It is sounding to me like having 2 separate bms with the discharge bypassed is just asking for trouble.
So it should probably either be a single bms with the balance leads paralleled between packs.
Or I need to find some room and run a separate bms for each pack and actually use the discharge lines.
That should allow each pack to isolate itself incase of any trouble.
I think I am leaning towards treating it like a single large pack with a single discharge bypassed bms and paralleled balance leads.
It seems like there is less chance for shenanigans with the 2 packs managing to get out of sync with each other somehow.
I do like the idea of powering something from the bms discharge that could act as a signal if something bad has happened.
Whether it be lights or something else I could add.
I see, that makes sense. Havenāt thought of the discharge leads being connected, I thought that in case of fail booth charge and discharge gets disconnected by the BMS.
I think the jury is still out on this. I wouldnāt state this as a confident fact. Thereās some complex shenanigans going on when you draw huge amounts of current from two drastically different sized, electrically connected packs.
If itās the same cells then it should be the same as building larger parallel groups
Different cell types with different IR and capacity ā then shenanigans
Well, itās literally building two different parallel groups (of series and parallel cells). But one is 4x the capacity. And then youāve wired them together.
Lots of people have paralleled two or more slightly different sized packs and seen no noticeable or significant repercussions, but Iāve never seen a 4x jump.
This question gets asked every couple months and Iāve read ever thread i could find here - thereās no 100% clear answer bc it requires significant testing. See the latest mooch thread on mixing cells for some background of what might happen
Ask this question - if a complete pack acts on some levels as a single, high voltage cell, whatās the difference between paralleling a single 1Ah cell with a single 4Ah cell and this?
The difference is that you know that the internal resistances will be proportional. 1Ah cell + 4Ah cell ā itās unlikely in the real world that a 4Ah cell of the same size would have 1/4th the internal resistance. But if both of these theoretical cells are made up of the same type of smaller cells (i.e., the 4Ah cell is physically larger ), then we know what the IR will be, and that it will be safe to combine them