You can use a wet cell battery like what came with the bike or get a better AGM type battery that will last longer as the acid is in a glass mat and won't spill out or boil out, fully closed case. Deka and Yuasa make a good one.
http://www.etx18l.com/
http://cyclemax.com/inc/sdetail/gl1000__gl1100__gl1200__gl1500_yuasa_hp_sealed_battery/158747/138559
http://www.etx18l.com/
http://cyclemax.com/inc/sdetail/gl1000__gl1100__gl1200__gl1500_yuasa_hp_sealed_battery/158747/138559