The only Garmin units that have rechargable batteries are the two wrist units they make. The Forerunner 201 and Foretrex 201. I have the Foretrex 101 which uses 2 regular AAA's, I get 15 hours out of a set and I keep a LARGE pack of replacements in my backpack. My Sony Clie PDA has a rechargable battery but with the color screen and always on backlite it doesn't last more than a couple of hours of constant use.
Rechargable batteries get a "memory" if they are not completely discharged before recharging, this shortens their total lifespan. We warn our laptop users where I work to occaisionally completely drain their batteries and then recharge them (about once a month) we have good results with this practice.
I'd rather trust in replaceables! IMHO, YMMV
There is a pleasure in the pathless woods;
There is a rapture on the lonely shore;
There is society, where none intrudes,
by the deep sea, and music in its roar;
I love not man less, but Nature more.