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I have a small problem with charging my battery pack.

The first problem is that I do not even know if it really is a battery pack. It’s just eight 18650 cells connected in series in order to get +- 33,6V (one cell 4,2V on full charge.)

I’m used to charging one cell with some TP4056 Li-ion cell chargers, but I don’t think that this same approach is fine even in this case. I can’t just connect one TP4056 to the output of the battery pack, right? Of course, I tried to google this and I found so called BMS circuits (although, I would like to avoid paying for one :-)). Just to be clear, is this a 9S battery pack or not?

battery

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  • \$\begingroup\$ Sounds like a 9S pack. If cells are in series, yes. Could be 3P3S - measure terminal voltage. || A BMS or manual balancing is essential. | A TP4046 will not handle 9 cells (only 1). || A CN3722 module will NOT QUITE manaqge (28V max) \$\endgroup\$
    – Russell McMahon
    Jul 9, 2021 at 13:11

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Eight cells in series is an 8S battery pack (8 Series) not 9S.

The problem with charging cells in series is balancing them. You need to effectively charge each cell separately to get it to its own individual full capacity, otherwise you end up with cells being over-discharged which kills the cell.

A balancing charger will monitor the voltage of each and every cell and keep charging until all the cells are completely full. Getting a balancing charger for that kind of voltage and quantity of cells will be tricky and expensive. I would suggest, if you don't need "online" charging, that you should split your battery into two 4S segments and charge each segment separately using a cheaper and more readily available 4S balancing charger.

Either that or consider changing your battery arrangement to be 4S but with each "cell" actually two cells in parallel doubling the capacity and current ability of each cell. Then to get the higher voltage use a suitably rated boost converter to raise the voltage to the level you need.

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  • \$\begingroup\$ Super, thanks a ton! I will try to get 2x 4S BMS circuits (it seems that they aren’t so expensive). And also sorry for that 9S mistake. I originally wanted to make 9S, but then realised that 8S is enough. Thanks again and have a nice day! \$\endgroup\$
    – Papouc
    Jul 9, 2021 at 13:37
  • \$\begingroup\$ Just one last idea. Can I use one TP4056 for each of the cells?? I have bunch of them just lying around. \$\endgroup\$
    – Papouc
    Jul 9, 2021 at 13:50
  • \$\begingroup\$ @Papouc Only if you want to separate out the cells and charge them all separately. \$\endgroup\$
    – Majenko
    Jul 9, 2021 at 13:51
  • \$\begingroup\$ Ok, thanks! I’ll get the BMS circuits! \$\endgroup\$
    – Papouc
    Jul 9, 2021 at 13:53
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Just open it up to see what kind wiring is going on there. Balancing is always required for Li-on / LiPo packs.

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  • \$\begingroup\$ Welcome :-) Although what you say is true, an answer here should try to address the specific questions asked above. I see two question marks in the question text above, but your answer doesn't seem to specifically respond to either of them (nor to the implied question about a BMS). You can always Edit your answer to improve it. Also, as a new user, I recommend you read the site tour and help center to see the site rules and how Stack Exchange differs from typical internet forums. Thanks. \$\endgroup\$
    – SamGibson
    Jul 9, 2021 at 12:27
  • \$\begingroup\$ Hi, thanks a lot, but I am sure about the pack being eight Li-ion cells connected in series, because I made it (I forgot to say that, sorry). But I somehow understand that I need BMS capable of balancing 9S battery packs. Is it right? Are there any alternatives (charging alternatives). Thanks! \$\endgroup\$
    – Papouc
    Jul 9, 2021 at 13:05

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