@luke My front light is a very old ebay special, that runs off a 3.7v battery pack. Originally it came with two 18650 cells, wired in parallel, which is slightly problematic. Li-ion batteries in parallel need to be matched, which is tricky, so buying matched pairs is quite expensive.
I can't be arsed doing the hard work to find two vape cells with the same characteristics, so I just check that their charged voltage is the same (enough), bung em together, and if they die in a couple of months then oh no, so anyway.
My back lights are an old Cateye set. The teeny tiny lithium cell in them gave up the ghost, so I wired an external plug to where the cell was connected to the circuit board, and run them off a single vape cell. The capacity of the vape cell is probably a dozen times the original so I only need to charge them once a fortnight or so (though since I'm paranoid about it I actually charge it every weekend). I also carry a spare cell in my saddle bag, because why not.
For charging them I use USB charging modules I buy of the internet (see below). This means that I can charge them anywhere I can find a usb plug, and it provides proper charging circuitry and overcurrent / undervoltage protection. If I wasn't so lazy I might 3D print a case or something, but instead I just hot-glue it all together and wrap it in electrical tape.