Not exactly home repair, but....

[mrcaptainbob]

What happens to a battery when it is 'over charged'? (Say this is a back up battery for a sump pump....)

[HayZee518]

When a NiCad gets overcharged it blows up! Same with a Lithium cell it overheats and starts to burn. The plates inside that exchange ions, negative or positive get saturated with their charges. Its like trying to put ten pounds of chit into a five pound bag. It won't go! A lead acid battery has spongy lead plates, a seperator and a different type of lead plate. The electrolyte is dilute sulphuric acid. The ion transfer that occurs during discharge depletes one plate until it can give no more. Charging, replenishes these ions back to the plate. Over charging makes the plate heat up and may even melt it causing a plate to plate short and you have a fire. A fully discharged battery is called a sulphated battery. it just won't accept a charge.

[mrcaptainbob]

Battery over charge

As usual, thanks again, HayZee.

[dha]

One thing that happens when overcharging a lead-acid battery is that it uses a lot of water and produces hydrogen. This is accompanied by heating and eventually a ruined battery.

[mrcaptainbob]

Sulfation.

So, the plates are 'sulfated'. What does that mean? The ions leave that medium and the medium is then converted to some other molecular structure? Are they recoverable? Permanently changed?

[HayZee518]

when the lead-acid battery is sulphated it means that the spongy lead plate has been destroyed. the plate looks like a waffle with the hollow spaced filled with a spongy lead compound. over charging melts the lead and is deposited at the bottom of the "jar" if enough of it comes off, it can cause an inter cell short. The plates and separators don't go all the way to the bottom of the jar but are about a half inch above the bottom. It is not recoverable.
each "cell" puts out about 2.25 volts. a typical car battery has 6 cells, inter connected by lead jumpers in series giving you 12 or 13.25 volts

[mrcaptainbob]

Non-recoverable.

Thanks for that explanation, HayZee. Much more clear now.