One 6V battery discharging really fast

wopachop wrote:
As mentioned above you might have lost a cell.

You can try what's called an "equalization charge" and see if the bad battery can still be used.

Wonder if the possible bad cell actually looks different if you shine a flashlight at night down the water holes.

I would not try to equalize a battery with a shorted cell. It likely has a mechanical failure which is very likely to create a spark which is a very bad thing with battery cells that are being overcharged. A cell adding no voltage to the battery doesn’t indicate a charging issue, but rather an electrical short.