one bank will continue to provide/receive current and therefore increasing the variance of the SOC between the banks.  When you attempt to reconnect the bank into the parallel circuit, the banks will attempt to rebalance transferring unrestricted current.
By using a Parallel Module with the BMS for each bank, it will govern the amount of current able to be transferred between cells and avoid any potential damage to the BMS/s or cells.
October 21, 2022 — Ernest Brindley