Do you have to have your iPhone connected to power?

[rbpeirce]

You get warned if you don't, but is the drain high enough that you might run out of battery power in a 20-30 minute session?

I usually charge my iPhone between sessions and that has always been enough for other apps.

[Harry]

It is a warning you get. Murphies Law tells you you will run out of battery exactly when you make your. Best lap ever. You can run LapTimer without a plug for 2 hours or so - certainly strongly depending on screen brightness, accessories connected and video or not.

Harry

[Fcat360]

I use one of those little Lithium emergency battery start packs which has a couple of USB charge ports on it. Last the whole day. Just leave it in the glove box with the USB cord hanging out and plug in the phone when need be. Also charges the DUAL GPS.