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Sorting sulfated AGM batteries

Maxbet

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Feb 15, 2020
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Hello all.

Several batteries in my AGM bank have given up. It's clear the whole pack will have to go and will be replaced by LifePo cells.

However, Since I'm a cheapskate it's coming from China on the slow boat, so this bank I have will have to be stretched until it arrives.

The bank consists of 8 80Ah AGM cells out of a backup lighting install, they were in a 8s configuration until I ripped 2 over heating units off and now I'm left with 6s 12v.

Those should have given about 2.9kWh of usable power, now they only take about 300Wh from fully empty. I can't imagine they all sulfated that badly in the relative short time I've been having trouble. So it's probable to me there should be some better and some less so cells in the pack. But how to find them?

I'm still using the pack since it's all I've got at the moment so I don't want to pull it all apart before I know what I'm doing. I thought about measuring the resistance but I don't know what kind of results I'd be looking for so I would appreciate any guidance you lovely folks could give me.
 
Batteries for backup/standby purposes usually don't have good cycle characteristics. They're designed to be floated for most of their lives with few cycles. If deep cycled, they will have very poor lifespan. They're often not much better than starter batteries when cycled.

Not sure where you're getting your 2.9kWh:

12V * 80Ah = 960Wh

300Wh may be good for ragged out backup batteries.

Screening process:
Charge from 50% SoC or lower - when they hit absorption voltage, check cell voltages and log them periodically. The HIGHEST voltage ones are suspect. You may find that one or more cells go OVER voltage as the others catch up. A handful of these charges can significantly degrade those going over volts.

Discharge from a full charge periodically logging cell voltages. The weaker ones will be at lower voltages.

The worst of the bunch will be those cells that are highest during a charge and lowest during discharge.

If you find all the cell voltages are in line with each other, you just have a bunch ragged out cells.

If you have the means of IR testing, you're looking for outliers, i.e., if they're not all equally bad, you'll likely see cells with notably higher resistance than the others.
 
Batteries for backup/standby purposes usually don't have good cycle characteristics. They're designed to be floated for most of their lives with few cycles. If deep cycled, they will have very poor lifespan. They're often not much better than starter batteries when cycled.

Not sure where you're getting your 2.9kWh:

12V * 80Ah = 960Wh

300Wh may be good for ragged out backup batteries.

Screening process:
Charge from 50% SoC or lower - when they hit absorption voltage, check cell voltages and log them periodically. The HIGHEST voltage ones are suspect. You may find that one or more cells go OVER voltage as the others catch up. A handful of these charges can significantly degrade those going over volts.

Discharge from a full charge periodically logging cell voltages. The weaker ones will be at lower voltages.

The worst of the bunch will be those cells that are highest during a charge and lowest during discharge.

If you find all the cell voltages are in line with each other, you just have a bunch ragged out cells.

If you have the means of IR testing, you're looking for outliers, i.e., if they're not all equally bad, you'll likely see cells with notably higher resistance than the others.
I took 480Wh per battery as usable capacity since that's 50%, I have 6 cells left in the bank so that makes 2880Wh with new cells.
So Tonight when they're just about discharged I'll disconnect them all and give them a while to settle. Probably gonna find some lower voltages then..

Edit: They're actually pretty decent for cycling. They're just eol and misused by me. I'm not at all disappointed in what they withstood in their time with me, especially since they were practically free.
 
Again, your numbers aren't making sense. A CELL is a 2V cell. You are saying they are 80Ah cells. That means each CELL has 2 * 80 = 160Wh of capacity. You have six cells, 6 * 160Wh = 960Wh.

When you say 6S 12V, that means six cells in series for 12V. If that's the case then you only have 960Wh.

If you MEANT 6 BATTERIES in PARALLEL, then that's a different story. If you have 6 12V 80Ah batteries in PARALLEL, then you have 6 * 12 * 80 = 5760Wh of total energy (rated). Half of that is indeed 2880Wh.

If you have 6 12V batteries in PARALLEL that only take 300Wh of charging from dead empty, than you have 6 JUNK batteries. No testing is needed. When you have batteries in parallel, you get to add their capacities together, strong and weak alike. Weak batteries simply take less of the load while the stronger ones take more. 6 parallel 12V batteries only capable of 300Wh of charge/discharged are destroyed.
 
Again, your numbers aren't making sense. A CELL is a 2V cell. You are saying they are 80Ah cells. That means each CELL has 2 * 80 = 160Wh of capacity. You have six cells, 6 * 160Wh = 960Wh.

When you say 6S 12V, that means six cells in series for 12V. If that's the case then you only have 960Wh.

If you MEANT 6 BATTERIES in PARALLEL, then that's a different story. If you have 6 12V 80Ah batteries in PARALLEL, then you have 6 * 12 * 80 = 5760Wh of total energy (rated). Half of that is indeed 2880Wh.

If you have 6 12V batteries in PARALLEL that only take 300Wh of charging from dead empty, than you have 6 JUNK batteries. No testing is needed. When you have batteries in parallel, you get to add their capacities together, strong and weak alike. Weak batteries simply take less of the load while the stronger ones take more. 6 parallel 12V batteries only capable of 300Wh of charge/discharged are destroyed.
Oh crap, I'm usually pretty good about not switching series vs parallel. I swear..
You're right, I have 6 batteries in parallel at the moment. Don't even know why I typed cells. Maybe because I was looking at lithium stuff before.

Anyhow, I'm going to try and see if there are any less bad batteries left in the pack. I know stuff can happen fast, but I still have some hope.
 
I can almost promise you... you have six bad batteries. The SUM of all the capacity of those six batteries is ~300Wh. Period. If there were stronger ones in there, they'd compensate for the weak ones.

The only way that's not true is if you have some batteries with absurdly high discharge, and they're pulling the others down. HOWEVER, since you're observing a 300Wh limit on CHARGE, it's unlikely this is the case. Best way to check is to charge them all up, break them down individually, and let them sit for 24 hours - compare voltages.

Since we're dealing with actual 12V batteries and not cells, you can apply a resistance load tester or a digital load tester to each and compare. I have these or highly similar units, and they are very effective at screening.
 
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