Be advised not the number of batteries will keep you in air for a long time but your RC's battery .
If the RC " feels" your phone/tablet running low on power will start to charge it cutting off the flight time . I just bought a 300k Ah power bank , output 5V1A or 5V2.1A . Every time just stick that in your RC (charger slot) and you'll keep flying . You welcome . . Bye .