cell_man
100 kW
If 2 packs are charged in parallel, each with a 3 wire bms, and either of the packs has a cell group/groups with an issue, then it is possible for that pack (specifically the problem cell groups) to be overcharged. Charge current will flow through the discharge leads and bypass the overcharge protection on 1 of the packs.
You risk blowing the fets in the bms if running 2 packs in series. If 1 of the 2 packs trips, the total pack voltage will be seen across the bms that trips and that voltage will generally exceed the operating voltage of the fets used. A diode should protect against this. If charging series packs with a single charger, the charge current will bypass the charge circuit of 1 of the bms, so you have no hvc protection on that pack (comments relate to 3 wire bms).
If you are going to series parallel packs, 2 wire bms should be used IMPO and fets which are rated for the total pack voltage should be used, or at least fly back diodes.
This is my understanding of it and i will always try to disuade people from messing with series/parallel and also split packs with a single bms. Several customers have wrecked the cells in a pack by plugging them together at different states of charge.
You risk blowing the fets in the bms if running 2 packs in series. If 1 of the 2 packs trips, the total pack voltage will be seen across the bms that trips and that voltage will generally exceed the operating voltage of the fets used. A diode should protect against this. If charging series packs with a single charger, the charge current will bypass the charge circuit of 1 of the bms, so you have no hvc protection on that pack (comments relate to 3 wire bms).
If you are going to series parallel packs, 2 wire bms should be used IMPO and fets which are rated for the total pack voltage should be used, or at least fly back diodes.
This is my understanding of it and i will always try to disuade people from messing with series/parallel and also split packs with a single bms. Several customers have wrecked the cells in a pack by plugging them together at different states of charge.
