Went to start my bike this afternoon; it cranked a few times than started die down. Put it on the charger and it fired right up. Then I rode it to the gas station about 1/2 mile away, filled it up and it started up fine. Rode it about 15 miles to my friends place and parked it. About 6 hours later I try to start it again, but it cranked and died down again. Bump started it and it fired first time. Rode it back, shut it off and immediately tried to start it again, but the battery went low again. Possibly a bad battery? Can anyone recommend a good replacement?