The
stuff on the
plates of
batteries that reacts with the
electrolyte to produce
electricity. In lead-acid batteries, this is usually a
paste form of the same material the
plate is made from.
The #1 cause for failure in
automotive batteries is
vibration, which causes the active material to fall off the
plates and collect in the bottom of the cell. Eventually, enough material collects to reach the bottom of the plate and
short out the cell.