5.7" ips screen on the 640xl is the main culprit, as ips screens use backlighting.
Thinking of getting another windows phone with a smaller screen. Loved the maps feature it was very handy for me so if i do get another phone i will me looking at the apps for mapping feature. Being in a regional centre like Gladstone it helped me a lot ,even cut down walking times from my apartment to supermarket and avoiding a huge hill. I think leaving the phone on with screen on standby could also drain the power too. As you mentioned a large screen drains power fast this is why i am thinking maybe a smaller screen size.
Strange the app to download Windows 10 disappeared when i arrived back in Adelaide. And i also noticed the phone tracked my movements from Gladstone back to Adelaide. So the GPS tracker does work. Same as the taxi coming home they use a Mapping terminal with gps ,wondered how he knew where i lived so fast.