Parallel p42a 12p and 3p batteries, how terrible an idea is this?

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.

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I see, that makes sense. Havenā€™t 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 wouldnā€™t state this as a confident fact. Thereā€™s some complex shenanigans going on when you draw huge amounts of current from two drastically different sized, electrically connected packs.

Hmm. Iā€™d be interested to know if that is actually what happens. I always thought that it only balanced while being triggered by the charge circuit.

If itā€™s the same cells then it should be the same as building larger parallel groups :woman_shrugging:
Different cell types with different IR and capacity ā€“ then shenanigans :slight_smile:

Well, itā€™s literally building two different parallel groups (of series and parallel cells). But one is 4x the capacity. And then youā€™ve wired them together.

Lots of people have paralleled two or more slightly different sized packs and seen no noticeable or significant repercussions, but Iā€™ve never seen a 4x jump.

This question gets asked every couple months and Iā€™ve read ever thread i could find here - thereā€™s 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, whatā€™s the difference between paralleling a single 1Ah cell with a single 4Ah cell and this?

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The difference is that you know that the internal resistances will be proportional. 1Ah cell + 4Ah cell ā€“ itā€™s 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 :smiley: ), then we know what the IR will be, and that it will be safe to combine them :slight_smile:

Edit: whoops I bumped an old thread, sorry

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