Earlier quoted context omitted.
A fun trick is that if you're sure the batteries are in series you can sometimes swap out a pair of AA batteries for a 3.7V 14500 Lithium rechargeable and a fake AA battery (just a wire in an AA-sized plastic shell). Drill a hole in the battery-cover of the device where the 14500's USB port is and now your device is plug-in rechargeable, assuming it can handle 3.7V instead of the normal ~3.1V that comes from a pair o…
I have a headlamp for hiking with a slightly different take on this. It will take 3x ordinary or rechargeable cells, or it will take it's Li-Ion pack that is the same size as the 3 cells side by side. Designed in from the start so there are no dangerous bits. Hiking headlamps are something where you do *not* want to be left with a dead battery!
There are a handful of these on the market and they're not common in retail stores. I'm fond of the Skilhunt H150.