Originally Posted by Bald Paul

Garmin does offer an extended battery that will mount below a Garmin GPS unit on an out-front mount that works very well. It also has a USB output port you can connect to any USB charged device. Anker makes auxiliary power supplies. I use one that is about the same diameter as my seat post, and attaches to it with 1/4 turn mounts. A USB cable can charge my Varia unit if the internal battery gets low.