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I have an 8S LiFePO4 battery pack for my ebike project which I tried to charge using my suspicious lithium battery charger. The charger applied a voltage slightly higher than the battery voltage at any moment to charge it along with 1.5 A of current. For example, if the battery was at 26.2 V the charger applied 26.5 V and 1.5 A, and the voltage increased in the same proportion as the battery voltage increased. This was constant current charging.

The thing is, the charger cut off the charging while my battery was only 85-90% charged(26.6V) and never intiated the constant voltage charge. I just read somewhere that I need a charger which applies a constant voltage of 29.4 V to fully charge a 24 V LiFePO4 battery.

Did I damage my battery by charging it through a charger that doesn't fully charge my battery and give out constant 29.4 V? I may be just paranoid but it could be a serious thing. Can I continue using this charger in the long term which never initiates the CV charging? I'm fine with not utilising the remaining 10% SOC. I just want to make sure this kind of undercharging and this charger doesn't harm my battery in any way.

And i also heard that many bms' tend to start balancing the battery when it reaches a certain VOLTAGE (full charge voltage), if this is true..... doesn't it mean that my battery can become unbalanced in the long run as the bms may never balance the cells?

Note: I'm skeptical about buying a new charger because my current charger also lists 29.4V as maximum, but it never reaches beyond 27.4V while charging. I'm scared the new charger would do the same thing with my battery.

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    Welcome to Stackexchange. This question is mostly about batteries and charging, so might be more on-topic on electronics.SE That it is for a bicycle is a side detail. If you want this migrated, just say so.
    – Criggie
    Apr 30, 2023 at 0:59
  • This is mostly about battery chemistry indeed. Li* batteries don't have the same 'learning'/'memory' problems as other chemistries like NiMH. A Li* battery should ideally be charged before it is empty and there's no problem not charging it fully - ideally the battery level should actually be kept in the middle regions. As opposed to a NiMH battery which you should discharge as much as possible and always fully charge.
    – Lundin
    May 17, 2023 at 14:08

1 Answer 1

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LiFePO4 need 3.50V-3.65V/cell to fully charge - 28V-29.2 for 8 cells. Without getting to 28V, the cells will never be fully charged. Unlike lead acid batteries this will not harm the LiFePO4 (and may even extend its life).

The charger you have not in any way 'smart' or 'special' - (Well, it might be 'special' in the way that some kids in the 1960's were called 'special'. It is a simple charger than can supply no more than 1.5A and (it appears) no more than 26.6V.

You have not harmed the battery in any way - within reason, the BMS is there to protect the battery from harm even if the charger could damage it. You can continue to use the charger until you get a better one. If the charger is specified as producing 29.4V and it is only outputting 27.4, you have a right to return it. Buy a quality charger, there are also other characteristics to consider (e.g ripple, open circuit voltage when first connecting batteries) that can be detrimental to batteries. Saving a few dollars on a charger can be a false economy.

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  • thanks for that detailed and fulfilling answer....can you tell me if the bms will continue to balance my cells even though they are never getting fully charged? I now understand from your answer that undercharging is not harmful for the cells but if the battery doesn't get fully charged, will the bms still balance the cells, is it true that bms only starts balancing once one cell is fully charged? Thanks for giving me your valuable time and efforts Apr 29, 2023 at 21:22
  • Sorry, cannot answer that as its beyond my knowledge. It probably depends on the BMS, but some require full charge. Presume yours needs full charge to balance.
    – mattnz
    Apr 30, 2023 at 2:38
  • @AkshatSharma The BMS should make sure that each cell individually gets charged with the correct voltage. This happens all the time during the charge cycle. Li* batteries are picky with charging voltages so it won't work any other way. Though please note that Li-Ion, LiPo and LiFePO4 are slightly different chemistries with slightly different charging voltage characteristics, so using a BMS intended for a different chemistry isn't advisable. How big a difference this actually makes in practice, I'm not sure.
    – Lundin
    May 17, 2023 at 14:12

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