My Garmin 705 started showing "low battery" after 2 or 3 hours when it was about 4 years old. I never tested it to see if it could actually run for hours with the warning, perhaps the older battery just had a slightly lower voltage, making the "low" warning show up sooner.
Replacement battery
I got a
newpower99.com battery almost 5 years ago. I didn't expect it to last as many years as the original, but it's still good, lasting at least 6 or 7 hours after all these years. I recharge after most rides.
Some Garmins are welded/glued shut, and not replaceable. My 705 was quite easy, with 4 small screws and a ribbon connector. The newpower99 video was useful. They supplied a tiny screwdriver and a plastic tool to pull the connector.
They have
your 520 battery.
but it's more complicated than my 705, needing to strip some wires, etc.