The charging is activated by a switch in the battery compartment, not the battery setting. When the switch is depressed the option to select battery type is not available.
Not quite correct there. The switch is only used when using Garmins own double battery. If using 2 separate batteries the switch is inactive because it is not depressed. Unless you do a modification. i.e use a bit of plastic under the batteries that holds the switch down and then the batteries can be charged through the gps unit. Although the said method is a bit iffy.