To determine if low battery voltage is causing the engine to crank slowly or not crank, perform the following steps:
Explosive gas may remain in or around the battery for several hours after it has been charged. Sparks or flames can ignite this gas causing an internal explosion which could shatter the battery. Flying pieces of the battery structure and splash of the electrolyte can cause personal injury. The battery electrolyte is an acid. Extreme care should be exercised to avoid skin or eye contact with the electrolyte. If you come in contact with battery electrolyte:
Flush your skin with water.
Apply baking soda or lime to help neutralize the acid.