My experience with the batteries in my Garmin gpsmap 66st is mixed. I began using name brand rechargeables and then charging them externally. They would only last about a day. I switched to brand name non rechargeable Lithium batteries and they lasted for multiple days, but very expensive. I then bought the Garmin brand battery pack. They also last about a day. However I do like having the option to plug in the gps through USB in my vehicle and charging the unit in route to the next cache. After all is said and done, I like being able to charge the Garmin battery pack in my vehicle but if I were out in an isolated area away from civilization where I REALLY relied on the GPS, I would have back up lithium batteries in my pocket.