suprised battery hasnt already exploded...
do what tom said about checking the output of the alternator to see exactly what its putting out. if its 12V go get a 12V battery, their like $40 from a farm store...or wait for sulfur and hydrogen gasses not to mention battery acid to come out of your battery if it doesnt blow up thru the floor board.
Ive been driving an electric cushman (has 6 12V batteries) when one internally shorted and blew up directly under my butt, thank god the seat bottom was 3/4" plywood, left a pretty big dent in the wood. Still hurt like getting hit with a paddle though. not to mention it was loud with earplugs in and acid was raining everywhere.