I use Battery Minder... on a trip yesterday for an hour, Plugged into car charger and running Google GPS nav and Tracks(GPS app)... at end of trip noticed that phone was hot... it's in a rubbery bumper too. Batteryminder reported 120 degrees F and Battery overheated... Turned off phone. waited... turned on later without any nav and continued driving / charger then charged from 50% to 100% and phone was cool. So I conclude that GPS uses alot of cpu's and somehow causes overheating. Because I could later charge without overheating, I do not suspect the charger (Cellet 700mA). Currently using the stock wimpy battery. BTW, outside temp in Maine was 72 degrees F and overcast. I put the droid in between the two gages MPH and Tach on a Subaru Legacy... so even if sun, mostly shaded with overhang. I think this is a better place ..closer to a heads up display.