I bought a 4-pack of AA "Energizer Ultimate Lithium" batteries to test. I'd like to use lithium batteries on backpacking trips because of their longer life, lighter weight, and better performance at low temperatures.
Weights on my scale (2 AAs):
Energizer Rechargeable: 54 g
Duracell Alkaline: 49 g
Energizer Ultimate Lithium: 29 g
First test. The first 2 lithium batteries lasted about 5.5 hours. They died at the beginning of my third day hike. I called Garmin tech support to discuss. They suggested a hard reset in case some unknown glitch was causing a battery drain. A bad battery is also a possibility (though I'm skeptical of that). Backlighting was never used. I had both the GLONASS and WAAS options turned ON. I know they drain the batteries faster.
Second test ended today. GLONASS and WAAS were OFF for the duration of the test. Backlighting was never used. Over 6 weeks I did 17 day hikes ranging from 2-5 hours, plus two 1-hour bike rides for a total of 57.1 hours on the 2nd pair of batteries (I kept track of the times in a spreadsheet to keep it accurate). I'm pretty impressed with 57.1 hours. I didn't expect over 40.
GPS alone gives me accurate tracks and decent position "lock on" times I think.
Third test. I might try turning GLONASS back on to see if that caused most of the drain in the first test.