Whoops!
A couple of years ago, I was given a broken (the case was broken, but not the electronics) B&M Ixon bicycle light, which I promptly tested, then stripped off the case and set aside for future putting into a light. Today, I decided that it would be a good time to solder it into a test harness and see if it would work. So I soldered a couple of wires to the + and - pads of the battery mounting points, attached a string of 4 AA batteries, then pushed the button to turn it on.
I immediately got a gratifyingly bright light, but, alas, this light was not from the LED, but from the circuit board, and it came along with a cloud of foul-smelling smoke. And, equally sadly, this only happened once; subsequent applications of voltage resulted in nothing but the circuit lying there sulking.
I tested the LED proper (by attaching it to a slightly-overdriven 3.7V cellphone battery) and got a whole bucketload of light (it took about 5 minutes for the receptors in my eye to recover from the overload) which I could repeat at will (but not for very long, because these 1 watt LEDs generate a lot of heat, and I didn’t have a headsink attached.) So, yay, I’ve still got a power LED. It’s not really usable yet, so it’s still a whoops.
In the worst case I can build a circuit that’s just the LED wired to a cellphone battery and a switch; it won’t be nearly as nice as one that gets power from a dynamo, but it would be better than nothing.