The internal memory battery for the Garmin GPS 48 normally recharges itself from the two AA batteries used to operate the unit. In addition, the memory battery is able to charge when the unit is connected to an external power source. Unfortunately, the internal memory battery will eventually wear out. When this happens, the unit will constantly display the "memory battery low" warning. At that point, the battery will need to be replaced, which is a job you can do it yourself with a few basic tools and a little know-how.
Pry the front and back halves of the GPS 48 unit apart using the screwdriver.
Locate the internal memory battery. It is silver, about the size of a nickel, with two green wires soldered to it.
Remove the battery using the soldering iron. Keep track of which wire you remove from the top of the battery and which you remove from the bottom.
Install the new battery by carefully soldering the wires back in place. Do not heat the new battery any more than is absolutely necessary. Be very careful to attach the correct wire to the correct side of the new battery.
Press the two halves of the GPS 48 back together.
Install new AA batteries in the GPS 48.
Switch the unit on. The "memory battery low" warning should disappear.