For Christmas last year I bought my Dad an LED lightbulb to replace the incandescent lightbulb in his work flashlight. The incandescent bulbs kept breaking whenever he smacked it against a pipe or a wall (he's a plumber) and at a couple bucks a pop he found himself spending a lot of money on lightbulbs. The LED lightbulb I bought him seemed like a great solution. It was a little pricey but I figured it would last a long time and totally be worth the price. One bulb would last longer than a couple traditional incandescent bulbs. However, the LED bulb didn't last more than a couple months before the lights started flickering.
I figured the schematic on the circuit board wasn't designed very well and so I gave it shot myself. I designed a new circuit board, four strings of five LEDs in parallel each with a 68 ohm resistor (four in total). I calculated the input voltage from the total charge of the battery, approx 20 volts, and used a nifty LED calculator online. So far so good. The light isn't quite as bright as I would have hoped but luckily BatchPCB threw in an extra board so I can do a test with some brighter LEDs.
In case you were curious, it was installed into a 18V Ryobi battery powered flashlight (the battery pops into a hand drill, hand vacuum, electric saw, etc.). I soldered long, thin male headers to the power and ground leads of the circuit board and used a custom bulb base from the older broken bulb that accepted these headers. For the next bulb, I'll have to cobble together something different.