I think I've proven to myself that you want to start out on the external battery so that it doesn't have to charge the Garmin at a high rate. I have some 2600mAh lipstick batteries that I use to keep the garmin charged. One of them is going to get me through the fleche tomorrow. I'm not sure it will even be fully depleted by the end. I think on 4am starts, the 2600mAh is usually depleted at about midnight or later. But then the internal battery is fully charged and gets me through. The lipstick batteries are a nice size to mount to the handlebar. I use a twofish lockblock to hold the battery. I carry extras, they are light.