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Growing voltage difference while charging LifePo4 battery at SOC 99% (Daly BMS)

Jordi

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Oct 13, 2020
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I am charging a 12V LifePo4 battery with a 14,6V LifePo4 charger (2 Amps). The battery has 4 LifePo4 cells (60Ah) hooked to a Daly smart BMS and a solar charge regulator PWM (battery connections). In the same battery connections of the PWM I have connected the charger which therefore can supply energy directly into the battery. PWM is intended for battery discharge and voltage visualization without Daly BMS app.

The charging goes smooth and whenever the voltage difference of the cells reaches 0,05V, the auto-balance function kicks in and reduces it to 0,01V.
When the battery reaches charge of 99% it stays there for a couple hours and then the behavior of the system changes.

After 2 hours of constant charging at 99% SOC, charging begins to be intermittent; furthermore, while the SOC remains at 99%, the battery voltage difference keeps growing over time regardless of the balancing function being activated. Eg. Voltage difference overserved = 0,12V.

I think that the SOC remains at 99% because the battery voltage according to Daly app is 14,2V; it remains under the SOC 100% voltage of 14,6V; but why does the last 1% of charging goes so slow? (Battery stayed 3 hours at 99% in which 2 the charging was constant and at 19W)

Then, why does the voltage difference keeps increasing over time? Is the voltage of full cells more difficult to balance? Could this have something to do with the fact that the cells are from Litokala Aliexpress and probably have been used already?

After one hour of intermittent charging and the voltage difference getting as high as 0,12V I have stopped the charging afraid of "destabilizing" the battery. Two cells were around 1,6V and two around 1,5V; charging just kept increasing its voltage slowly.

Since in the theory cell voltage can go as high as 3,65V I though that maybe I could let the charging process continue. But then I was concerned about voltage battery being 14,2V in the Daly app and 14,7V in the PWM display. At which charging voltage can I start damaging the LifePo4 cells? I think however it is just the PWM measuring bad.

When I disconnected the charger, the voltage of the battery and cells went way down to 13,3V (3,35V per cell) and the voltage difference became as low as 0,005V which got me thinking that maybe I could have safely left the charging ongoing to reach the 100% while ignoring the voltage measured by the PWM. The PWM seems to measure always a higher battery voltage. The charger I have should stop charging at 14,6V as it is made for LifePo4 battery charging.

Some volunteer to give me green light on 100% SOC charging despite the observations?
 
After 3.45 volts per cell you aren't adding much power to the battery just adding voltage. You can let it sit and stabilize over time or charge less or add on an active 5 amp balancer.

I have a 500 ah pack that I only charge to 3.45 volts per cell and still get 100% charge on.
 
After careful consideration I let the charge continue. The voltage difference grew up to 0,3V while the SOC did not go higher than 99%. After disconnection it took hours for the batteries to have a similar voltage again; I could see that the batteries charged to 3,7V took longer to reach the stable nominal level of 3,3V. There is one cell though, the one that seems a bit blotted, that can not have a voltage higher than 3,52V and that reached the nominal voltage faster; this might be the cause of the voltage difference and the reason why the BMS can not reach the 14,6V.

And of course, all this is probably happening because they are second hand cells even If Alixpress says grade A and bla bla bla...
 
Charging a 4S battery to 14.6 volts is a shorthand for saying, -> charge each cell to 3.65v, but there are many ways to sum 4 voltages to 14.6, you discovered one of them. what really matters is the voltage on each cell, not really the sum of the voltages.
you have one cell that is lagging in voltage hence to sum to 14.6 the others must be >3.65. This cell lagging cell is still at a lower state of charge than the others ( but at 3.52v is “full”). You could try to selectively charge it, or discharge the others to balance everything. Or just set the pack voltage to 14.0v and use the pack. The tiny balancing currents of the BMS should eventually even things out a bit with a 60ah pack.
 
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The closer you charge to 3.65 the more likely you are going to expand a cell.....Best to limit to 3.5 or 3.45 cell voltage and you will still be fully charged.
 
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