2 lithium packs 2 bms to put in parallel?

Hi everyone.. Great forum you have here, I have been lurking for over a year and just reading and using the search engine for my questions but have not found a answer for this yet.. I have now constructed several different 18650 packs for my personal use and know much of the basics. I would like to know if I can jump positive to positive and negative to negative between packs to put them in parallel without any negative affect on the bms or cells?

My packs..

4s15p spot welded.. Bms on one side for drain and other side I have no bms just neg and pos rails and a 4s balance harness to plug into a Isdt smart balance charger.. One pack is MJ1 and other is Sanyo Ncr18650ga, both similar aged cells under a year old.

I just want to jump them for draining and charge them individually.. What would be the best or safest way of doing this? Jump bms to straight pos/neg rails of other pack or jump straight pos/neg from each pack and drain from one of the packs bms?

nightlife416 said:

I just want to jump them for draining and charge them individually.. What would be the best or safest way of doing this? Jump bms to straight pos/neg rails of other pack or jump straight pos/neg from each pack and drain from one of the packs bms?

Click to expand...

Well, the best and safest way is to parallel first, at the cell/string level, by jumping the pos/neg from each pack (BEFORE the BMS) AND jumping the balance leads, then only BMS to controller. The two packs would electrically become one big 30p4s pack, which would preserve full BMS functionality.

Without paralleling first, next best is straight pos/neg from each pack. Overcurrent/short and pack-level over/undervoltage all still work, but cell/string-level over/undervoltage protection only works on the original BMS pack.

Hi Fatty thanks for the info, I never thought about jumping the balance harness between packs too, would it be something similar to the photo? I'm also wondering about those commercial lithium packs (Dakota, Ionics etc) they advertise as jumpable to parallel or series and only have a single pos/neg post.. So are they not jumping bms to bms?

Maybe even something like this to jump