I have a phone I leave it my car at all times also. I carry a Blackberry for work, but I don't always have it on me. If you don't turn the phone on (i.e. outgoing calls only), then the battery will stay charged for a long time. I only need to charge it a few times a year. Of course, if you leave it on for incoming calls, that will likely require weekly charging.
Sony XBR65X930D | Denon AVR-X1300W | BELL Fibe | Panasonic DMP-UB900 4K | XBOX One X