In reply to Bjartur i Sumarhus:
Sorry, can't help with ios.
Just used it on the walk home, the maps seem to move slightly clunky and no matter what options I select I can't get the screen to come on at junctions so unlikely I'll see much battery life improvement.
I'll try it on another, newer phone in case it's android / manufacturer specific but it looks like I'll be sticking with MMTracker and a power bank for now.