Introduction: 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


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


D6equj5 (author)2018-01-11


I've done everything in your instructable but my batteries still don't hold a charge and struggle to even get to 7volts. Any ideas?



RandyD66 (author)2017-07-19

Many people mistakenly believe that if a 12v battery charges and reaches the voltage of a charged battery (12.8v - 13.2v) that that indicates the battery is good. That is not the case. You can have an 80Ah battery indicating 13.2v, but nearly ZERO CAPACITY. Capacity is the ability of the battery to store a charge...not just reach a voltage point. Adding water to a badly sulfated battery is not going to help.

If you have a mildly sulfated battery, you can try connecting it to a 12v 1 amp solar panel for about a week. If the battery is salvageable, this will help. If after a week the capacity has not risen measurably, the battery should be properly disposed of.

amiir (author)2017-03-08

hey guys. i have 12V SLA battery and it wasn't use 1 years. after charging its not save power as it should. the voltage was 3.6V and it dies after 10Sec. i collect some rain water and clean it with cotton filter. refill the 6 cells and charge the battery for 5H. in first hour it takes 0.3 ~ 0.9A and in next hours 2.6Amps. is it normal? what was my mistake? the voltage is 7.78 now and its not die fast. it can provide 7.78V for a LED lamp almost 3hours. what should i do to make it 12V or near 12?

ciprkfupm (author)amiir2017-04-06

I've recovered four SLAs three 6Volts and a 12V sla. First of all if the battery voltage is 0V or say less than 2Volts just filling it with distilled water and connected it to the original charger will not work especially due to massive sulfation in the battery. Also the elctrolyte present in the battery after refilling is mostly water and almost no acid. To get your battery to atleast 80% of it previous state I did the following:

1. Pry open the battery cover and remove the rubber caps and fill with distilled water, but make sure that its slightly flooded inside the cell compartments but not overflowing.

2. Now you will need to desulfate the SLA, which means you have to apply a larger volatge at least 18 Volts with 2Amps or above. I usually connect the SLA to a 19V 6.5A laptop adapter till the battery starts bubbling and after some time the battery becomes slightly hot remove the 19V supply.By now some of the water is converted to sulphuric acid, now you have to proceed with extreme caution.(NOTE: This step should be done only with the rubber caps open)

3.Once the voltage is 5.7V+ for 6V SLA and 10 to 11V for 12V you can connect the battery to your original sla charger. It will charge like normal. Remove after an hour or so. Close the rubber caps and seal the plastic lid with epoxy glue and once set pu the sla back on original 6V or 12V charger and it will work fine.

4. After a few cycles of use yor battery capacity will improve greatly like mine. I recovered a Panasonic, 2 Fiamm GS and a BB Battery.

aduy (author)2013-05-10

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.

ryanrocks714 (author)aduy2017-02-12

JonathanR185 (author)2016-09-11

Does an SLA battery have an expiration date if I never fill it with acid?

cmerriman1 (author)JonathanR1852016-12-04

Do not add sulfuric acid. Just distilled water. You only add acid to a brand new battery that you buy DRY. Sulfuric acid does not evaporate so it is still in there. When you add the water it will mix with that water. As that water evaporates the solution will become more acidic.

KostadinD (author)2016-04-21

The author says: "Warning, You have to estimate how much water to add, Adding too much will cause it to leak/spit when being charged." There is no reliable way to estimate how much water to add. It depends at what degree the battery dried-up what obviously is unknown. It is only my guess to compare the weight of brand new battery, which probably will be not available, and the old one. The difference will be the amount of water to add.

AndyS19 (author)KostadinD2016-05-09

From my experience, usually it's not that difficult to figure out the amount of water it needs, with proper tool (syringe with needle). I usually just shine a light into the hole while I'm slowly refilling it, just right above the element/sponges/whatever it's called. You'll see it inside the hole. and then wait for 30mins or so for the water to get absorbed (it it has been dry for a while), and then put more water, same as before, just right above the element.

MahdiH18 (author)2016-04-13


For Tmyrbatry UPS should have enough knowledge in electronics

Or read articles and books in the field

UPS battery repair is not easy

You can also visit the articles on this site read -ups and Ernie get help

with respect

Thomas055 (author)2016-03-10

Definitely going to be using this! It looks great! Thanks man!!

Kinnishian (author)2016-02-29

So I have a red optima I'd love to give this a shot with. The guy who gave me it said he thinks he accidentally boiled off some water while charging. I don't really need much capacity out of the battery, but I need it working-ISH (plan to use it to sink some current during small lipo discharge tests.)

Anyway, I am surprised to only find TWO vents in my battery. Do you know what's up with that? They are both underneath a porous disc and look like your vents, but there are only two of them. Do you think I need to do more digging?

Kinnishian (author)Kinnishian2016-02-29

Update! More reading suggests yours is unusual for having so many vents, and mine is more typical (two vents on opposing sides for various vent-tube configurations.) If I am correct, that's cool (for me at least.)

However, new question: Is there any typical suggested amount of water to start with? I don't even know what the maxinum content of water is in there :). If you have any suggestions I'm open ears.

chandranariyandath (author)2016-02-24

Very useful information regarding revival of old &used SLA battery etc are given in simple launguage easily readable to any type of peoples.Procedure mentioned also can be easily followable.Tnx for the discriptive deatils in a simple manner to understand easily for even a lay man.

chandranariyandath (author)2016-02-24

Very useful information regarding revival of old &used SLA battery etc are given in simple launguage easily readable to any type of peoples.Procedure mentioned also can be easily followable.Tnx for the discriptive deatils in a simple manner to understand easily for even a lay man.

manukeshav (author)2016-01-09

Sir, when I opened my battery some gas was coming out and creating pressure inside .The battery was nearly dead.

Minura lakvin (author)2015-05-09

RajeshbAdoor (author)2015-03-10

Hallo I Am going to do this work in my computer UPS battery its damaged

OutspokenJew (author)2014-12-02

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.

PuneetS (author)2014-08-11


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 :(

toyota4k (author)2014-06-25

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

czarmatra (author)2009-06-22

Is it not acid like in the 12v car battery? with a 1275 SG. Or just plain old clean water?

thermoelectric (author)czarmatra2009-06-22

Well, You can add acid, That's probably better, But you can use distilled water, i'm pretty sure...

BruceLyn (author)thermoelectric2009-09-09

Hi, can I add the swimming pool acid instead of water? thx

thermoelectric (author)BruceLyn2009-09-09

Hydrochloric acid? Yes, But not too much.

en2oh (author)thermoelectric2014-04-12

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.

chamunks (author)thermoelectric2011-06-06

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?

cpreble (author)chamunks2011-06-06

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.

BruceLyn (author)thermoelectric2009-09-10

Thank you.

czarmatra (author)thermoelectric2009-06-23

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)czarmatra2009-06-23


baglady201 (author)2014-04-02

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

astral_mage (author)2013-12-24

holy crud flat man 220 comment an growing.

hansonsux (author)2013-01-12

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.

Somoldguy (author)2012-12-29

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.

PCelec (author)2012-12-03

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.

Objectivityiskey (author)2012-09-01

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.

chnaveed.alam (author)2012-07-05

thanks for nice upload
can i charge 6 v lead acid battery with 12 volt charger

It's not advisable unless you watch the voltage closely, it wouldn't take much to boil the electrolyte right back out.

ron12 (author)2012-03-04

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

The 4th Doctor (author)2012-02-04

i heard somewhere that epsom salts can be good for desulfating batterys

Electronics Man (author)2012-01-27

So if I have a couple of old SLA's that are completely dead, Will doing this revive them?

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!!!

ajazmd13 (author)2012-01-28


cenzo188 (author)2011-12-02

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.


dsandds2003 (author)2011-11-30

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.

Callum Snowden (author)2011-11-17

Have just refilled one of my 'reused' electric scooter batteries so hopefully it will charge. Fingers crossed...

anuragmahajan152 (author)2011-08-08

Thank You Very Much

About This Instructable




Bio: I am a high school student in Cairns, Queensland. Most of the time I am either at school, sleeping, doing stuff on my computer, making ... More »
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