In a cell chemical energy is converted into electrical energy.

Cells may be either PRIMARY or SECONDARY types.
A primary cell is discarded when its chemical energy is exhausted.
A secondary cell can be recharged.
The most common primary cell is the zinc/carbon (Leclanche) as used in torches, portable radios etc.

The zinc and carbon react with the ammonium chloride ELECTROLYTE to produce electricity.
The manganese dioxide absorbs hydrogen gas produced around the carbon rod which would insulate it from the electrolyte and stop the cell working.

The most common secondary cells are the lead/acid and nickel/cadmium (nicad).
Lead acid batteries need a constant voltage charger.
Nicads must be charged with a constant current charger.

All cells have INTERNAL RESISTANCE.
This is not an actual resistor but a characteristic of the cell.
Internal resistance increases as the cell ages.

When current is taken from a battery, voltage is dropped across this internal resistance and the voltage at the battery terminals falls.
The diagram shows that as the current taken increases the terminal voltage decreases.

This is called POOR REGULATION.
It occurs in any type of power supply.
Battery voltages must therefore always be measured ON LOAD, i.e. with the radio etc switched on and drawing current.