You can also use NiMH batteries as they are available in AAA size. They won't run for quite as long, but does not require any modifications to be done on the headlamp.
@@winniepeg2020 its a fun way to learn the way things work and you learn to upgrade things in general, it’s nice. The fact that you save money helps all this out.
I've done several of these myself, and like you, opted for external mounting! But I found that recharging them inside the unit like a phone doesn't work bc phones have circuitry to handle the charge properly. Also for a crank type LED lantern the li-ion is brighter and stays charged longer. I don't think it charged faster bc the li-ions have more capacity.