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Cells top out at 3.31v

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dstra1

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Feb 11, 2024
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I have the opportunity to pick up a large amount of 100Ah cells cheap. Unfortunately they have been sitting in their original box for over a year and were around 2.56V. I have 4 of them and have spent the last 2 days charging them separately (at 3.65V) and as a 12V pack (at 14.6V). They are currently not taking additional charge and are sitting at 3.31V. They are perfectly balanced but obviously not a max charge.

So if I'm correct these batteries are only taking about a 75% charge. Are there any tips/tricks to trying to get them to take additional charge? Or is it likely that's the best they will get? I'm going to discharge them and recharge to see if maybe a little exercise will help but looking for any other advice.

If they only charge to 75% what do you think a good price is per cell? There is close to 100 cells so even if they don't charge all the way up if they are cheap enough they would make a decent sized 24V bank. Or should just walk away?

Thanks for any input
 
dstra1 said:
I have 4 of them and have spent the last 2 days charging them separately (at 3.65V) and as a 12V pack (at 14.6V). They are currently not taking additional charge and are sitting at 3.31V.
Click to expand...
Lets explore that.

What are you using to charge the cells?
What do you mean when you say "They are currently not taking additional charge"? Are they still taking current but the voltage is not increasing?

I ask because 3.31V sounds like they are in the middle of their charge curve. If the charge current is low, it could easily take days to charge, and in the middle of the curve, the voltage will change extremely slowly.
 
FilterGuy said:
Lets explore that.

What are you using to charge the cells?
What do you mean when you say "They are currently not taking additional charge"? Are they still taking current but the voltage is not increasing?

I ask because 3.31V sounds like they are in the middle of their charge curve. If the charge current is low, it could easily take days to charge, and in the middle of the curve, the voltage will change extremely slowly.
Click to expand...
I was using a Victron blue smart charger. I initially used the lifepo4 setting but it was going into absorption. So I switched it to power supply mode and set it at 14.6V. It was taking about 10A and declined to 4A for a few hours and then they stopped taking any charge. At that point I unhooked the charger and let them sit for about 5 min and measured the individual cells which were all at 3.31V.

I'm currently discharging them and will charge them up individually. I have a HP 6268B variable supply and will set it to 3.65V at 30A and see what happens.
 
not sure what voltage your expecting I believe they idle around 3.35, which means your very close.
 
I think they will be fine. If the capacity test isn't good I would use the HP power supply and charge them to 3.65V until current drops below an amp or two.
 
dstra1 said:
I was using 10A
Click to expand...
Was it 10A current or was the current limit on the supply set to 10A?
How many cells were you charging at once?
Were they in parallel?
Was there still current when you gave up?

Four 100Ah Cells in parallel with 10A of charge current will take at least 40 Hours to charge. Eight in parallel will take 80 hours. Furthermore, they will sit at 3.2-3.3 volts for most of that time. The voltage will only start going up at the very end, and then it goes up very fast.
 
FilterGuy said:
Was it 10A current or was the current limit on the supply set to 10A?
How many cells were you charging at once?
Were they in parallel?
Was there still current when you gave up?

Four 100Ah Cells in parallel with 10A of charge current will take at least 40 Hours to charge. Eight in parallel will take 80 hours. Furthermore, they will sit at 3.2-3.3 volts for most of that time. The voltage will only start going up at the very end, and then it goes up very fast.
Click to expand...

I charge them up for about a day in parallel. Then put them in series today and used the Vicrtron charger at 12V. At that point the cells were at 3.28 up from 2.56. In series they were pulling 10A from the charger. Then it dropped to 4V for a while and then it was not pulling anything. I had the charger in power supply mode at 14.6V as using the LiFePO4 charging profile it was just immediately going into absorption and not really pulling anything.

I'm discharging them now and will put them back on the other power supply in parallel at 3.65V @ 30A and leave them for it sounds like 2 days or so. Hopefully they will get beyond the 3.31. If not then I need to decide what to do. Right now the offer was $20/ea. I'm thinking if they don't fully charge maybe $10/ea? If I don't take then he was going to recycle them so whatever we come up with will be a win.
 
yea not sure your understanding how charging works, did you not see them hit 3.65? doesn't the victron have some kind of graph? charger only flips modes when it hits set voltage. what makes you think they didn't hit 3.65? nominal voltage is 3.2, you can put a 500A charger behind them, the battery is only going to take what it wants.
 
MakeMePower said:
yea not sure your understanding how charging works, did you not see them hit 3.65? doesn't the victron have some kind of graph? charger only flips modes when it hits set voltage. what makes you think they didn't hit 3.65? nominal voltage is 3.2, you can put a 500A charger behind them, the battery is only going to take what it wants.
Click to expand...

The charger went into absorption for 2 hours and it was showing the voltage per cell at 3.31. Never saw the voltage at 3.65 or anywhere near it. Thought that was really strange which is why I switched it to power supply mode and just fed it until it was no longer drawing any amperage. This leads me to believe the cells sat too long discharged and may have capacity issues. I'm going to sit tight tonight while it discharges and try again tomorrow.
 
When I woke up this morning I cranked up the supply to 30A. As of 3 PM they are at 3.345V and still pulling 11A from the supply. All the cells were within .001 of each other. My calculation puts them close to 90% charge.

Last nights discharge test was under whelming. Didn't have a shunt on it but with a 15A load they only lasted around 2 hours. Hopefully with this better charge the results will be better. I'm digging out my old shunt and will do a better job of monitoring the discharge. Looks like there may be some hope.

Thanks for all the recommendations so far.
 
dstra1 said:
When I woke up this morning I cranked up the supply to 30A. As of 3 PM they are at 3.345V and still pulling 11A from the supply. All the cells were within .001 of each other. My calculation puts them close to 90% charge.
Click to expand...
3.345V is in the flat range of the charge curve. Consequently, any calculation of SOC based on that voltage is questionable. The only thing you can conclusively say based on 3.345V is that the cells are not empty and the cells are not full.

I know many charts on the internet claim to show voltage vs. SOC for LiFePO4, but IMHO, they are all pretty much worthless.
 
Last edited:
Update,

I let the charger run all day and night again and the cells are showing 3.4 on the charger and 3.38 resting. When on the charger they were pulling 10A all day yesterday but this morning it's down to around 4A. Going to keep an eye on them and see if I can get them up to 3.4 resting. Then run a discharge test and see what kind of capacity they have.

Depending on the results of the discharge test what Ah rating should I consider buying them? Battery Hookup is selling used 72Ah cells at 90% for $20/ea. If these test at 90% or better I was thinking somewhere around $20-25/cell? If they put out 80% and I get them for $15 or so they would still be a decent deal. Any thoughts?
 
you need to do one cell at a time, go back to the victron so the graph can actually show you what's going on, i don't know how much your charging, but with lead acid, when I had a battery that would never stop charging, it was self shorting and heating itself, and could potentially run away as the short gets worse and you will see the current start climbing again.

the battery will rest where it wants, you can't dictate that
 
The batteries did end up making it to 3.42V each. Ran a full discharge and they came in at 95%. For a $1/Ah or less I may have ti gamble on a few dozen.

Thanks for all the info
 
Final update, I was able to get 76 working cells from the deal. It worked out to about $12/100Ah cell which seems to me to be a good deal. Now to build some batteries!
 
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