Wet SLA battery filled with distilled water to the top.... unfortunately :(

Thread Starter


Joined Jul 9, 2013
Hello everyone,

This is my first post here so I`m sorry for any mistakes. I have searched the forum about this specific subject and found no information which is relevant for me.

The basic question is how to get rid of the extra water added to a wet-type SLA... (I turned it in flooded LA).. Below follow lots of details, should anyone care to read them (sorry, it`s a long post).

I have an APC (CS650) with a 2+ years-old APC battery. A couple of months ago, during a power failure, I observed that its capacity decreased to ~66% (held like 5+ minutes instead of 9, same PC, same OS, same screen).

I then opened it and (not knowing that SLA is only a "wetted" lead-acid battery) I filled it almost to the top with distilled water..... Electrolyte concentration definitely decreased....

I only found out like 2-3 weeks ago what is the difference between wet SLA and flooded LA (GEL is simply irrelevant for the context).

I have never had a power failure ever since, only some ugly mains voltage fluctuations (you know, winter is coming and humans in my neighborhood are using electric heaters with fans... what can I say... but the common power lines of the apartment building I live in are from aluminum...). Anyway, little off-topic, sorry about this.

I started to hear the APC UPS crying (that cicada sound, UPS asking for its battery to be replaced). I turned it off and... surprise: the battery was VERY VERY hot (not swollen/deformed/round... checked it with a carpenter`s elbow, all straight edges and 90-degrees corners)... I waited for it to cool down and removed it. Surprise: it vented little acid through the vent near + terminal (slight smell of rotten eggs, + terminal plastic cover eaten about 50%, contacts are full copper now - no longer silvery plated).

My question is if there is anything that can be done for this SLA to restore it to its former glory. I tried to build a 555 timer 1khz desulphator but its inductors nearly produced light (they were almost incandescent-hot, skin-burning), so little success with that... Need to rebuild it. Anyway, not sure the battery is sulphated, but I have no clue why it heats (should be too much charge current, but why?)... Will check the charge current later on today.

Battery voltage is somewhere between 11 and 12 volts while rested (not too good...).

I started to charge it with a smart charger last week, 700mA setting (motorcycle battery), all vents open, and 2 cells bubbled for a long time, but he battery got only surface charge and the charger stopped charging in less than 2 hours, without the battery to get hot... Then the battery voltage dropped again. I will measure the individual cell voltage with two solder pieces and see if there is any imbalance (I suppose there is). Leaving the charger connected to the battery overnight I saw that in the morning it was charging the battery again (same 700mA setting, stopped 7~8 minutes later).

Any help is much appreciated!


Joined Sep 9, 2010
I think your battery is toast. It was probably not adding water that did it, it was running it to the point where it was low on water. I hope someone else may have a different opinion for you.


Joined Oct 29, 2013
SLA stands for sealed lead acid, you're not supposed to open them or add anything to them. My guess is it's ruined, time for a new battery.


Joined Mar 30, 2015
If you want to be able to depend on your UPS; just buy a new battery and recycle the old one.

Two years may seem short, but it really depends on how many times the battery was discharged deeply. I typically lose power long enough for the batteries in all of mine to run down several times per year . The batteries in my 1000VA unit lasted for over a decade; but the batteries in my 500VA units seem to only last for a couple years.


Joined Oct 2, 2009
It was junk the instant you unsealed it. Melting it down sealed its fate...

After you get a new one, check the final voltage right at the battery terminals about 24 hours with the UPS running after you install the new battery. It has been my experience that UPSes float SLAs at too high a float voltage....


Joined Nov 30, 2010
too high a float voltage....
I have seen this. The cure I used was an LM317L regulator. 100 ma charge limit, float voltage programmed by myself. It's good enough for a little 5 or 10 amp-hour SLA. For the larger batteries, there are regulators in the 1 amp to 5 amp range.

I live where power drop-outs are usually in the range of under 3 seconds. Once every few years, it might take an hour or three to get the service truck to reset a breaker. The only thing that matters is that I have 2 or 3 minutes to shut down the computer in an orderly manner.

Thread Starter


Joined Jul 9, 2013
Well, the battery has now (yesterday actually) 1 swollen cell. Wifey connected the ups yesterday morning, I didn`t tell her what was wrong (why I bypassed it). She told me that it cricked all day after she connected it, but I guess it is too late now. Battery is dead meat. Ordered a new replacement battery this morning.

I am anyway curious how can I regulate the charging (since you all say that UPSes float their batteries too high).. I don`t like UPSes which eat VRLAs (SLAs)... I have at most a total of 5~10 power failures of more than 10 minutes per year (I live in big city, you know...), but there are periods when for 2 or 3 weeks in a row the power fails in the middle of the night (at 00:00) for about 10~30 seconds, maybe some maintenance work or who knows what.... That`s why I got the UPSes. And for protecting my sensitive/expensive equipment from over/under-voltage situations.

Another SLA which I flooded doesn`t protest too much. It is some no-name-brand (not like super-brand APC!!!) and it is older than 3 years. Hosted in another UPS of the exact same model (APC CS650). Never had a problem with it, but I flooded it as well... backs up my NAS, security system and router and still works (after 2+ months in "flooded" state).

Question remains open: Is there anything that can be done to save a wet-type SLA if it was accidentally (due to stupidness/lack of knowledge) converted to flooded-type? (I really thought it dried out, that is why I added water... I expected to see some water level...) charging it to bubble all extra water out isn`t really what I thought to..

Thanks for everything!
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