batteries and pentlites

Batteries and penlites have the function to deliver electrical energy. So they must contain energy: chemical energy.
The substances in batteries and penlites must be rich in energy. Otherwise they are useless.
Petrol is an energy-rich substance, but you never meet petrol in penlites. Other substances are used, staying quietly in their position, being not so dangerous (explosive).
In module 7 we will talk about those substancies full of energy and how they deliver that energy during chemical reactions.
You must know that batteries and penlites suffer chemical reactions and that chemical energy is transformed in to electrical energy.

An "empty" penlite is not really empty. The energy rich substances are used, transformed into energy-poor substances. Recharging a battery is a process wherein energy-rich substances are reproduced.