20s lipo is 84V fully charged, but it's 74V nominal, as I just posted in my last post. You can use 18s if you want. It's actually a 66.6V nominal pack that will fully charge to 75.6V. if you have a 72V controller, either will work unless it's really not a 72V controller and only labeled as such.
Mongoose 26" FS MTB bike $99, yescomusa.com 48V 1000W rear hub kit $276, Hua Tong 72V 40A controller $35, 10ah 24s lipo $275=40+mph, range=45 miles @20mph