I went to charge my lawn mower batter I took the battery out and went to charge it I connected the positive then when to connect negative and it sparked even though charger was not plugged in?

It is possible it was charging a capacitor that is inside the charging system. Unless there is high heat or melting then I wouldn't worry too much.

You don't want sparks occurring after you have been charging the battery as that can cause an explosion. Be careful to use all precautions like gloves and safety glasses when working with batteries.

Hi,
You are connecting what is in effect two sides of the coil of a transformer. If is a more expensive solid state unit, it probably will not spark. If it is a cheaper model, you probably are connecting the two ends of the secondary coil of a transformer. If it has a ammeter, you will see the needle jump.
Peace,
Clarke