Over time the battery in the Garmin GPS will become less effective until it no longer holds a charge and the GPS is useless unless plugged in. If your Garmin battery is at the end of its useful life, you can open the GPS and replace the internal battery. Opening the Garmin is a relatively simple process that requires a few special tools. You can find most of the tools you need, including the replacement battery, by purchasing a Garmin battery replacement kit.
Unplug the Garmin GPS system and lay it face down on a flat surface.
Remove the two hex screws at the bottom of the unit using a hex screwdriver.
Insert a thin flat device such as a case puller or plastic scribe anywhere into the seam between the front and back of the case.
Work your way around and pry it apart, separating the front and back of the Garmin case. Open the case. Note that there are two wired connections between the front and back of the case so be cautious not to damage the connections when opening the case.
Disconnect the two wired connections from the front half of the GPS. One connection belongs to the battery and the other belongs to the speaker, both located in the back half of the case.
Pull the old battery off of the back of the case. Note that double-sided tape secures it.
Remove the double-sided tape from the old battery and transfer it to the new battery.
Place the new battery in the back of the Garmin case, exactly where the old battery was.
Connect the speaker wire and battery wire to the front half of the Garmin.
Place the back half of the case over the front half, lining up the edges. Press the two halves together until you hear it audibly "snap" closed. Replace the screws.