How to Revive Old Lead Acid Batteries




About: My name is Alen and i study electronic engineering. I love to make anything that needs electricity, hack and mod things as well building anything AWESOME...

If you are like me you probably have old lead acid batteries sitting somewhere probably discharged

If you dont use lead acid battery always charge it before and recharge it every 3 monts

I ve tried this method on maintenance free lead acid, sealed lead acid and lead acid batteries , only difference is that maintenance free and SLA have hidden caps

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Step 1: You Will Need


-Adjustable PSU or Taper Charger (Smart chargers wont work)

-Dead battery

-Distilled water (you can also use rain water)

-Rubber gloves




-Super Glue

-Paper towel


Step 2: Measure Battery Voltage

Connect multimeter to your battery and check voltage

Your battery shoudnt be lower than 11.8v (30%) in open circuit

If you read lower than that you will need to revive it if its higher than that try to just charge it

100% = 12.73 v
90% = 12.62 v

80% = 12.50 v

70% = 12.37 v

60% = 12.24 v

50% = 12.10 v

40% = 11.96 v

30% = 11.81v

20% = 11.66 v

10% = 11.51v

Step 3: Inspect the Cells

Place paper towels on your working areas

Now if you have SLA or maintenance free battery you will need to remove lid with screwdriver

Put on gloves and remove caps

Do this in well ventilated area

Wipe any wet spots with a paper towel

Then look inside cells and you should see white fabric that is dry

If you see brown fabric with white lumps your battery is probably sulfated and your chance of reviving battery is really bad

Step 4: Add Water

Take a syringe and fill each cell with water and look if fabric is absorbing water

Slowly shake battery and let it rest for 10 minutes

Step 5: Turn on Your PSU

Now you will need to connect your multimeter to show you how much battery is drawing

Set your multimeter to 10A and connect it in series with charger and battery

When you connect it to the battery an it should draw 100-200mA

Thats sign of dead battery

Set your psu to 14v or connect your charger and let it sit for day or two

After that your battery should draw 500-1000mA

Then just leave battery and calculate how long battery should charge

Ah capacity = Amps that your battery is drawing x hours

For my battery 8Ah = 0.39A x 20,5 h

Step 6: Testing Battery

Now leave your battery for 1h and hook it up on car hadlight or any 12v bulb

Voltage of battery shoudnt drop lower than 12v

Step 7: Edit: 28.04.2019

At the time i wrote this instructable i was 15, i didn't have a lot of equipment at that time or knowledge. Over the years i gained a lot more experience with batteries and how electronic things work so i decided to test what i wrote 5 years ago.
I got 2 old batteries that my uncle used with portable speaker and both were dead (4.5V when measuring on terminals). I charged them up slowly and tried to discharge.
First battery only had 402mAh (same one i charged in, it's about 4y old) which is way too low for 7Ah battery and second one got 4424mAh which is kinda fine. I opened them up, instead of distilled water i used sulfuric acid that i got from old lead acid battery. I used pulse charger that i made and first battery didn't want to go over 10V which is clearly a sign of a bad cell. Second one was about 2 days on charger and when i tried discharging it i got 6682mAh which was really good for 4y old SLA battery. Same pulse charger was used on starter battery from motorcycle (12V 3Ah) and after few charge/discharge cycles it was good enough to start engine and charge at decent rate.

I can't say that pulse chargers (or desulfators) work but in my experience they did help with two battery while i had no luck with third one. I also tried simple 12V transformer and bridge rectifier and i had no luck bringing that first battery to life, it simply got hot and voltage dropped even further down to 6V which is a sign that cells are most likely damaged.
Desulfator/Pulse charger is simple 555 timer with 1khz signal turning mosfet on and off (1ms turn on time) with about 18V pulses up to 10A.
Method shown in this instructable works but your result will vary a lot. You might have luck and restore your battery or it may be damaged way beyond repair. Pulse chargers may work but if your battery is beyond repair just get a new one (you will also get discount by returning old one when buying new). Capacity will also vary, some batteries might be close to original capacity but if plates or electrolyte are bad it will have low capacity.

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    34 Discussions


    3 years ago

    I know you put a lot of passion and effort into this project. I commend you on this. However when lead acid batteries fail, regardless of the variant, they simply cannot be revived and expected to produce even 40% of their original capacity. They are truly disposable after failure. Also, do not interfere with lead acid battery insides; you don't want to risk your future with lead poisoning (it can happen easier than you think). Be safe and keep up the great work!

    6 replies

    Reply 3 years ago

    I revived few lead acid but they usually die after 5-6 months and then you cant revive them
    Also they cant deliver same amperage as they used to


    Reply 3 years ago

    I abandoned Lead Acid technology about 3 years ago. I use exclusively the Lithium Iron Phosphate chemistry for my home solar bank, my car engine and also all my portable power sources. Environmentally safe, safest Lithium chemistry in production, low self discharge and far longer service life. I love LiFePO4 with a passion!


    Reply 1 year ago

    Hi. Does this change to use LiFePO4 requires some change on charger or it works the same as original lead acid alternator/rectifiers in the car?


    Reply 1 year ago

    Fully compatible with OEM alternators.


    Reply 3 years ago

    I started using 18650 becouse you can make packs how you like
    If you need less voltage or more, maybe more capacity


    2 years ago

    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?

    4 replies

    Reply 2 years ago

    buy a replacement battery or spend the extra money to purchase the acid necessary to properly have a specific x to y ratio of water to acid.

    If you keep adding water, you're diluting and messing up the acid to water ratio. and that is BAD.


    Reply 2 years ago

    What capacity is your battery? 2.6A should be normal (if its more than 7ah, 2.6A is fine), charge it till it reaches around 12,6v


    Reply 2 years ago

    hey tutdude98. thanks for answering me.
    that battery was died. i have another one. 12V 1.4AH Maintenance-free sealed lead-acid and dry. i fill
    the cells with rain water. wait for 24 hours. and charge it with 15V. in
    first minutes it takes just 0.1A (100mA) from power supply. but in 3
    hours it reached to 2.15A. when it was on 2.15A it seems like boiling
    water inside of battery and rubber caps jump from cells and make a bad
    smell. after charging i use the battery with 12V 2A Car Lamp and it make
    it glow perfectly. voltage was 11.08.

    this is how the voltage get down.

    4:15PM : 11.08

    4:35PM : 9.09

    4:45PM : 6.58

    and finally after 24Hours its get down to 0.0Volt.

    i cant understand this battery!

    any help would be appreciated.

    thanks again for answering my comment.


    Reply 2 years ago

    Sounds like you have shorted cell, or one really bad cell
    Also, if you charge 1.4Ah battery with 2A, it should take less than 1h to charge it and you should disconnect when water starts to boil
    Keep in mind that after you revive the battery it should lost part of its capacity ,1.4Ah battery might drop under 1Ah so you might not be able to draw 2A from it longer than half hour


    3 years ago

    Keeping the battery full of water is part of the maintenance it shouldn't be low. If you have to add water you haven't been taking proper care of it.

    3 replies

    Reply 3 years ago

    Yeah but SLA should be "maintenance free" becouse they have hidden caps and you dont need to add water


    Reply 2 years ago

    The only way Sealed Lead Acid batteries can loose fluid is by over charging when pressure inside needs to exhaust, those caps are in fact one way valves. A pulse charging can clean the plates, and restore plate surface .


    Reply 2 years ago

    I had several SLA batteries that were not charged and sitting in garage, they were all somehow empty.

    Never tried pulse charging or desulfation but i noticed that "dumb" charger with only transformer and rectifier works better when restoring old batteries.


    2 years ago

    very interesting, I will be trying one based on your instructions. We have oldecided lead acid batteries lying around. Thanks.

    Gaia Slut

    2 years ago

    can anyone confirm this actually works? i mean im sure theres a way to do this just i find this topic confusing as i never thought this was possible

    1 reply
    Cekpi7Gaia Slut

    Reply 2 years ago

    i tried on few batteries and it worked, but they didnt have back their original capacity, they would lose something like 30-40%


    2 years ago

    Don't forget to wear goggles. I have a SLA I can't wait to try this on. Thanks.