1 of 5 strings overheating BADLY!

[CallMeChaz]

I have a 4 year old, 24V bank of 20 Roll S-530's, charged by 2 stacked Xantrax and one Outback PV charge controller. There are five strings of 4 batts. Fo the last two year I have been charging them as Rolls recommends: charging at 15% of c6 rate which is 296 AH per battery, using their voltage settings. I usually charge the bank at 225ADC split between the two Xantrax, or about 112ADC each. The setting computation in AAC according to Xantrax is 25% for my 24V system, or 27AAC on the panel.

Problem: The generator runs too much. And for some time, batteries seem to be quite hot when charging, but I only have a thermostat in the middle string (and THAT seems to be the culprit!). I can barely get the bank up to 1.200 SG. The charging system was not originally set up for the aggressive charging that Rolls recommends. I suspect they were badly sulfated from undercharging for the first two years, and probably not watered well. I used the recommendation that Rolls has for corrective charging for heavy sulfation: equalize at 5% of c6 (1/3 normal rate) for as long as it takes to reach the charging system's max Volts of 32, then 4 more hours.

It looked good for a while. After 8 hours, the slow charge rate was keeping the batteries at 94F. I thought I could continue the process without the overheating I experience during normal equalization, and clean up the bank. Unfortunately, over the next 8 hrs, the middle string (ONLY the middle string) overheated, badly!

After 8 hours, V=27.2, T=94F, SG=1.146 (avg of 4 test cells)
After 8 more, V=25.1, T=150, SG=1.178.
SG went up in all but the overheated middle string, and overall volts was down.

What's going on?? The water in the other strings has some occasional bubbles, like you might see in a glass of resting soda water and was quite clear, and the strings were a balmy 95F. The middle string was gassing profusely: the color was muddy like when I do a normal equalization.

Why just the middle string? Any explainations?
I can't recondition the bank with the overheating. I'm thinking of removing the middle 4, and trying to recondition the rest.
Am I ever going to get this bank back into shape, or is it junk? Any suggestions?

BTW, I am using an expensive electronic SG gauge, so the reading are accurate.

[Sunking]

Couple of comments. To my knowledge C6 is too high for Rolls, C/8 is the max and C/10 is the sweat spot.

OK you need a good hydrometer t0 find your bad cells. Sounds like you have several of them and time to replace batteries. When you do, do not buy such small batteries so that you have to use several parallel strings. Buy the Amp Hour capacity so you only have one string to avoid the problems you are having, and extend battery life significantly.

[CallMeChaz]

Thanks, coming to same conclusion

I'm coming to the same conclusion. I think there are bad cells in the other four strings, and the majority of the charge is going into the middle string. I have an expensive refraction hygrometer. The chemistry is so screwed up from the bad charging that the readings are very hard to interpret. However, measuing across individual cells I found cells at 2.1 volts with one around say 1.7. I've been pretty sick for the past week, and have not had a chance to check all of them. I will post my finding when I have time.

I have access to 2, 48 volt Xantrax inverter/chargers. I think I'm going to trash the four worst batteries, and build two strings of 8. Then I will have to see if I can perform corrective equalization with overheating them. Even if I have to replace them all, I would rather have the 48 volt units.

Thanks for your insight.

[Sunking]

I am no tsure but I do not think a refraction hydrometer is the right tool for batteries. I could be wrong but they are used for aquariums and wine making for the lighter water liquids. You need one calibrated to read heavy water of between 1.1 and 1.3 with temperature compensation. With that said I would recommend a good lab quality hydrometer with built-in thermometer. You can pick them up for $20 or less. Heck even a decent Deka Battery one for $6 is decent. I use the the Deka Professional Model for all the batteries job I do

Not sure about your confusion, but the SPG will tell you exactly what is going on. Do not rely on voltages alone, lean on the hydrometer. Sounds like you have both shorted and open cells. There is no way to fix that. Most of time fails fail open, and no amount of voltage applied will force current into them. As soon as you hook up a charger it will shut off and indicate full charge, when in fact it has no charge at all.

What I bet you find is some cells with very low SPG and the others with very high SPG. Weed out the low ones and try to bring the high ones down by discharging. The low ones are likely sulphated and at the point of no return, but you can try by grouping them together and apply a charge. IF they take little current and trips the charger off saying fully charged, they are done, get rid of them.

Shorted cells are most common in sealed batteries like AGM and Gel and is the cause of thermal runaway. As th ebattery heats up, causes the impedance to drop which results in more current and then more heat, then more current, then more heat....... Runaway.

[CallMeChaz]

Appreciate the response. Pretty much what's happening is shorted and sulphated cell. BTW, my refraction instrument is designed specifically for battery acid. It is extremely accurate, far more than a flotation type. But it comes at a cost--$350.