New BMS does not balance all cells - faulty BMS or Cells

LowkeeRider

100 mW
Joined
Jul 17, 2012
Messages
49
I have headway 38140 12A LiFePO4 battery pack, has about 3000km on it and could charged to 56.6v. Few months ago, I unwrapped the pack but made a mistake of damaged the BMS. There was no problem making the 25km to and from work without a BMS. Start voltage 56.3 end 52.7 volt. At the end half the cells has 3.31v and the other half has 3.28v. Not sure how many watts used but the cells looks good, I guess.

So I bought a new 16s 45a BMS from Aliexpress but it could not balance all the cells. It charged to 54.3 volt, I thought that's not right so I let it charged overnight it only increased to 54.85 volt. I then discharged it to 52.37 volt. Individual cell voltage before and after discharged:
1. 3.32 3.27
2. 3.57 3.28
3. 3.32 3.26
4. 3.31 3.27
5. 3.32 3.28
6. 3.57 3.28
7. 3.58 3.28
8. 3.58 3.28
9. 3.31 3.27
10. 3.32 3.27
11. 3.32 3.27
12. 3.30 3.28
13. 3.58 3.28
14. 3.58 3.28
15. 3.29 3.24
16. 3.58 3.28
So my question is, is it a faulty BMS or cells but if it is a BMS then why seven of the cells able to charged to 3.58 volt?
 
Could you show us your BMS?
And a link maybe.

Also with such a huge voltage delta you will have to balance that pack for a week.
 
What is that supposed to mean?

You have 16 cells in series, so you need a 16S Lifepo4 BMS.
If you search 48v Lifepo4 BMS you will get the correct one.
 
eee291, I took your advice. I leave the pack charged for 48 hours it is now fully charged and balanced to 56.6 volt just like when it was new.
 
For a LiFePo4 pack, that's not badly balanced. It's not in balance, but its not badly out.

Unlike LiPo, where 1/10th of a volt is way out, there's only about 5% capacity between 3.3v and 3.6v, so at a guess from best to worst cell would only be about 10% out. 48 hours was probably more than enough, a week way too long.
 
The balance current in the BMS is very low, so would take a really long time (days) to balance that pack. The charger would need to maintain output the whole time but many will shut off as soon as the current tapers off.

Manually charging the individual low cells is the fastest way to get it balanced again. Once balanced, the BMS should be able to maintain the balance if all the cells are healthy.

With LiFePO4, if the cells are bad, they tend to develop high self-discharge and lose voltage quickly after charging.
 
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