•
The only commercially available electro-chemical
compound that can old its charge longer is in the
well known alkaline batteries.
The reason is the inner resistance of the compound;
it is so high that it has the longest expectable shelf life.
But there is a bone: one can't hope to recycle quickly
an energy hungry flash but is ideal for the less hungry
wall clocks for example.
The black Ennelop Pro (the old XX) made of another
electro-chemical compound, have a lesser inner resis-
tance so they can charge any flash much faster and
may cheaply be recharged.
The best power source, and also the most dangerous,
was the NiCad cell. Dangerous because it could dischar-
ge all its energy in a few seconds (when short circuited)
and cause very severe burns.