How do batteries re-energise themselves?

Question

working with battery drills alot i notice that even when you run them flat you can let them sit for a while and the battery seems to re-energise a little.

How does this happen?

Answer 1

First, most battery chemistries are damaged by excessive discharge.  By doing so, you may be greatly shortening their useful lifespan.  Nickel cadmium needs a complete discharge once in a while, but not a discharge all the way down to nothing.

When the battery first quits, you have used up all the mobile (unbound) ions involved in the reaction.  With some rest time, some of the more tightly bound ions find themselves in a lower concentration solution, so they gradually dissolve/free.

Answer 2

The electricity is made using a chemical reaction.  The chemical reaction has a maximum rate.  Once you deplete the battery it takes a few minutes for the chemical reaction to catch up. The chemical reaction is reversable by recharging the battery.