Exploding LEDs? I've fried a few and am still around to tell. I bought a couple strings of fairy lights on eBay and requested 12V strings so I could use a battery pack I already own or a 12V DC converter scavenged from old electronics. Instead they sent strings designed to work on 5V and they fried instantly when I flipped the switch. These strings have a resistor built into the wiring. The only difference between a 12V string and a 5V string is a higher value resistor in the 12V circuit. If you are into DIY LED projects, this site allows you to determine the correct resistor size and watts:
LED series parallel array wizard
Different battery types deliver different voltages. Carbon and alkaline batteries yield 1.5V, rechargeable NIMH 1.2V, and Li-ion batteries 3.7V. The resistor matches the voltage required by the LED to the battery or battery array. That's why some LED flashlights using a single alkaline battery (1.5V) will not work with rechargeable NiMH batteries. Not enough voltage to drive the LED. A string of lights designed to work on 12V DC will work on 3 Li-ion batteries in series (11.1V) but will be a bit less bright.