The older gps12 had an internal battery that needs to be kept charged. If the 4 batteries inside the garmin run flat, or are taken out for the winter, then the internal battery can go flat and you loose your data. I think the standard charge for Garmin to look at the gps and replace the battery is around Ł45, but if you ring them they will tell you. They are a helpfull company to deal with.

I think someone told me that the later 12's stored the data in a different format and did not need an internal battery.