I really like my Elecraft K1 QRP rig. Mine’s the 4-band model (40, 30, 20, and 15 meters). I also have the KAT1 internal ATU and the K1BKLTKIT backlit display installed. It’s a great rig for QRP CW in the shack or in the pack. Mine is a staple of my Field Day excursions.
Typically, when I hit the trail with my K1, I pack a 2-AH gel cell to power it. Works great, but the gel cell is kinda heavy and bulky. So, in a moment of boredom (I had the itch to build something, I guess), I ordered the KBT1 internal battery option and installed it.