Bill,
I have used an Ipad to display leadsheets since the very first Ipad came out. I have NEVER plugged it in on a gig. When fully charged I will get up to 4 one hour gigs with it without having to worry about it quitting on me. Even then it usually will have about 40% charge left. I have often left the house with only a 50% charge and have never had a problem. Keep in mind, my gigs are usually one hour in length - sometimes two hours.

Deane