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this is a puzzler here. I have a 48V bank of Battleborn lifePO4 batteries connected to a huge 10kva inverter.

When I tried to charge one of the 12v batteries while the pack was still connected to the inverter, the charger went berserk immediately, I don't understand why this happened.

When I had LEAD ACID batteries (same 48V bank), connected to a smaller inverter, I used to be able to charge any of the single 12v batteries while the bank was still active on the inverter. So, lets say the inverter had finished charging and the pack wasn't balanced, I used to be able to just pop on the 12v charger to bring up the lowest in the pack without having to disassemble anything.

Why can't I do that now? It's a simple series connection, but could the fact that it's a huge inverter be playing a role?

The batteries are wired to a start cap before the inverter, I believe it's called a CLS500 and the negative is wired to a grounding rod buried underground.

For those reasons, I just don't feel comfortable having to disconnect the batteries every time i wan't to balance them out.

Please someone explain to me why I can't charge them individually while they are still connected. I can't afford to lose my new charger to the same failure as I am in Nigeria and it will take ages to get a new one if I mess this up.

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  • \$\begingroup\$ Click on edit and the schematic symbol. \$\endgroup\$
    – winny
    Commented Aug 17, 2020 at 21:04
  • \$\begingroup\$ Imagine a circuit with a bunch of cells in series totaling to 48v and a filament lightbulb. Now remove one of the cells (say it fails high impedance). Measure the voltage across the missing cell - you get pretty close to 48 volts. Think about that for a while and what sort of nasty situations a charger might experience as a result from even the most transient impedance bumps such as a protection circuit opening. High voltage series packs are no joke, you have to think about the entire pack voltage, not just that of a cell or a few in a submodule. \$\endgroup\$ Commented Aug 17, 2020 at 21:17
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    \$\begingroup\$ Also be careful that there isn't an unrecognized path to ground through the charger and inverter; but even if there isn't, using it on part of a larger series pack is simply a bad idea, at minimum until the series path is broken. \$\endgroup\$ Commented Aug 17, 2020 at 21:22
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    \$\begingroup\$ I have a theory, related to @ChrisStratton's comment about "unrecognized paths to ground". I bet your lead-acid charger had the battery negative isolated from the wall ground, but your lithium charger has the battery negative connected to the wall ground. If so, that means charging any battery but the first in the string (the lowest in your diagram) is shorting the positive terminal of the previous battery to ground, through the charger. This would be expected to blow up the charger. (If this is the case, there's an obvious workaround, but I don't know how unsafe it is.) \$\endgroup\$ Commented Aug 17, 2020 at 22:43
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    \$\begingroup\$ @ChrisStratton Care to make an answer from your comment - plus perhaps other comments on your comment. \$\endgroup\$
    – Russell McMahon
    Commented Aug 17, 2020 at 23:50

2 Answers 2

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  • Charger MUST float with respect to battery overall negative terminal.

  • Floating charger MUST have enough isolation to withstand battery voltage.

  • Charger may be unable to handle situation where battery voltage being charged is > charger voltage. (Some can, some can't).
    Adding a diode to charger output MAY help. This will probably cause charger regulation to be poor or worse, but if you are manually balancing by eye this may be acceptable.

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You put two chargers in parallel, you the risk of the smart 12V charger failure or voltage source fighting each other or reverse charging the charger blowing the reverse diodes. Possible series R or diode maybe.

You needed a BMS system for any large pack when they were new and balanced <1% now that they are used and over/undercharged the imbalance has increased which occurs all the time as it is impossible to balance any pack without a BMS.

Why? Because the weakest cell maybe 10kF @ 4V with the lowest capacitance will charge up the fastest dV/dt=Ic/C and discharge the fastest. the more energy mismatch, the more expensive high power dump load you need to bypass energy while the others are in CV mode or worse in CC mode.

For now you can make a current source by using a larger voltage and a heater to make it a quasi current source then use a comparator switch to cutoff at4.1V.

In the meantime you can program your system to only go between 20% SOC or higher and 90% using a lower CV level by 100 to 200 mV. sure you have lower capacity but in the long run , longer life span. See battery university .com or ask CADEX. this will reduce the risk of exponentially killing the weakest cell . Like a slow thermal runaway in fewer lifetime charge cycles but in electrolyte/electrode decay.

if you used between 50% and 90% , you can get much more than 10 yr life if kept cool from a good supplier. So you learned some expensive lessons. Mitsubishi cars do it all the time.guaranteed.

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