How Can I Determine What Charger Is Needed To Charge A Garmin GPS Battery?

[JAS]

I have a Garmin Zumo XT GPS. Lately, I have been charging the internal battery and using it on my e-bike. With certain chargers, it never seems to fully charge the battery. For example, a charger with a Mini USB port is rated at 5 volts and 1 amp and never seems to fully charge the battery. However, if I use a charger that has 5 volts and 2 amps output, it seems to fully charge the battery. How can I determine what is needed for a given battery? Is their a formula?