SLA battery... Dead or not?

I know this isn’t anything to do with Arduino, but I didn’t know where else to ask it.

I bought an air compressor at a yard sale, and the battery was “completely dead” as the previous owner had said. So, I brought it home to try and fix it. I turned it on and the analog meter needle read about halfway. So the battery was not dead. So, I hook up my bench power supply to the charging port, set it to 12v (the voltage printed next to the port), and let it sit overnight.

I woke up the next morning to an unchanged meter needle. I thought maybe something was wrong with the charging circuitry, so I cracked it open, and there was the rather large SLA (Sealed Lead Acid) battery.

I disconnected it from everything and hooked up my multimeter. Instead of seeing the expected 12v, the battery read 8.3v.

I hooked the battery itself up to my bench supply, set it to 12v, and left it overnight again. When I got up the next morning and checked it with my multimeter, the battery read pretty much the same 8.3 volts as before.

Is the battery recoverable? Am I doing something wrong charging it? Or do I just need to buy a new battery?

The battery is a 12v 7Ah SLA battery.

How can I recover this battery, if possible?

I’m not entirely sure of the exact age, but the battery is definitely old. TASK FORCE # 202754 is the compressor.

SLA batteries are pretty unforgiving.
In use, if they drop below about 11.3V, they must be recharged IMMEDIATELY
Any delay whatsoever will result in internal damage to the electrolyte and electrodes.

This the reason good quality SLA chargers are relatively expensive.

They can sag under load, but the quiescent / idle voltage must be maintained.

So does this mean I need a new battery? Or is there a way to recover this one?

Pretty much - yes.
It would cost you the same amount to find out, or to end up with a confirmed dead battery.

SLA are cheap nowadays, just remember to take care of them ! They typically last 3-5 years in continuous service if maintained well.

I would try and set your bench power supply to 13.8-14.4V limted to 0.8-1.5amps to charge the battery, If you’ve set it to 12v this will not charge the battery.
At least you can try it before you bin it.

Lead acid batteries use a set of cells that each have a voltage around 2.0 volts. So six in series give 12V.
If you are only seeing 8V then 2 cells have died. They arent recoverable. And if a battery at 8V wont charge from a 12V supply then it wont charge from a 14V supply.

Tried this. Left it for a few hours, the voltage on the battery actually dropped to 8.2v.

What voltage do you read across the battery terminals while the 12V is connected?

If it is 12V with battery connected, it means the internal resistance is way too high and the battery has;
< sarcasm >
Gone to God.
Gone to meet its maker.
The battery is no more.
Bereft of charge.
Kicked the bucket.
Shuffled off ‘is mortal coil.
Run down the curtain and joined the bleedin’ choir invisible!
(To quote Monty Python…)
</ sarcasm >

Tom… :grinning: :+1: :coffee: :australia:

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Well, the battery has gone to meet Shimastu (The company that made it) I didn’t want to measure its resistance with my meter in case it was damaged by the voltage, but when I connected it to my power supply, there was no voltage drop.

You measure internal resistance by comparing the open circuit battery voltage and a battery voltage with a known resistance loading it.
It is not a directly measurable quantity.

Tom… :grinning: :+1: :coffee: :australia: :woozy_face: