I don't think the 520 has a replaceable battery. And lithium batteries don't last forever. You can send it in to Garmin, I think.

705
My old 705 got a replacement battery about 3 years ago, after the first one only lasted a few hours on a charge. It was easy to replace, since the back uses tiny screws instead of glue.
from newpower99.com. Here's the 705 instructions: youtube link.

My 705 has annoying problems. But I still depend on it to reliably work with a loaded route in unfamiliar areas.

The saved rides have occasional bad data embedded in the middle, making the ride file unusable unless I edit out the bad XML entries.
The Routes method of following a route never worked from day 1.
The Courses method works reliably.

I've had it almost 8 years! One of the 4 side buttons split it's rubber cover, so there's a patch of gorilla tape over it. Otherwise, it's still good.