Refilling SLA's (Sealed Lead Acid battery), Like Refilling a Car Battery

Picture of Refilling SLA's (Sealed Lead Acid battery), Like Refilling a Car Battery
Have any of your SLA's dried up?
Are they low on water?

Well if you answered yes to either of those questions, This Instructable is for you


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Step 1: Materials/Tools

Picture of Materials/Tools
  • Safety Glasses (So you don't get mild battery acid in your eyes (like I did))
  • Funnel or something to put water into the cells
  • Very small flat-head screwdriver
  • Needle-nosed pliers
  • Battery Charger (optional)

  • De-ionized water (you can use tap water but it's not reccommended)
rimar2000 says:
You can also use rain water without problem. But it must be very clean. You can collect it whit a clean plastic sheet as funnel, and a clean plastic barrel as container. IT IS FREE!!!
  • Dry or nearly empty SLA

Step 2: Pry off the cover

Picture of Pry off the cover
Use the small flat head screwdriver to pry off the cover, There are usually some slots that it will fit in.

This will not work on the SLA's with a single cover for each of the valves unless they have pry holes.

Step 3: Take the cell vents off

Picture of Take the cell vents off
For this step use your safety glasses

Using your needle nosed pliers, Pull off the rubber vent covers.

Be careful because this could spit battery acid at you (NOT GOOD!)

Step 4: Add water to the cells

Picture of Add water to the cells
For this step use your safety glasses

Add De-ionized water/water to the cells by using your funnel or similar.

Warning, You have to estimate how much water to add, Adding too much will cause it to leak/spit when being charged.

Step 5: Put the rubber caps back on and charge

Picture of Put the rubber caps back on and charge
For this step use your safety glasses

You should probably charge it before putting the caps on so the water can boil off......

Put all the caps back on the cells.

Connect to a battery charger and put a towel or rag over the SLA in case it was filled to much and needs to vent, If it vents it will probably pop off the rubber caps, Just put them back on.

Step 6: Put the cover back on

Once you have checked the battery is good, Put the cover back on.

Use any type of mild to strong adhesive to stick the cover to the battery, remember not to fully seal the top so that the gases can escape.

Now you're done!

Report how good/bad your refilled battery has performed.

Remember to comment I like comments
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Hallo I Am going to do this work in my computer UPS battery its damaged

OutspokenJew3 months ago

Please don't do this - sealed lead acid batteries are not made to be refilled for a reason.... While you may be able to coax a bit more life out of it, it can also be very dangerous. As lead acid batteries age, the plates corrode, deform, and break apart. This, along with crystal growth between the plates can cause shorts, overheating, overcharging, and potentially an explosion.

It's good to know how to do, but unless you're stranded on an island, it's just not worth the risk.

PuneetS7 months ago


When i added distilled water to the SLA, it started heating!. Not sure what went wrong or is this expected behavior? I was scared to attach it to charger :(

toyota4k9 months ago

I did this to my SLA battery adding distilled water and it's working well now, I was just wondering if I need to bore holes on the caps since they are solid-screwd to the cells allowing no vent for gases to escape

czarmatra5 years ago
Is it not acid like in the 12v car battery? with a 1275 SG. Or just plain old clean water?
thermoelectric (author)  czarmatra5 years ago
Well, You can add acid, That's probably better, But you can use distilled water, i'm pretty sure...
Hi, can I add the swimming pool acid instead of water? thx
thermoelectric (author)  BruceLyn5 years ago
Hydrochloric acid? Yes, But not too much.

seriously? NO!

Don't do it. You can't just switch/mix acids. The chemistry of a battery is based on the electrolyte and the plate material. You aren't just supplying protons. The chloride ions don't play nicely with sulphate ions.

I have both Hydrochloric acid and Sulfuric acid and some high accuracy ph test strips would you recommend trying to be a chemist here and actually adding acid? Like would it really be a big benefit with all of the risk involved?
i have tried a combo, you all know the RISK of combining acids. I had learn t along time ago that some work and some are rather explosive. I also can maintain a 14.8 volts in a 12 v lead acid battery, downsize the alt, or eliminate it all together. I play with Tesla theories for the recharge of the battery, and gain HP. When doing this you must have everything across the board. your # are very spot on. But can be ramped up.
Have gone to 24. with very min loss and better performance for cars all around, they also act as my solar gen and inverter backups.
This is a very good Instruct, but outside the box, there is alot more you can do.
I wish to thank you for this.
Thank you.
i am starting to recycle. so far mostly car batteries, now i have a bunch of these, all dry, wasnt sure if they were a gell sealed, or just ad acid. There was nothing to take out of the thing, so i assumed acid, but i hate to assume. And thanks for the instructable
thermoelectric (author)  czarmatra5 years ago
baglady20111 months ago

Will this worl for deep cycle wheelchair and scooer batteries? Mine are black, ow do I know if they need fluid?

holy crud flat man 220 comment an growing.
aduy1 year ago
ok so im helping a friend repair his electric skateboard, its got four of these 12v 8ah batteries SLA. now the voltage is supposed to be 24 but wheni measured it, it was around 8 or 10. the charger wont kick in to charge it up so i put a dummy load on the line and have gotten it up to about 20v, but with a small load on the batteries it still drops down to 10 volts. i have now tried adding water to all of the cells, and im currently charging it up to see how it does. are the batteries just dead or can they be salvaged? btw the batteries are 2 in series and then two of those in parallel to make 24v 16ah.
hansonsux2 years ago
I have my doubts this will work, because unlike other Lead batteries, "Sla" batteries like the ones in Ups's or scooters etc. are AGM type. The GM is "glass mat". The mat is in between the plates and holds the electrolyte like a sponge. There is the possibility the battery is dry, but more likely the batteries are sulfated anyways. Even if you did icrease the capacity of the battery, sulfation damage will likely limit your sucess. In AGM sla's, larger sulfate crystals grow and press against the glass mat, and when they get really big, warp/destroy the pates. Minor sulfation can be reversed in AGM's, but any larger crystals that haven't destroyed the plates yet will not be able to fall away and remain stuck in the glass mat. Which will mean a permanent rise in internal resistance.

"Refilling" is a bit of a misnomer, since AGM batteries are a "starved electrolyte" system whereas there is no loose electrolyte beyond what is soaked up into the glass mat. If you take the case off of one of these batteries, there will be no electrolyte leakage as it is entirely contained within the glass mat.
andersonhdj2 years ago
Don'y know if anyone has posted this, old lead acid batteries can be desulphated by charging with a Glauber's salt solution.
Thereafter a thorough rinsing with distilled water to wash all the loose sediment out , allowed to drain and refill with electrolyte.
Used to recover motorcycle batteries this way.
PCelec2 years ago
may give a try got 7 old batteries form 3 old UPS units. 3 are relitivly small aprox 7/5AH 2 of them sound like they fell apart inside and make rataling noises. the other one i may try this, the other 4 are heavy duty say 17AH/20hr think those are defintly worth a try fixing. oly one ups would still run off batery power. rest were bad so probly use the bateries for solar projects.
If the battery is "dry" try filling it with a solution of distilled water and Alum, but first rinse the battery acid out with a solution of distilled water and backing soda until the acid is nutralized. Much safer than acid, and now you have a rechargeable alkaline battery. Try it, and you will be pleased! Don't forget to understand the word "solution" when mixing the water and Alum. Alum is sold in the spice section of most food stores.
thanks for nice upload
can i charge 6 v lead acid battery with 12 volt charger
thermoelectric (author)  chnaveed.alam2 years ago
It's not advisable unless you watch the voltage closely, it wouldn't take much to boil the electrolyte right back out.
ron123 years ago
How can I tell if my sealed lead acid scooter battery needs fluid. How can I tell how much to add. I don't see any fluid in the cells. I do not want to over fill. Thanks Ron
i heard somewhere that epsom salts can be good for desulfating batterys
So if I have a couple of old SLA's that are completely dead, Will doing this revive them?
thermoelectric (author)  Electronics Man3 years ago
I can't say it will revive them, but if they're dead, there's no harm in trying. Chances are they're probably sulphated though. Try a desulphator after refilling them?
Thanks for the help!
I successfully revived 5 batterys today and three of them have been sitting dead for at least a year. The other two just needed topped off. I'm so glad I found this instructable, anyone can do this!!!
ajazmd133 years ago
cenzo1883 years ago
I just did this with an old alarm system battery that I use whenever I happen to need 12 volts out in the field. It was totally and completely dry. I filled it just as stated here and charged it with my power supply set to 14 volts current-limited to 1.5 amps, the voltage and current that was specified on the battery label.
So far, it's held its charge for 24 hours and can supply adequate current.

dsandds20033 years ago
You can use distilled water. You can buy it, get it out of your dehumidifier or from your air conditioner. You can also boil water and let it cool. Some auto stores also still carry battery acid.
Do NOT fill above the plates. You can use a needle and syringe from an ink filling kit.
When charging you should use a charger that shuts off when it reaches 13.8 to 14.4.
If you fill above the plates the battery will get HOT and start to deform the battery.then it is junk.
Just some tips i have learned over the last year.
Keep up the great work.
Have just refilled one of my 'reused' electric scooter batteries so hopefully it will charge. Fingers crossed...
Thank You Very Much
ive overfilled mine its just leaking now :( what do i do should i keep chargeing it?
how do you know if a battery is low on water?
nverma3 years ago
great ......I really thank you for grant me the info at free cost.............thanks see you soon after d work is done
Fragmaster3 years ago
I just reconditioned a pair of 12V 2.3Ah batteries with great results! These 2.3Ah batteries are holding their charge much better than the 12V 7Ah battery that I reported below. The 2.3Ah are hovering around 12.58V while the 7Ah has fallen to 9.05V on its own. I think I'll try a desulfator circuit to improve the 7Ah battery's performance and post my findings, that'll take a while though.

Still an excellent tutorial! Last year, my aunt got a used electric mini motocross bike, the batteries wouldn't charge, and I made the $60 mistake of suggesting that she buy new ones. Thanks to this tutorial I'll make sure to never shell out money for new Pb s until I give refilling a try.
Fragmaster3 years ago
I used distilled water, a 12V regulated power supply, and a Thunder AC6 charger to monitor charging time, voltage, current, and power.

Breakthrough! My battery works great now! I was using a car battery charger with dual rates, but I think that charger couldn't handle the resistance of the newly added water (I think I over-filled it). Recently, I bought a Thunder AC6 multi-charger [It's absolutely great! I highly recommend anyone dealing with batteries, especially RC enthusiasts to buy one.] and it's safety features wouldn't charge the newly filled SLA battery, it was giving me a "low voltage" warning. So I hooked my battery to a 12V regulated power supply for about 30 minutes so the battery's voltage could reach about 11v. Then I immediately set my AC6 charger to 1.5A of charge current and connected it to the battery. It worked! The AC6 took 368 minutes to pump 9000mAh into that battery [overnight, so I couldn't check it all the time], evaporated the excess water, and magically saved me $35! The battery is now holding 11.851V and seems to be as good as new! I'll put it through a discharge/charge cycle to get it over 12V, but so far so good!

Thank you thermoelectric! Great money-saving Instructable!
vdanvung6 years ago
hi there, good hack. just wandering i got one which when i charge my charger say its full charge, but when connected there only 5v. the battery is 6v 12ah. do i need to refill.
12V vdanvung5 years ago
i had exactly the same problem with my 12v 7.2ah and my 6v 4ah.
i connected the 12v one in series with 1 12v Christmas bulb and connected it to a 30v power supply for 3 minutes and immediately** connected it to the charger, it worked though it did not hold as much power as new.
exactly the same with 6v bu you MUST USE 2 bulbs otherwise they will blow.

**less than 2 minutes.
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