The battery capacity on the Garmins tends to degrade quite rapidly-- after maybe 50 charge cycles, the battery will be at maybe 75-80% of its original capacity. After 100+ cycles, it's often 50% or less.
Add in the fact that GPS about doubles battery consumption, and an external pack becomes mandatory for any unit approaching it's second birthday-- unless it has been used sporadically.
With backlight off, GPS on smart, and no NAV running, my 2+ year old 520 will just squeeze out 6 hours. When it was young, ~7 hours while running NAV was pushing it. It really is a tiny battery.
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