A “boost” program applying a gentle charge current to wake up the protection circuit often restores the battery to full capacity.
The graph shows self-discharge of a nickel-based battery.
Among rechargeable batteries, lead acid has one of the lowest self-discharge rates and loses only about 5 percent per month.
With usage and age, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates.
Lithium-ion should not be discharged below 2.50V/cell.
The protection circuit turns off and most chargers will not charge the battery in that state.