Batteries are rated for a set number of charge cycles. If you recharge at 70% instead of at 35% you can expect half the life with the battery. I recommend using the bike and checking the amount of discharge after each ride. This will provide a good idea of how much the battery charge level has been depleted to cover a certain distance.
My bike has eco, sport, and turbo modes and I set the eco mode for the percentage motor contribution I want and use it 100% of the time to maximize the miles per battery charge. On hills I drop into a lower gear rather than use a higher power mode. I get more exercise and the battery should last longer with fewer recharges needed.