I started out with a 700 and bought an external battery for it after suffering a complete electrcial failure in flight. I continued using the backup battery when I upgraded to the 720.
For what it's worth, once tablets became a viable option, the tablet with its built-in battery became a backup to the 720, and I retired the 720's external battery, glad to be rid of the hassle of ensuring it was charged before flight. (Eventually, the tablet became the primary device. Today, the 720 is a tertiary backup, with my two newest tablets taking the primary and backup roles.)