So I bought a truck from the salvage auction a little while back. The batteries in it are of an unknown age. They were completely dead! So I tried to charge them. The automatic charge settings wouldn't do anything. So on a whim I tried the manual 2 Amp setting for a while. I have one back up over 12 volts. The other is still only about 6 volts. I noticed that it might read 12.6 when I take it off the charger but then settles some over time. Like to maybe 12.2 V. Is that normal for them to settle at a lower voltage? What is the likelihood that if I get them up to 12.6 volts that they will actually be good for anything?

I suspect the only thing they would be good for is the core return on a new battery. Lead-acid batteries don't like to be taken down completely dead and usually suffer permanent damage. Probably shorted cells internally.

1 hour ago, Captain Obvious said:

I suspect the only thing they would be good for is the core return on a new battery. Lead-acid batteries don't like to be taken down completely dead and usually suffer permanent damage. Probably shorted cells internally.

Thats always been my understanding but if it makes 12 volts, then I wouldn't think there would be any shorts

12 is half capacity.  When lead acid batteries get fully discharged bad things happen.  I had one in the garage that I kept charged but it went dead a couple of times.  Then it was toast.  Wouldn't hold a charge.  Apparently the plates get coated and don't transfer the lead to the acid.  Like having a smaller battery in a big package.

There are internet stories about how to save them.  Drain the acid, rinse the plates, put new acid in.  At the end it might work and you have a bunch of nasty lead contaminated acid to deal with.  You can also find methods of charging at high amperage to regenerate the plates, but you have to be careful of hydrogen gas and boiling acid.  You can probably gamble on five used batteries and find a winner for less pain.

It's a fun subject to study but it gets complex.

https://www.rvtechlibrary.com/battery/bat_volts.php

Thanks guys

Just to pile on...Lead acid batteries work because of the semi-reversible chemical reaction. The lead plates eventually react with the sulfuric acid to produce lead sulfate since that compound has a lower energy state. The speed and finality of the reaction is dependent upon how much and how long you discharge the battery. Eventually semi-reversible becomes irreversible.

So why does capacity drop so much with little changes in voltage? Think of battery as a water tower. The pressure is proportional to the height of the water tower. As this particular water tower gets older, the inside is getting filled with sediment, and the sediment is also going down the pipe and sticking to the inside of the pipe. When you measure the water pressure (voltage), it still looks good, but with the pipe out of the water tower getting clogged, you don't get any water flow (current). Since the inside of the tank is also filled with sediment, the water empties quickly, and you have no pressure. (The voltage goes to zero.)

13 minutes ago, SteveJ said:

Just to pile on...Lead acid batteries work because of the semi-reversible chemical reaction. The lead plates eventually react with the sulfuric acid to produce lead sulfate since that compound has a lower energy state. The speed and finality of the reaction is dependent upon how much and how long you discharge the battery. Eventually semi-reversible becomes irreversible.

So why does capacity drop so much with little changes in voltage? Think of battery as a water tower. The pressure is proportional to the height of the water tower. As this particular water tower gets older, the inside is getting filled with sediment, and the sediment is also going down the pipe and sticking to the inside of the pipe. When you measure the water pressure (voltage), it still looks good, but with the pipe out of the water tower getting clogged, you don't get any water flow (current). Since the inside of the tank is also filled with sediment, the water empties quickly, and you have no pressure. (The voltage goes to zero.)

Good analogy!