In my eTrex Venture Cx, I use regular AA. It will last the whole trip to NRT, TPE, HKG, PVG, KLA or SIN with some on the ground use.

The FRA-SFO track I posted above was taken with an older Garmin eMap. It also use AA. The proprietary memory card has only about 128MB capacity. I left this unit in taxi's etc several times but was able to get it back. On that FRA-SFO trip, I had an exit row seat and placed it in plain view on the door window. FAs and passengers came and went and never said a thing the whole trip. Some took a look with interest.

While traveling in the U.S., I take a Garmin Quest. This unit has internal lithium rechargeable and I bring a car cradle. On the plane, it has a battery life of about 20 hours. This unit has non-removable internal memory of about 110MB.