hwse
Solar Enthusiast
- Joined
- Jan 2, 2021
- Messages
- 585
@Evan BateAn 8s BMS expects to check the voltage of each cell by measuring the difference between consecutive balance leads. So if you have an 8s BMS is has 9 balance leads, and to check the voltage of cell 4 it will look at the difference between the B4+ lead and the B3+ lead. Now if you try to hook up to 2 separate 4S batteries, it will assume that the B5+ lead (the sixth of your nine balance leads) will indicate the voltage of the 5th cell, by comparing it to the B4+ lead. That won't work, and the BMS will try like hell to balance and it can't. Moreover, it will probably shut down with an under-voltage since it will look like cell 5 is something like -9V.
I watched the video you are talking about from Andy some time ago. I don't have time to watch it again right now, but as I recall the "trick" he did was to use an 8S BMS (or maybe it was 16S) on a 4S pack of cells. For some BMS's that will work, if you leave the unused balance leads unconnected to anything.
In Andy's video, he used his 24s JK to run a 4s configuration. The JK allows you to tell it how many cells are in the battery but it still has a minimum operating voltage. For the 24s that minimum voltage is around 44v minimum so the boost converter needs to be a 12v/48v. For the 8s the operating voltage is about 20v so to use it for a 12v battery you need a 12v/24v boost converter.