Can I use a single 1s-3.7 V BMS circuit for a 4-cell parallel connection?

Question

I want to make a 24v-7s4p lithium-ion battery.

May I use single 1s-3.7V BMS circuit for each parallel connection of 4 batteries?

Answer 1

A BMS can have several distinct tasks, and not all tasks are required in all applications. Your proposed solution would be suitable for some of these tasks, but not for all.

• Protecting against overvoltage. Charging to a too high voltage can cause fire.
• Protecting against undervoltage. Discharging to a too low voltage can cause damage to the battery, increasing the probability of fire later.
• Protecting against overcurrent. Drawing too much current can cause fire.
• Protecting against temperature. Too high temperature can, again, cause fire.
• Balancing the cells. If the pack is charged and discharged as one, the current must be stopped once any cell reaches the upper or lower voltage limits.

Depending on the single-cell BMS you use, your solution might be able to do the protection-tasks, but balancing will need to be preformed by a separate charger or some other system that can consider all the cells.

Considering the parallel connections: Once connected, you can mostly consider the parallel connection to be a single cell. However, if you want to push the limits on current draw (i.e. draw more current from your battery than a single cell can safely provide), you have to consider the matching between the cells, since minute differences in internal resistance can mean that the current is drawn more from one cell than from the others. This matching is also important during assembly, as the cells needs to have very closly matched voltages when you connect them, to avoid large inrush currents when you make the connections.

Answer 2

Since this BMS is being used on a 1S configuration, yours is equally effective as any other BMS so long as power-handling is not a factor. Making sure your parallel cells are matched for mAh and discharge rate is important, though; having one cell with very-low internal resistance and three high internal resistance will allow the BMS to charge the low-X cell to capacity, stop charging until the internal resistance allows for proper balancing.