Also, if a battery is really bad, you can prepare a 5 gallon bucket with a couple boxes of baking soda, then pour out the old acid, rinse the inside of the battery with DISTILLED water, and then fill it back up with NEW acid.
Charge at the lowest rate possible.
7 out of 10 times it will work.
The idea is to clean out the crapola that has settled and shorted out the plates, and then use clean, new acid to refill.