Blog - BMS Systems

FAQ: Using SBM with a reduced number of cells

Question: We have a special project where we need to use 13 LFP cells. We want to use a simple SBM for the pack protection. Can you supply a SBM for 13 cells? Or can you modify the existing type of SBM to be used for 13 cells only?

Answer: Well, it is possible to make a customized SBM for an irregular number of cells. However such a solution requires many weeks to proceed and the costs to make just a few units will be very high. In reality, it is not worthy and reasonable to make such a customized SBM.

Also it is not possible to take a ready make SBM and to modify it for a different number of cells. The SBM is always designed for a fixed number of cells.

For proper operation of the SBM, all the input terminals of the SBM must be connected to the battery pack.  If the terminals are not connected to the battery, the SBM will detect the missing voltage at the input and will stay disconnected, preventing from a proper operation.

It is not also possible to change the number of cells. The SBM from its core design is always fixed to specific number of cells, typically multiples of four: 4 cells, 8 cells, 12 cells, 16 cells, etc.

Hint: there is an unverified proposal from our customer that suggests using a mixed pack with small cells, that are used only for keeping the SBM working. The main current is drawn only from the main cells (e.g. 13 cells) and the auxiliary cells (e.g. 3 cells) are only acting as a dummy supply to make the SBM feel the voltage on the terminals. Because the SBM draws some little current from the auxiliary cells (especially if they have only a small capacity), the auxiliary cells need to be replaced or charged after some time.  The customer suggested to use two AAA cells (2x 1.5V = 3V) for each dummy cell to make a 3V supply replacing the LFP cells. See the drawing of the design.

Note: This suggestion was not verified by GWL. You may need to make some tests of your particular SBM to see whether the HW design of your SBM supports this type of mixed installation.