While I'm at it, this thing started life as a Weller BP650. I made it probably over 12 years ago. Recently it started flaking out. I thought it was giving up the ghost... perhaps because I soldered to the battery case with plumber's flux (12 years ago, I didn't have a bottle of RA flux, and I washed it best I could). But I found my spare tips, put one in, and it's still going strong.
You can see I just spot-welded the batteries together with solder beads. And there's a little bit of corrosion, evident. If this ever shorted, I can only imagine the flames shooting out the sides.
(These are salvaged cells, so I'm not positive they have safety fuse, but the positive terminal is pretty well protected).
I wish I remember what the white label is at the bottom. It's either faded away, or it was always a blank piece of Brother label? I imagine it said "4.2V" at one point.
I think I did pretty good on the energy density. I've never run out of batteries, anyhow.
Hold the button for 2 seconds to turn it on (the heating starts the instant you press, but it won't latch unless you keep it held for the full 2 seconds). It start up with full 3.6V power for the approximate time it takes to reach 200C. (Or if the iron was already hot you could press the button to immediately enter standby). Then it switches to 25% duty cycle standy, which is about what it takes to keep it at 200C. Red/green LED indicates the status. Tapping the button will give full power for about 2 seconds, or you can doubletap and hold to keep full power on, indefinitely. It shuts down after 2 minutes, if unpressed. Or you can hold the button down for 2 seconds to shut it down. It's very simple, but it works great on protoboard or SMD stuff on double-sided pcb.