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

General Discussion about electric bicycles.
Post Reply
LowkeeRider   100 mW

100 mW
Posts: 46
Joined: Jul 18 2012 1:32am

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

Post by LowkeeRider » May 21 2018 10:40pm

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?

User avatar
eee291   100 kW

100 kW
Posts: 1046
Joined: Nov 05 2015 4:35pm
Location: Austria, Burgenland

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

Post by eee291 » May 22 2018 3:48am

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.

User avatar
eCue   10 kW

10 kW
Posts: 632
Joined: Nov 03 2017 5:14pm
Location: Vancouver Island , Canada

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

Post by eCue » May 22 2018 8:10am

Im thinking it could be a 48 volt BMS
Solar charge station on wheels join the Revolution

User avatar
eee291   100 kW

100 kW
Posts: 1046
Joined: Nov 05 2015 4:35pm
Location: Austria, Burgenland

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

Post by eee291 » May 22 2018 9:09am

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.

LowkeeRider   100 mW

100 mW
Posts: 46
Joined: Jul 18 2012 1:32am

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

Post by LowkeeRider » May 31 2018 12:58am

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.

User avatar
Sunder   100 MW

100 MW
Posts: 2871
Joined: Sep 06 2011 11:24pm
Location: Sydney, Australia

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

Post by Sunder » Jun 01 2018 6:41am

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.
eBike: Q100H on 16S with Phaserunner FOC Controller
eMotorscooter: Vectrix VX-1 Died... Electrified Ninja in Progress!
eCar: Mitsubishi Outlander PHEV... Warramty expired. Still not game!
eHouse: 6.2kw On-grid solar with battery backup coming soon!

After 5 builds, the best advice I can give, is start with high quality products. I prefer http://www.ebikes.ca

User avatar
fechter   100 GW

100 GW
Posts: 14647
Joined: Dec 31 2006 3:23pm
Location: California Bay Area, USA

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

Post by fechter » Jun 01 2018 8:38am

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.
"One test is worth a thousand opinions"

Post Reply