base2 

I looked into this once on an old Edge 520. If you are looking to rehab a Nuvī or something, maybe my experience will not apply.

The trouble was finding a battery of the same capacity. All the ones I could find were quite a bit smaller. Then there was the issue of making the connection. The battery is factory soldered to the leads. I didn't trust my ability to solder anything that small and delicate. It ain't no ni-cad. In the end I dropped the idea & got a new Garmin Edge 820.

Maybe battery tech has come a long way since then? The instructions are readily available online. Separating the case is pretty easy with a jewlers cross-tip screwdriver.

Of course with a Nuvī there is no guarantee of any function beyond navigation. The cool thing about cycling computers is the cycling specific features and the ANT+/BlueTooth functionality.