U could lower your cell balance threshold level to say 3.0 volts . Then the bms would do it for u.
Might take a while though
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Ill try that. waiting for a few dry days so I cant charge it outside
Or just charge it and wait a long time for all the other groups to slowly get balanced down to that one group. Just watch the voltages as u charge.
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Ill give that a go. Is there any reason to be concerned about using an old battery that not been charged for years (besides the cell imbalance)? Itâs been stored in a cool dark and dry space.
Better to just lower the bms balancing voltage as said above.
I dont think old batteries are a problem
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ok, ill have a fiddle with the smart BMS app. thanks for your help @hummieee @hatman and @jack.luis
IMO cells being unbalanced at that low a voltage is no concern. A pack thatâs been in room temp storage for years with all of the cells still above 2.5V is no concern.
What can be a concern is BMS that doesnât properly balance the cells when you charge the pack. Do it s.l.o.w.l.yâŚ500mA is goodâŚand stay close by the entire time. Make sure no part of the pack gets noticeably warmer than the rest. Make sure that as the cells approach about 4.1V or so that they start balancing.
You can switch over to 250mA charging, even 100mA, once youâre up above 4.0V in order to give the balancing a better chance to âstay aheadâ of any unbalancing that occurs. The slower the charging the more of an effect the limited balancing current can have.
If possible, and you are sure youâre doing it correctly, you can try setting the chargerâs voltage to 4.0V/cell and set the BMS to match (start balancing at 3.8V or so).
Once that works and the BMS has finished balancing at that voltage then you can set it all up for normal 4.2V charging and monitor that.
The odds of something going violently wrong are very low but the consequences can be very bad if it does happen so be prepared for troubleâŚjust in case. You want that pack outside ASAP if catches fire and you wonât be able to be near it so getting it on something that can be pulled outside by a rope or on wheels (a metal kids wagon?) can work well.
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Hi Mooch, my charger has 2 modes: On and Off. Will i have to get a programmable charger or something to feed it power in small sips (at 500mA), or is this done another way? My only charger is output rated at 50.4V 4A
and I definitely will be charging it outside. Waiting for a non-rainy day to do it.
Yes, you will need a charger which outputs the desired voltage and amperage. You will also need a charger which performs the proper CC/CV cycle for charging liion batteries. You can not make your current 12s4A charger output 500mA.
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Thereâs no easy way to get that charger to go slower or charge at a lower voltage. You would need, at least, a low current 50.4V charger or a fully programmable one so you can choose the voltage and current.
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I can reach most of the balance lead tabs, but a few are completely filled with a hard epoxy/glue, but the sides of the cells facing the outside are completely covered in nickel. Doesnât seem possible to test each individual cell without disassembling.
Was thinking of building my next battery⌠all I really need is a welder. More toysâŚ
No need to get to the individual cells yet, just proper readings from each p group. Once you have good reads we can see whatâs going on with those weird voltages. The total voltage being low isnât a great sign though - did you measure the voltage at the battery output or just on the bms header?
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At the XT-90.
I got the list above by connecting the negative electrode on the black wireâs pin in that white wire harness, where the leads attach to⌠and the positive electrode on each pin adjacent it.
Sorry if Iâm slow to understand⌠are saying to put say black electride on the wire lead tabs (connected to each pgroup) and red electrode on the nickel strip (opposite side) of the same pgroup?
Thanks!
I dont suppose you could suggest, or even better, link an example of a low-current charger that i might get? I would be guess that a fully programmable one would be out of my budget
Dont suppose yo could link an example of the type of charger you have mind? I am really on a budget so a cheaper one would be preferred if possible
I donât think a low current charger is going to be especially helpful in your situation.
Your bms is only going to bleed one cell at a time at about 50ma at best, charger or no charger, its going to take days/weeks for the bms to bleed the 11 high cells down to match the low one.
Your best option in my opinion is to use what you have available to you right now.
Set the bms to âalways balanceâ, you may need to lower the balance start voltage as well. This will start bleeding the high cells whether the pack is charging or not.
When time/weather permits, put the pack on charge when you have the ability to monitor it the entire time.
The bms should cut the charge when any cell hits the overvoltage limit, usually default is 4250mah.
If it doesnât, iâd be suspect that there may be something wrong with your charger.
In any case, once any cell has reached 4.2v, disconnect the charger. The reason we want the pack close to full is that the low cell should come up to a closer voltage than it currently is, and hopefully reduce the time we have to wait for the pack to balance.
Hope that makes sense!
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Extremely help advice, thank you. Ill use my existing charger then the bms setting to regulate things. awesome 
would you say thereâs a use-by-date with regards to batteries (like the 40T cells that I have) - like how long can unsused batteries go before they shouldnt be used?
Thereâs no definitive answer to that unfortunately.
The best you can do is balance that pack out and observe it over a few cycles to see if it still behaves. If it doesnât, youâll need to reassess then.
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Heâs right, that does sound unsafe
This doesnât offer any current control, defeats the purpose of trying to carefully charge one group in isolation. You could add a series resistance that would limit the current, but not by a predictable amount
Ideally youâd want one of the following:
- An adjustable power supply (aka lab power supply), that lets you set voltage and current limits. Set it to 3.6V and 0.5A, itâll use the full 0.5A for a while then reduce as it gets closer to 3.6V.
- A 1S lithium balancer/charger (or a bigger balance charger that can be set for 1S) would also work
Generic lab PSU on top. Then IMAX B6 balance charger, those are cheap enough and (I think, please fact check me) pretty solid. Itâs a configurable 1-6S charger
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