only 2 hrs on battery

I bought this app to use it on the motorcycle. The turn by turn voice guidance works well and the balance between enough information and unnecessary comments is well done. However, after only 2 hours my fully charged iphone 3GS is out of battery when using this app. I did all the tricks (turning off push, no wifi, screen off...) but 2 hours is it. Even worse, there is no notification by the app that it is shutting down due to low battery. Can lead to bad surprises when driving longer distances.
Probably a nice app if you stick to using it in the car with a car adapter.

ebbi55 about TomTom Western Europe