They are 15A BMSs so I have to run them bypass. They share a charge lead so they will end up the same voltage all the time, but one will start leaching 0.12V and dissipating it as heat when they balance
That statement is incorrect. You donât have to, you are choosing to.
It will likely be fine. But in a high drain situation, you will be at risk of running the 3p pack below the optimum LVC.
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Touche
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Whats âoptimum LVCâ?
If itâs better I can run one pack at a time and swap them when they are low, but thatâs more strain per cell when in use
Low voltage cutoff
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30Q cells are okay to 2.5V right? I am planning to set the low power mode to start at 3.2V/cell average and cut out at 3.0V/cell average
My opinion is that discharge bms is not needed. packs are in parallel, so they will always have same voltage. maybe during discharge, one pack will lose voltage a little bit faster than another, but they will quickly balance again.
also, you have same charging port for both bms? I donât know how smart this is. I would wire separate charging port for each bms, and then one cycle charge through one port, next cycle through second port.
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2.5 is a bit too low
you would be eating away at cycle life
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Yeah i would think 2.7 absolute minimum. But having hard cutoff at 3 sounds good.
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Since they share an output, wonât they always read the same voltage across the charge terminal? The extra energy will transfer via the output directly to the cells, it just wonât balance or anything
I mean if the hard cut at 3.0 average and they vary by 0.1 at full discharge I am not running serious risk of trouble right? I have no intention of getting that low
yeah thats fine
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I am trying to think really hard about that but I am not smart enough. maybe you are right and I am just paranoid?
Yeah I was staring at a large pile of connectors and wires for about 2 hours yesterday trying to make sense of it, one second and I can try and make a diagram for you
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Everything shown is supposed to stay permanently wired and sealed in a box, was gonna lock it and leave it in theory
yeah just like I thought. what bothers me the most is this: since li ion chargers are CC/CV (constant current/constant voltage) and you have two pack with different capacity - one pack will charge (rise its voltage) faster than other, so I am assuming this will confuse charger? I really find it hard to make any conclusion here
EDIT but then again, they are in parallel, so they will always have same voltage. 
but I am assuming here that bms could not properly balance one pack, because current from another pack will be stronger than balancing current. I hope you can follow my thoughts
From what I vaguely remember from passing an electricity and magnetism course is that the current will split based on the capacity (assuming they charge similar to a capacitor to some degree) so the 3p pack will take 8A*(3/7) and the 4p pack will take 8A*(4/7), and they will have the same voltage since the XT90s bridge will have to be equal voltage
the red wrapped bullet is the connection of the positive terminal of the 2 XT connections, it just made assembling easier to have it as 2 separate harnesses
since your battery is easy accessible, try to check groups after some cycles and let us know if they are always balanced. I am really curious to see how this works.
Will do! Since the cells are all top to bottom and the series connections go down the length, I can cut a âwindowâ through the heat shrink and fish paper on the sides facing away from each other (case is just plastic) to check everything without issue, then I can kapton tape the holes for peace of mind
The best choice I ever made was putting packing tape handles to pull the blocks out, its a lot easier
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