You're gonna need some stout batteries to provide 100A in a compact 72V configuration, either 6 AGM batteries or take your chances with Lipo, either that or maybe two of the strongest 50A rated 72V LiFePO4 bike battery packs run in parallel for 100A total. It's either heavy or risky or expensive, take your pick of poison. 12V 20Ah AGM batteries weigh about 20-25 lbs each, so a 72V pack would be quite the lead anchor, which causes the motor to pull more amps and run them down faster than lighter batteries, the RC Lipo can really rip, but it needs to be set up correctly and the cells charge balanced, discharged properly, and monitored carefully. The big LiFePO4 packs are the best balance between weight, maintenance, ease of charging, and reliability, but they are also astronomically expensive.