I found an old Black and Decker Snake Light at a thrift store last year, I thought they were kind of cool when they came out in the 90's. I quickly modernized it with a 1w LED in it immediately which improved the battery life, but it still used plain old C batteries, still I passed on the later Versa Pack version at the store.
Not too long ago I was in a discount-discount store and I saw a Power Bank that extends the life of a phone and I had an epiphany. For $5 bucks I picked up the rechargeable battery that charges batteries.
Step 1: Tools and Materials
Step 2: Disassemble Power Bank
With the small screwdriver I carefully pried the case open, it just snaps off, no glue. I found a 18650 3.7v Li-ion 1800mah cell with a PCB on it.
(You really need 5v to charge a phone, this just prolongs the life of the phone in an emergency, when you can't find a wall outlet.)
Step 3: The Build
Before I mentioned that I put a LED in place of the old krypton bulb or whatever was in it. It's a cheap 1w LED on a 20mm star board with a 3.2 forward voltage. I didn't even use a resistor with it because the C batteries made 3v.
Since this battery is 3.7v I would need a 1.5 ohm 1w resistor for this 1w LED bulb (1w resistor would dissipate heat better than a 1/2 or 1/4w). A 24 ohm resistor is the smallest I had at the time I did this. I want to try a 5w bulb so I would try a 1 ohm 1w resistor.
Here's the calculator I used... ledcalc.com
They also have direct fit LED flashlight bulbs like the Rayovac brand you can pick up a Walmart. I just don't know what the specs are on them though.
Step 4: The Test
Well it works as a rechargeable flash light now while keeping the vintage looks, which is what I wanted. It's also nice that I can use a ubiquitous micro USB to charge it. And as a side affect doubles as a phone charger / prolonger.
I wouldn't use the light while charging the battery, I'm not sure if the Power Bank would like it in the long run. Don't leave the light with its Lithium-ion battery in the blazing sun either.