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Please help me restore this old Lead acid battery

Hello everyone,
I just salvaged a small, 50AH FLA battery from a Tractor. The Battery has seen 2~3 calendar years. It was showing 11.38 Volts sitting on the shelf since 3 months so I thought it was good and I brought it for a bargain.

I brought it home and connected it to my Intelligent charger which claims to support (Automatic)-

Deep discharge Charging
Boost Charging
Absorbtion Charge
Tricke Charge
Equization Charge

Automatic Charge Current and voltage regulation with Temperature compensated charging.

The charger is currently showing "Boost charge" at about 12 Amperes. Problem is,the Charging voltage seems to Jump from 13.1v all the way to 14.2V (like under 10 sec) I hooked my DMM to the battery terminals which comfirmed the same.
Sometimes the voltage also reaches 14.4v and the Charger goes into "Equalization Charge" but falls back to Boost charge at 13.5v or something odd.

After 3 days for 'Cycling"

I tried a eq charge twice. it seems to be improving.

the Battery voltage on charge is still jumping across 13.6v to 14.2v. if I draw a load it goes back to 12.3v.

admirably, I connected the Battery to our DC load yesterday night and it ran Led street lights (load 4a ) for 5 hours before I switched it off. 20AH is decent progress for 50ah old Battery right?

do you think it's sulpahation that's causing voltage jump?

I ran the Battery down to 11.5v yesterday. in the morning with sun up, the Battery is slowly progressing from 12.8v and above... very stably. the charger shows'absorbtion charge' at 8-9 amps. 8 amps are good for 50ah Battery right?

else how do we test the ah of Battery?

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Well, the easiest would be to take it to battery joint and do a full load test there.

You need mssive resistors (or better shunts) and they get hot, nothing you could do with home made stuff.

Fluctuations in the charging voltage are normal and depend on the type of charger you use, some even work with spikes of over 16V for the trickly charge.

For an estimation take a fixed load of let's say 5amp.

after 10 hours you have reached the 50Ah mark and if your battery goes under 10.8V before that it won't have the full capacity anymore.

bhvm (author)  Downunder35m2 years ago

Thanks for the reply,

Unfortunately I don't have access to Battery Joint. Anything homemade can be done? I have access to a DC amp meter and a Car headlight that pulls 10 Amps or some LED lamps that pull 2 amps...

being an old battery I think i;ll be happy at 40 or even 35 AH of capacity

Just do the math: Resistive load (like a lamp) in Amps multiplied with the hours.

But this will only show if the battery still keeps a decent load, it won't tell how badly it collapses under the maxium load it is rated for.

Check how much your lamps actually draw and calculate.

Give the batteries a full charge and connect the lamps.

Monitor until the voltage reaches 10.8V.
Multiply the hours till then by the amps of the lamps.
Now you now how many Ah are still in the battery.

bhvm (author)  Downunder35m2 years ago


I may try that on a weekend. A Headlight bulb and DMM,i'm good to go.

However this battey seems to have a lower voltage than other NEW batteries. resting is bout 12.4v. If i put a load on it, It quicky goes down to 11.8V and then to 11.6v. However It will hold all night on 11.5v with light loads 2~3 A.

What does that say in Battery terms? Sulphation? Grid corrosion? Weak electrolyte?

Any solutions to bring up the voltages slightly higher?


1.2 Volts per cell, if it is 10 cell battery the value you have is correct.

For a 12 cell battery you should see more.

Considering the age I would simply use it for a few weeks and check if the performance goes up again.

If there is sulphate build up you can add 3-5ml of phosphoric acid (rust remover) per cell, this will break down the sulphate over time and increase the performance.

But sometimes old just means that, old...

bhvm (author)  Downunder35m2 years ago


1.2v per cell? aren't lead acid cells 2.1v per cell?

Mine is a 6 cell battery, which makes it 12.6v by theory. I can see 12.44v on multimeter when fully charged. i think the performance is going up with use and care (restoration).

Do you think its worth keeping it?


As long as it is stron enough for your needs keep it.
And, yes, with proper charging and discharging it should perform a bit better after long storage times.

bhvm (author)  Downunder35m2 years ago

I tested the battery after 24 hours of sitting. It reads 12.33v now. I put it on a charger and it shoots up to 14.1v right instantly, without pulling much amps. Then Is floats happily at 13.6v all day long.

I guess it should simply parallel it to some of the better batteries as a 'Support" ?

Thanks again.

To me the battery seems to be fine so use it as you please :)

bhvm (author) 2 years ago

anyone??