That is an excellent idea. And yes the battery could run out, but by taking the photo in the first place, it is also forces you to look at the row number and section name so you have a better chance of remembering it in the event that the batteries do give out. Paper can get lost and writing on your hand, well that can get washed off over the course of the day. Unless of course you don't wash your hands and then all I have to say is YUK. But even the bottles of water and soda make your hands wet. You have a better chance with the photo.
I guess I've been lucky because I've never lost my rental car in the parking lots. But is very easy to do that because the colors are pretty much the same in a given year from one rental car company to another, not to mention mostly light colors. I have a dark color at home, so I always have to remind myself I'm looking for a white/gray color and not burgundy.