Lifepo4 Resting voltage ..

[DanF]

Charging a bank of Lifepo4 cells (24v, 8S) I started to see some runaway at around 3.5v/cell so I stopped charging. I am assuming that they are B rated cells. After disconnecting the charger (No BMS) all of the 8 cells dropped to 3.3 volts and have maintained there. These cells were originally bottom balanced by me when I got them. Could this be why they go back to 3.3v?
My BMS is coming Monday and I want to top balance these cells now. Am I wasting my time? I wanted to get them to around 3.6, connect them in parallel and use a bench power supply to get them top balanced to 3.65 as per Will's directions.
Maybe this is not possible? Any help is much appreciated!

Thanks,
Dan

 

[RCinFLA]

Cannot make any judgement without knowing AH of battery and charge current.

If your voltmeter is accurate, 3.300v would be about 50% state of charge.

There are two things that cause cell voltage to rise under charge current. First is impedance of cell times charge current. Older used cells have higher cell impedance. Second is kinetic voltage which is greater the greater the charge current. It is the overhead power to move ions, positive voltage during charging, negative voltage when discharging.

Voltage due to cell impedance is immediate, like a resistor. It can be measured by initial voltage drop when current applied (or loaded).
Voltage due to kinetic voltage has a longer time constant of several minutes to reach steady state.

Drawing is for discharge. Charging is similar just flipped to voltage rise instead of voltage drop.

Going from 3.5v to 3.3v is extreme but possible if charge rate was very high (like 3x C rate in amps).

I should mention surface charge. If battery is brought to full charge with current dropping to very low level and allowed to sit on charger for a period of time an electrolytic capacitance surface charge can develop between layers of cell. LFP has very low surface charge capacitance and dissipates quickly in about 0.001% C discharge. (about 500 mA discharge for 30 seconds on 280AH cell). It can raise appearent cell voltage by up to 0.15v.

 

[DanF]

180ah Calb cells 12 amp charger

 

[RCinFLA]

180ah Calb cells 12 amp charger

Assuming you measured at battery terminals I would say battery is bad with very high internal impedance.

If you measure 3.3v open circuit put a >10A load on it and see how much voltage drops An aged but still useable cell of 180 AH should not be greater then about 1.5 milliohm impedance. Starting at 3.300v, a 10A load should not drop more then 15 mV. A new 180 AH cell should be in 0.35 to 0.5 milliohm range.

All this assumes the 3.5v was at battery terminals and not at power supply terminals with small gauge wires voltage drop to battery. If battery terminals were at 3.5v wiith 12A charge current and dropped to 3.30v after taking off charger then cell impedance is in 10 milliohm range.

 

[DanF]

Assuming you measured at battery terminals I would say battery is bad with very high internal impedance.

If you measure 3.3v open circuit put a >10A load on it and see how much voltage drops An aged but still useable cell of 180 AH should not be greater then about 1.5 milliohm impedance. Starting at 3.300v, a 10A load should not drop more then 15 mV. A new 180 AH cell should be in 0.35 to 0.5 milliohm range.

All this assumes the 3.5v was at battery terminals and not at power supply terminals with small gauge wires voltage drop to battery. If battery terminals were at 3.5v wiith 12A charge current and dropped to 3.30v after taking off charger then cell impedance is in 10 milliohm range.

Thanks for your quick reply!,

When I had these cells bottom balanced I never had an issue with using them in a 24v bank, but even when fully charged they would always drop after removing the charger and they all would come to rest at 3.3v

I have a total of 16 of these batteries so I think I'll tinker with them and try to pick the best ones out of the bunch.

They have plenty of usable life, but I didn't get what I thought I paid for. Lesson learned!


Thanks again,
Dan

 

[DanF]

I did a true capacity test (Finally!) with 8 of my calb 180ah batteries in series (24v) configuration. This has been bugging me for some time now. These cells I purchased are mismatched "B" rated cells. I was really bummed to find this out the hard way, but I have some encouraging news for some that are in this situation!
I attached an overkill.com BMS to the cells and ran a drain of a very steady wattage out of my 3kw inverter and it lasted for approx. 6 hours. I then did the math and it came out to around 160ah by my calculations..
After a few beers and learning the app for the BMS.. lol.. I found out that the BMS already figured that part out! 162ah! So... my settings are at 3v/cell minimum shut off the pack. None of my cells would reach much over 3.4v before another would run away and I had to shut er down.
This works out to 90% of the rated amperage of these cells and this is in the range that ANY cells should be operating at for longevity.
With the BMS in place I will see if it balances a bit better but I'm happy with them already!

Just thought I would pass this along.

Also,
Thanks Will for all you do! You are a good soul brother!

I have been into solar and batteries for many years now. I took a course offered at our electrical school about 6 years ago and dropped out after running numbers that could never add up to solar being efficient back then because of a 20 year payback period. Now with way lower cost of solar, LiFpo4 and even wind generation.. It is VERY doable!

Absolutely love this BMS by the way!

 

[Ampster]

Your BMS will continue to balance them at the top of the charge cycle. You already parallel top balanced them and sounds like you solved the problem. The bottom balance may have contributed to the problem you had at 3.5 and above. There is not much energy beyond 3.4 volts so that is a safe setting and a good setting for B grade cells. That will have little effect on the voltage they settle at.