Twins tear up batteries. Takes a healthy battery to tick the cylinders over. Have the battery fully charged and give it a load test, by either having a voltmeter and keeping it on the batteries terminals before, during, and after the start attempt. Should be reading around 12-13.5 volts. If it dips down under 11 with the starter pressed- the battery needs to be replaced. Or just take the battery to Batteries Plus, Interstate, etc. They can tell you the same with their Load Test equipment. Note- a charged battery does NOT mean it's a good battery.