I have a 20hp Murray Widebody that now won't start when keyed?

I have a 4-5 yr old 20hp Murray Widebody that now won't start when keyed. I charged the battery was charged to 100%, and it would make a quarter revolution on three separate start attempts, now won't turn over at all (no clicking or noise at all). The battery has +- 15 hours on it and reads at 100% on the charger. I pulled the starter and had it tested at an autozone shop, they said it drags a little but does spin good. The mower is only used once or twice a month during the season and hasn't been started since September 06.
Ideas???