Batteries losing 20% capacity permanently with each discharge cycle

[Mikzi]

Diffusion does not break up stratification... at least not on any time scale that is useful.

In order to break up stratification the batteries must be held at or above the gassing voltage. Float voltage is usually not above the gassing voltage... 14.6 volts is above gassing voltage for most batteries, but I would call that an absorb voltage, not a float voltage.

--mapmaker

Very interesting. Thanks for clearing that out, Mapmaker.

[mapmaker]

When I put the batteries in storage the concentrated acid at the bottom mixed with the less concentrated acid at the top through the slow process of molecular diffusion.
<snip>
Furthermore, the acid is supposed to be mixed in the float step of the charging process, but that seems not to have occurred. A float voltage of 14.6V should have been enough to mix up the electrolyte and avoid stratification. Does anyone have an explanation for this?

Diffusion does not break up stratification... at least not on any time scale that is useful.

In order to break up stratification the batteries must be held at or above the gassing voltage. Float voltage is usually not above the gassing voltage... 14.6 volts is above gassing voltage for most batteries, but I would call that an absorb voltage, not a float voltage.

--mapmaker

[Mikzi]

I think I've found the explanation: severe acid stratification. That would explain the loss of capacity and the mysterious recovery. When I put the batteries in storage the concentrated acid at the bottom mixed with the less concentrated acid at the top through the slow process of molecular diffusion. So what happened was the diluted acid at the top limited plate activation and reduced capacity more and more with each cycle until I was left with only 10% capacity. The high voltage measured came from the concentrated acid at the bottom, fooling the chargers into thinking the batteries were full when they were not.

Acid stratification normally occurs from repeated cycles of partial charging and discharging. However, during my tests I always recharged the batteries fully before trying another cycle. Furthermore, the acid is supposed to be mixed in the float step of the charging process, but that seems not to have occurred. A float voltage of 14.6V should have been enough to mix up the electrolyte and avoid stratification. Does anyone have an explanation for this?

[Mike90250]

many automotive chargers, will not get deep cycle batteries to a full charge.

The only thing that can change while a battery sits, is the temperature, so that may mean your hydrometer is not being thermally compensated.

[Mikzi]

Have you placed a load on them yet?

Yes, I tried the batteries today. They are able to handle heavy loads again just like when they were new. They really seem to have recovered on their own.

This time around I will avoid discharging the batteries below 50% SOC and see if this helps avoid the loss of capacity.

[russ]

Have you placed a load on them yet?

[Mikzi]

Update

I have an update. After charging the four 12Vx100AH batteries with a car charger for a day without getting the gravity beyond 1.13 I concluded that the batteries were toast. So I put them in storage while I went on to test another battery type. A few weeks later I came back to the 100AH batteries and noticed that the green eye indicator had appeared on all four batteries. I checked the acid level, and to my amazement the batteries had recovered most of their lost capacity on their own. Whereas before I had been unable to charge the batteries beyond 1.13 gravity (10% SOC) the batteries had now recovered to 1.20 gravity, 70% SOC. I put a car charger on the batteries and after a night they were back to full capacity, 1.24 gravity.

Does anyone have an explanation for this?

[inetdog]

"It's 10:00, do you know where your batteries are?"

[PNjunction]

But if the actual capacity of the battery, after abuse, is only 40AH, then any current over 2A may destroy whatever capacity they had left.

Wow - I'm glad you pointed that out! That's one thing I never thought about. That same issue can also be directed towards neglected / abused ups-style agm's too. Normally they can take a 0.3C max inrush, but that is at rated capacity. Throw in some bad neglect that drops the capacity to half due to hard sulfation, apply .3C current, and you are now charging at twice the max rate!

That's a great point and I'm glad you brought that up. Moral - take care of your batteries!

[inetdog]

...The maintenance free aspect is also a nice bonus.

And you get the same bonus without all of the other drawbacks from an AGM type battery which is much better suited to RE use. GEL batteries have a niche, but it is generally in UPS or battery backup applications where cycling and charging will be limited.
For solar you often need to do as much charging as possible in a short number of sun hours (high latitude in winter for example) and so AGM are superior even to FLA for that niche. GEL, no way.

[Mikzi]

I did a discharge test when I first received the 4xFLA batteries which showed that they had their full capacity, 24Vx200AH. Based on that I limited the charge current to 20 amp. This is a first test before I buy a much larger quantity of batteries, and so I carefully monitored the amp and voltage of the batteries during the five days of charging and discharging when their capacity was reduced from 200AH down to only 80AH. The voltage or current never exceeded Yuasa's recommendations (14.4V, 20A) so I think the batteries must have been damaged from the start. At least this does not sound like an ordinary occurance.

It's true that gel batteries may be more vulnerable to mistreatment than other battery types. But if you have a charger able to handle gel batteries and limit their charging current I think the end result will be increased cycle life. The maintenance free aspect is also a nice bonus.

[inetdog]

The results are in. All four batteries are equally bad. I've tried recharging each one of them with a conventional charger, for up to 24 hours, but the gravity for none of them will rise beyond 1.15. Not even if I increase the voltage. At this stage I'm pulling the plug on their life support and declaring them dead. The mistreated five year old battery in my lawn mower is healthier than all of them put together. The MPPT charger isn't to blame either. It is able to charge and recharge car batteries without any issues.

I'm going to go with Inetdog's "Incredibly rapid sulfation" theory, seeing as how the batteries could in fact deliver 200AH, if one for the first cycle. Or perhaps the batteries were sulfated from the start but had gone through some procedure to revive them before being sold, with the shortcoming that their capacity would quickly deteriorate again, unbeknownst to most customers. The retailer is legit, but he may have gotten a bad pallet of batteries for some reason. I'm going to do a warranty claim and go with another brand and go for gel or AGM batteries instead.

Thanks for all the help.

The other possibility that I did not mention is that if some of the plate capacity was lost (i.e. sulfation) by the time you got them, then an attempt to charge the battery at the nominally OK C/20 rate was actually producing higher current density in the remaining good part of the plates and developing gas that blew the gel away from the plate surface.

If you have a GEL battery which was originally 100AH and therefore you try to charge it at 5A, that works fine for a new battery.
But if the actual capacity of the battery, after abuse, is only 40AH, then any current over 2A may destroy whatever capacity they had left.
Did you do a discharge test when you first got them? What was the AH compared to the nominal?
What charging current (maximum) did you use for your first recharge?

These and other quirks and vulnerabilities of GEL batteries are very good reasons not try to use them for RE applications.

[PNjunction]

Gel is an all around bad idea for solar generally. Typical C/20 max current charge (slow) in bulk, and on the back end low charge acceptance rate in float. Toss in some voltage abuse and pow.

Even if a retailer seems legit, make sure they are in fact "authorized", meaning that genuine warantee claims are backed by the factory, and not just the retailer, who may take his gripes to some 3rd party. I've seen Optima batteries sold as new, when in fact they were just cleaned up, and cranked on with some kludge of a recondition charger. Basically they got intercepted on the way to the recycler, and pawned off on ususpecting customers or distributors.

[Mikzi]

The results are in. All four batteries are equally bad. I've tried recharging each one of them with a conventional charger, for up to 24 hours, but the gravity for none of them will rise beyond 1.15. Not even if I increase the voltage. At this stage I'm pulling the plug on their life support and declaring them dead. The mistreated five year old battery in my lawn mower is healthier than all of them put together. The MPPT charger isn't to blame either. It is able to charge and recharge car batteries without any issues.

I'm going to go with Inetdog's "Incredibly rapid sulfation" theory, seeing as how the batteries could in fact deliver 200AH, if one for the first cycle. Or perhaps the batteries were sulfated from the start but had gone through some procedure to revive them before being sold, with the shortcoming that their capacity would quickly deteriorate again, unbeknownst to most customers. The retailer is legit, but he may have gotten a bad pallet of batteries for some reason. I'm going to do a warranty claim and go with another brand and go for gel or AGM batteries instead.

Thanks for all the help.

[inetdog]

I will follow Sunking's advice and try to charge a single battery for 24 hours with a conventional charger to see if that helps increase the gravity beyond 1.15.

If one battery charges properly and the other does not you will have your answer.
PS: The battery that does not increase in SG when charged separately is the bad one.