Motorola :: Milestone Doesn't Keep Up Charging When In Heavy Use

I took a road trip last week and weekend and noticed that when under heavy use, my milestone will discharge even when plugged into the charger. From a functional standpoint, my milestone did great. I listened to an entire baseball game steamed with the MLB app over ATTs (mostly 3G) network. (Several places in S GA on I75 dont have 3G but the streaming app did pretty well over EDGE). I also use Google Nav and had my screen on since it was sitting on my car dock. After 4-5 hours of use my milestone was pretty warm and had quite a bit less battery life even though it was on the charger the entire time. I am guessing the GPS is a huge power draw, but what about having the screen on all the time. I would think that as long as the device is plugged into a power source, it should be good to go.