Batteries explode ?

[5555tj]

i have a bank of 20 * 12v batteries as we arent on grid and two look like they have eaten all the pies from tescos - ie they are totally out of shape and bloated. they were fine a week or so ago.
this is obviously not good so
1 whats the safest way to remove them from the bank. - apart from reduced ah is there anything else to worry about?
2 would the generator have over charged them ? they are old and we are abusing them at the mo as generator auto start/stop isnt working

many thanks

Tim

[martin]

find the vent holes and clear them pdq! - They should be fine (and return to their proper shape) if you do it swiftly!

(but be very careful when you do it - rubber gloves and goggles at the very least job!)

[DominicJ]

Are they the first or last (or first and last) batteries in a string?

[rogeriko]

Are they gel batteries? if so you have overcharged them. I have seen this before, when 1 battery goes short circuit in a string then the controller keeps on charging the others because it sees a low voltage and they all swell up then go to the scrapyard!!

[EccentricAnomaly]

What sort of batteries? What's the configuration (e.g., 10 strings of 2 for 24 volts or whatever)? How are they wired? What's in common for the bulgers - e.g., same string? How's charging controlled?

[5555tj]

Martin,
thanks will do it first thing tomorrow - they are in my barn with no lights so probably best not to try to do it in the dark.

DominicJ,
first pair

rogeriko,
not sure what they are - will check model etc (we inherited system when we moved in)

EccentricAnomaly,
yes 10 string of 2 ,
we have a trace c40, outback, 10kwa generator and a mere 780watts of solar

thank you all very much

tim

[SimonHobson]

Do they by any chance look like these ?


In the first, look at the cell furthest away on the battery nearest the camera - and the cells on the battery upper right.
In the second image it should be obvious - but do look down the whole stack !

These are AGM batteries (aka "dry", VRLA). This is (or rather was) the battery bank for a UPS running at 8kW, two strings of 10 for a nominal voltage of 120V. It was fairly noticeable one morning from the "sulphurous smell" pervading the office when I opened up. The effect you see is (I believe) fairly typical of batteries that have been overcharged/used in a warm environment. They need very little abuse to lose water from them, and being AGM have very very little to start with. Once a cell loses a bit of water, it's internal resistance goes up and so it has a higher cell voltage than the rest of the string (while charging) and has a lower cell voltage than the rest under discharge. This means it will get hotter than the rest, and so develops a feedback cycle with the bad cell getting even more abuse.
If they are not too bad, then you could try adding a few drops of de-ionised water. Don't go overboard, I tried it with the better string of these) and I think I used a tad too much The result is that they promptly tipped some of it overboard.

You are lucky that you can drop a pair off the system fairly easily, though leaving reduced capacity. For us, it needs a complete new set (1 string wouldn't be up to the job), and that's well over a grand in cost

[Outtasight]

Yep, some kind of overcharging is most likely.  But some batts can recover from the bulge.  My gel ones actually have soft cases and bulge quite a bit with every full charge.  When they've been on float for a while, the bulge goes down (through gas recombination inside) and when heavily discharged they can even look a little bit "sucked in".  The valve on the top of all AGM/gel batts should open at a couple of PSI so would prevent a blow-out / split in the case.  The valve is one-way.  Gas can only come out.  Opening the valve to add a bit of water will harm gel and AGMs further as the valve is designed to only open for excess pressure and opening it at any other time allows air in that causes oxygen poisoning of the cell.

With 24V or 48V strings of AGM/Gel cells, you should look into active bank balancing.  I had some trouble with out of balance 12V blocks on my 24V bank (one going high Volts before the other and then gassing a bit - venting that acrid smell).  I fitted 12V balancers that shunt up to 1A from top or bottom half of the string to keep the blocks within 0.05V of each other while above 12.8V (so only active when on charge).  The blocks all behave themselves again and no gassing.  The dodgy block that lost some gas (water) seems to be holding up 6 months (180 cycles) later.

Despite the fact that my solar array can pump out up to 75A charge current, the 1A balancers seem to do their stuff.  Goes to show, only small imbalances in charge current and Voltage can have a big effect with sealed batts.

I could have bought 6V balancers from the same maker (half my bank is made of 12V blocks and the other half from 6V blocks) but balancing at 12V granularity seems enough. You can use the balancers on longer strings of cells because they are floating (3 wire connection to the (+), centre and (-) of a pair of blocks).  You can just daisy chain them together on 48V banks.

[SimonHobson]

With 24V or 48V strings of AGM/Gel cells, you should look into active bank balancing.  I had some trouble with out of balance 12V blocks on my 24V bank (one going high Volts before the other and then gassing a bit - venting that acrid smell).  I fitted 12V balancers that shunt up to 1A from top or bottom half of the string to keep the blocks within 0.05V of each other while above 12.8V (so only active when on charge).  The blocks all behave themselves again and no gassing.  The dodgy block that lost some gas (water) seems to be holding up 6 months (180 cycles) later.

Ideally balancing needs to be done on a cell by cell basis, and this is where I dislike AGM batteries. With wet cells, you can overcharge them much more freely - fully charged ones will gas, undercharged ones will catch up. With wet cells you've got the reserve of water so you can afford to lose a bit - but with AGM you can only gas at a very limited rate or they just dry out.

I realise the advantages of AGM - particularly the significantly reduced risks from not having any spillable liquid acid - but they definitely aren't the magic answer to all ills some people think they are. I deal with a fair number of (customers') UPSs at work, and we replace quite a lot of batteries that I reckon are just dried out. By the nature of what they are used for, they are often used at higher temperatures than is good for them - I've just changed one (small, 24V 7AHr) pack that's been in for just 18 months since we last changed it. But it's stuck in a small gap with the server, and most of the time is probably into the high 20's or 30's temperature wise.

Similarly, I've read multiple comments over the years that "maintenance free" car batteries really aren't, and in hot climates you do have to break open the seals and treat them as ordinary batteries if you want them to last.