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)
-Distilled water (you can also use rain water)
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 https://www.instructables.com/id/Portable-Power-Supply/, 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.