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Balance charging lithium cells

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OM222O:
so what I'm getting at here is to test the capacity of each cell, then make the packs so they are as closely matched as possible, then balance at the full charge point (4.2V) and actively monitor cells to protect against over discharge  ???

as you can see from my first post, my main goal was to reduce the waste as well, so can you please share how you managed to reduce the balancing current using resistive balancing?

Siwastaja:

--- Quote from: OM222O on April 30, 2019, 03:09:59 pm ---so what I'm getting at here is to test the capacity of each cell, then make the packs so they are as closely matched as possible, then balance at the full charge point (4.2V) and actively monitor cells to protect against over discharge  ???
--- End quote ---

Yes, matched for same capacity, then balanced so that voltage matches at 4.20V. It won't match perfectly at low end, hence the cell-by-cell low voltage cutoff.

So, monitor each cell and stop discharging when any hits LVC (I'd recommend 3.00V as a baseline). Charging is a bit different and depends how well you can control the charger. If you can control the current, do a proper CV phase based on the highest cell voltage - dynamically adjust current to maintain the highest voltage at 4.20V, stop when your current tapers off (C/20, for example). If you can't do that (i.e., if your charger has fixed CV output voltage), just let the charger do the CV phase based on total pack voltage, and use the cell-level OVP as an extra safety measure, but you'll have to adjust it a bit higher (say, 4.25V) so that it won't cut off prematurely, preventing full CV charge.

If you don't need 100% charge, just skip the CV phase completely, and you'll have a battery at around 90% charged, and a simplified procedure.


--- Quote ---as you can see from my first post, my main goal was to reduce the waste as well, so can you please share how you managed to reduce the balancing current using resistive balancing?

--- End quote ---

By using longer balancing time compared to the competitors. Instead of dissipating for a a few minutes during the end of the CV phase, like most products do, I just measured the voltages at the end, calculated required shunting times per cell based on imbalance voltage measured - this could be hours, and the balancing could run while discharging, or doing whatever you do. The required amount of balancing charge is the same, but during a longer time, with lower current. This makes sense, because very seldom you need a lot of balancing at all.

OM222O:
oh! so it doesn't actually reduce the waste, just makes it over a longer time (presumably for heat dissipation factors ? and safety like the fuses you mentioned?)
by that definition I would just choose the normal method (higher current over shorter time) if the balancing is not required that often (lets say once a month or something) as it does not produce a huge amount of waste  :-/O

I watched the video about the "DIY Powerwall" that was posted and it leaves a lot to be desired (using cheap pre made BMS? 7 segment leds for voltage indication?) and it's pretty rough around the edges but I had a similar idea so might take some inspiration. My end goal is to have my bench running all on DC from this sort of DIY power wall so there won't be massive current draw (100s of amps or so) and they won't be abused, maybe something like 20A would be a worst case scenario (for charging super caps  :palm:). currently I'm just using switch mode PSUs that you can buy online but they are a bit too noisy for some of the analog projects  :-DD it might end up becoming a modular system like he proposes but that's later down the line. it'll also save me quite a bit as I can run it from solar and have free electricity in my work area. Thanks everyone for their advice.

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