Have tested battery an it reads fully charged but when I put a battery charger on it it still takes a charge . What could be wrong
Nothing is wrong. You can always pass a current "backwards" through a battery if you connect a higher voltage across the terminals.
If the battery is fully charged and you force a high current "backwards" through it, you will heat it up, produce potentially explosive gas, and evaporate some of the liquid inside it, but a battery charger won't cause that sort of damage unless the charger itself is faulty.