4s Lifepo4 battery bank cell diffeence at near full charge.

larrylwill

New Member
Joined
Aug 22, 2021
Messages
75
I built a 4s battery and top balanced the cells. After several problems I started to use it in my Motor home. The BMS is only 150a but the battery is 280Amp so I have to charge through a Renology 60 Amp DC2DC charger. The battery has been kept at 80% untill I started driving, I have the BMS set to 3.6v for the cells and 14.5v for the pack.when I get near full charge I get 3.493, 3.435, 3.554, 3.560 which is 125ma difference. (Readings taken after charge shut down). The pack shows 14.04v Somewhere around there the BMS shuts down charging due to overvoltage cell (blutooth monitor). The monitor shows the battery at 280A capacity.
After sitting a day I get pack at 13.3, cells at 3.328, 3.323, 3.328,3.327 and 0.004 difference.
Should I rebalance, or just let it alone?
 

sunshine_eggo

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Oct 26, 2021
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I built a 4s battery and top balanced the cells. After several problems I started to use it in my Motor home. The BMS is only 150a but the battery is 280Amp so I have to charge through a Renology 60 Amp DC2DC charger. The battery has been kept at 80% untill I started driving, I have the BMS set to 3.6v for the cells and 14.5v for the pack.when I get near full charge I get 3.493, 3.435, 3.554, 3.560 which is 125ma difference. (Readings taken after charge shut down).

There is a relatively small imbalance. It's good that all cells are above 3.4.

If time has passed since you did the top balance, it's likely the cells lost charge at different rates and are slightly out of balance.

The pack shows 14.04v Somewhere around there the BMS shuts down charging due to overvoltage cell (blutooth monitor). The monitor shows the battery at 280A capacity.

Recommend you set charge voltage to 14.0V and float to 13.8V. Set cell protection to 3.65. Set BMS to balance only during charging, 3.40V and higher, 20mV difference. Run it this way for a week or so. Increment charge to 14.1V and see if it charges successfully. Repeat the process every week until you get to the charge voltage you want.

After sitting a day I get pack at 13.3, cells at 3.328, 3.323, 3.328,3.327 and 0.004 difference.

Once cells settle into the working voltage range, they are almost always identical. It doesn't have any meaning beyond, this is how LFP is.

Should I rebalance, or just let it alone?

If the above process is unsuccessful, you can simply attach your 10A power supply to each cell individually and charge to 3.65V without breaking the battery down.
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
2,317
Sunshine_eggo is right about 14.0v. When you get up to about 3.55v per cell, there is normally a cell (or more) that takes off ahead of the others. If you hold 3.5v per cell for long enough, the cell is fully charged.
 
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