1
\$\begingroup\$

I have a 2200 mAh, 3S 40C/80C (11.1 V) Lithium-polymer battery pack. After a few days of usage, one of the three cells became defective. The remaining two of the three cells are in normal condition. I have the following questions:

  1. Can I use the same three cell, 2200 mAh 3S Li-ion battery pack as a 2s (two-cell) battery?
  2. When it comes to charging the Li-ion battery pack (one cell defective and two cells in normal condition,) can I charge it to 7.4 V? That is, each of the two cells with 3.7 V.
  3. Is it safe to charge such a Li-ion battery pack (one cell defective and two cells in normal condition), or should I dispose of the whole battery pack even though two cells in the pack are in normal condition?
\$\endgroup\$
3
  • \$\begingroup\$ Without knowing exactly what’s up with the defective cell it can’t be a good idea. \$\endgroup\$
    – Frog
    Jan 9, 2023 at 8:22
  • \$\begingroup\$ In what way is the cell 'defective'? What position is it in the pack? What would you use the 7.4V for? \$\endgroup\$ Jan 9, 2023 at 11:13
  • \$\begingroup\$ Charger's LED indicators shows 2 GREEN lights (indicating charge complete for respective Battery Cells) and 1 RED light (charging incomplete for one battery cell) even after several hours of charging. Initial plan to use 7.4v was for RC application. After learning about danger of using such defective battery pack, I dropped the idea of using the 3s battery pack as 2s one \$\endgroup\$
    – Deepak
    Jan 10, 2023 at 14:18

2 Answers 2

2
\$\begingroup\$

If your pack has a BMS then it will shut off the output once the defective cell hits the lowest voltage. It will discharge faster than the good cells so you will not be able to use full capacity. Charging them is an issue as well - it will become an unbalanced pack. Don't use this.

If the pack doesn't have a BMS then it is very dangerous.

  1. You can't use this as 2S unless it is modified for 2S. Doing this needs skill.
  2. Never do this.
  3. Don't charge nor use this pack like this. If you have knowledge and you know what you are doing, then you can pull out those batteries and use good ones individually, or turn it into 2S pack or 3S/4S pack by buying new separate battery. But, as I said, this needs lots of knowledge, application type, and lots of other things and its very time consuming. Building a 2S pack with it still less time consuming. Using them individually with proper protection in place even have less technicality.
\$\endgroup\$
2
  • \$\begingroup\$ Thank you very much for the safely guidelines for handling partially defective battery pack. \$\endgroup\$
    – Deepak
    Jan 9, 2023 at 12:13
  • \$\begingroup\$ Going to dispose this pack for safety reason.I have no idea of tweaking this LIPO pack. As it is for RC application, I will buy a new pack. \$\endgroup\$
    – Deepak
    Jan 9, 2023 at 12:36
2
\$\begingroup\$

The only safe option is not using this battery pack.

You can't treat it as a 2S battery pack, because there is still that third defective cell in the circuit that does't magically disappear because it is defective. Charging to a lower voltage does not remedy this either.

You may be able to re-use the two good cells individually or in a new battery pack, but your 3S battery pack as it stands now is toast and possibly dangerous to use.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you for the suggestion. For safety reason, I am going to dispose the defective battery pack. \$\endgroup\$
    – Deepak
    Jan 9, 2023 at 12:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.