I found a couple vintage flashlights on eBay and replaced the guts with a high powered LED and a rechargeable Lithium Ion battery
It looked bigger in the picture...
Jokes aside, I had originally planned to cram a lipo charger into this guy as well, so all I'd have to do is unscrew the rear cap and plug in a micro-usb. After the thing arrived I realized this was unlikely to be possible unless I wanted to spin my own board. I do not want to spin my own board, so the I'll have to remove the battery and charge it with an external charger like a peasant. I'll leave it as a future upgrade path if I come back to this later...
For this particular flashlight, the brass casing is part of the conduction path - the negative terminal on the battery was originally 'connected' to the casing via a spring in the back cap. The switch, in turn, allows current to flow from the casing to a small brass tab on the inside of the casing, which was in direct contact with the side of the bulb. The positive terminal of the battery was pressed right up against the bottom of the bulb.
The spring in the back cap was completely broken off, and there was some pretty substantial corrosion on the interior preventing good electrical contact. The switch would slide, but it would never make electrical contact. A healthy dose of rubbing alcohol, a wire bristle brush, and a wire tube brush took care of most of those problems.
A handful of challenges presented themselves:
To replace the damaged spring in the rear cap I designed a small bracket which would act as a spacer between the cap and the battery. Battery terminals installed into either end of the cap would ensure contact between the battery and the chassis.
I designed a second bracket which would hold the heatsink and LED driver in place on the other side of the battery. I used JB Weld and thermal paste to secure the LED to the heatsink (no room for screws!), and the back side of the heatsink slots into the bracket. The bracket has a small slot for inserting the LED driver.
To integrate the existing switch, I used another battery contact positioned such that it seats against the brass tab which protrudes into the interior. Unfortunately, there wasn't enough room for the battery terminal, the brass tab, and the heatsink, so I ended up removing 3 fins from the heatsink. The forward bracket is a tighter tolerance so it has to be pressed in place. This ensures it won't fall out or move around.
The LED can take up to 3 amps, but I'm using a 1 amp constant current driver. It;s certainly bright enough at this level, and hopefully that reduces the thermal output enough for the mutilated heatsink with no airflow to keep up.