Balancing a 48V 20AH Yesa LiFePo4 battery pack

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decibel1
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Balancing a 48V 20AH Yesa LiFePo4 battery pack

I have a one-year-old 48V 20AH Yesa LiFePo4 battery pack which has gone significantly out of balance. After a typical charge, the three batteries at the positive end of the string are over 4V, the middle batteries are at about 3.3V and the three batteries at the negatave end are all about 3.4V. I have tried using an individual battery charger to charge the middle batteries to about 3.7V but after a short time they come down again to 3.3V.

Anyone out there have any ideas on how I might re-balance this set? All the batteries seem to hold a charge ok.

antiscab
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Re: Balancing a 48V 20AH Yesa LiFePo4 battery pack

Gday decibel,

its normal for a LiFePO4 cell to go back down to 3.3v some time after a charge (because anything high is just capacitive effect within the cell).
as long as you charge them above 3.7v, and the current has fallen below c/100 (or 0.2A in your case) it should be fully charged

just a word of warning, if you only have one single cell charger, wait until all cells are back to 3.3v after charging before use, otherwise you will be stuck with balance issues again.

do you still use the BMS that comes with your YESA pack?
I would suggest just using that to balance out the cells. It will take a long time though (days), the YESA BMS isn't meant to shunt alot of current.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

decibel1
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Re: Balancing a 48V 20AH Yesa LiFePo4 battery pack

Matt:

Thanks, this is helpful. My problem is that a few of the cells go above 4V during the charge cycle, and that shuts off the charger before the other cells get to 3.7V. I am using the YESA BMS, but after a couple of days of letting the pack sit at float the >4V batteries have dropped to only about 3.9V. Do you think I just need to wait longer?

antiscab
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Re: Balancing a 48V 20AH Yesa LiFePo4 battery pack

after the couple of days, have the other cells increased in voltage at all?

the Yesa BMS doesnt balance particularly quickly, but it should get it done (eventually).

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

decibel1
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Re: Balancing a 48V 20AH Yesa LiFePo4 battery pack

Matt:

Last night I took the pack off the charger entirely since I figured that holding the pack at 55V might make impair the balancing by the BMS. The high voltage cells have gone down a bit over the last 24 hrs, though I haven't seen much increase in the 3.3V cells. I'll try watching it for the next few days. Thanks.

antiscab
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Re: Balancing a 48V 20AH Yesa LiFePo4 battery pack

The BMS can only balance when the charger is on.

unless the design of the BMS has changed, on each cell a load is added when the voltage of that cell exceeds a certain level (usually 3.7v or there abouts, though can be as high as 4.0v).
The load on the individual cell also increases with voltage.
The BMS usually also signals the charger to slow down charging, so the heat dissipated isnt as great.

so the balancing action may be as little 0.05A.
The balancing action can only occur when the cells of higher SOC are above the cut in threshold, and the lower SOC cells are below.

what you will notice is eventually the lower voltage cells will increase, while the higher voltage cells will stay the same, while on the charger.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

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