Bad cell help

[bamfords54]

Hey all, I have a battery bank of 8 deka L16 batteries in 24 volt . I believe one cell of one battery is bad as the SPG of that cell will not raise at all. I let the water get too low this summer,the top of the plates were just covered. The bank is about 4 years old. My question is can this cell be brought back or is it gone for good? Right now I am running on 4 batteries and need to do something for winter. The batteries have been  equalised at least 3 times a year at 30 volts. I noticed the water was low,added  a little water,equalised,added more water,let it go for a week charging on solar and topped off the water. Right after that the charge would not stay up,this is when I found the bad cell.Any help is welcome

[TomW]

Don't mistake me for an expert but...

I would pull that one out and try a proper equalizing charge on it  [depends on the specific battery] and "boil" it some while keeping an eye on the fluid level. It might just need to be stirred up a bit. Several hours to several days at higher than normal voltage should stir it up and maybe convert some of the sulphation back into the electrolyte.

Was it from the middle of the string?  My guess is it was.

Just what I would try with a flooded battery before I recycled it.

If it did not dry out enough to expose the plates it should not have damaged it being "low"

Again, I ain't no expert.

Tom

[bamfords54]

Tom, no,the battery was from the end of the string. I did equalise the whole pack for about six hours before it went bad, it almost seems like that made it go.It's weird, I never heard of anything like it before.

[Madscientist267]

If you noticed it immediately following an EQ, it may be that the cell was close to developing a short. Equalization has a tendency to produce a bit of heat, particularly if they are run for a long time, which can trigger a short if it is already close to forming anyway.

Does the battery still 'work' under a load when it is isolated (obviously at a lower voltage than rated)?

If so, you're probably looking at exactly that, a shorted cell. You might be able to 'cook' it out, as Tom suggested, but thats delicate, requires some insight (for lack of a better word) by watching meters closely and adjusting charge rates, insane amounts of patience, and can even be dangerous - If you manage to get it cooking and it is gassing, and the short decides to suddenly show back up... I really don't need to say what can happen next, do I?

If not, its an open cell, and theres absolutely nothing you can do about that.

Since your bank is not individual cells that you can manipulate by themselves by isolating them, there's little you can do with the damaged battery. Best thing at that point would be look into a new battery (and personally, I'd look into a new bank, not just a single battery... but thats just me.)

Avoids the headaches of mismatch that will crop up on you in the near future due to differing ages and subsequently, capacities.

That decision will ultimately come down to how expensive that replacement is. If you have a smaller capacity bank, just fork it up and be done with it. You won't regret it later.  If it's a sizeable (and therefore expensive) bank, weigh your pros and cons and do some tests to determine the health of the others before making your final decision. If one cell failed, others may be close to failure as well, but if they can be determined healthy, there may not be any point in replacing them yet.

Just remember, an RE system is only as good as it's battery bank! (as you can see...)

Just my $0.02  

Steve

[dnix71]

Three times a year equalization on flooded batteries ain't enough. Once a month maybe. Maybe even once a week if they see hard use. 30 volts might be too high depending on how warm the batteries get, or not high enough if the climate is cold enough. It's hard to equalize strings of 4 like that. It's also going to be awkward replacing one battery in a string of 4 like that with a new one. Maybe it's time to load test the whole string and pull the weakest other battery and buy 2 new ones. When the next one fails do the same.

Have you pulled the caps and checked the SP to verify the bad cell?  What kind of use do these batteries see and what temp range are they stored in?

[Madscientist267]

Good point - I didn't necessarily mean the entire bank.

Since you are running series-parallel, keep the good string of 4, and replace the one that failed and the other 3 that were in series with it.

Steve

[bamfords54]

thanks for all the replies. The batteries are in a temp controlled room, never drops below 50 F. I guess I may need to bite the bullet and buy a new bank, I really need more capacity anyway. I'm looking into some telecom batteries I could pick up for 800 dollars,so that may be my next step.I just checked the voltage tonight and found 5.4 volts in the battery that was low before.It was down to 4.0 two weeks ago,what the heck? maybe I'll try a slow charge for a day or two just to see what happens.BTW the charge is temp compensated.

[Madscientist267]

Yeah sounds like theres definitely at LEAST one shorted cell... maybe two, depending. The fewer the cells, the easier it is to determine, but alas, a dead cell is a dead cell, and does you no good.

Irony turns a page your way that just a few days ago, I initiated a thread based on this very problem. The only difference is I am toying with SLA (or VRLA actually) and not FLA. My main batteries are all FLA, and still in good health (keeping fingers crossed).

In case you're curious of my findings and wonder what outcomes bring, even though on a much smaller scale, check here:

http://fieldlines.com/board/index.php/topic,144491.0.html

Good luck man. Just be careful. On FLA, shorts can REALLY come back to bite you in the ass, particularly if yours happens to be facing their way when they decide to strike. 

Steve