For almost a year now, I am experimenting with junk truck batteries. When disposed, often one out of a set of two of these flooded lead/acid batteries is still fairly good. Half of my battery bank consists of former truck batteries, and it looks like they do fine and sometimes even benefit from the easy life they get now.
I've had a few junk batteries that read 12.8 volt after sitting for a week. Seems good, but specific gravity can show a number as low as 1.20 on all cells. Periods of equalizing followed by periods of sitting and vise versa improves the gravity slowly to 1.26, sometimes 1.28. When equalizing, the voltage rushes from 12.8 to 15.5 volt within a minute and he batteries mildly gas. The batteries are rated 225 Ah and charged at about 12 amp during equalizing. My charger can do 25 amps, but the batteries never take the full number during equalizing.
Now my question: what's wrong with a battery that shows an excellent voltage, but a not corresponding low specific gravity?