Originally Posted by
mwatkins
Personally I prefer not to mount on my helmet lights that use lithium based rechargables which can
explode.
Yes, there is a chance that your LiIon cell can explode. There is also a chance you'll get hit by a car on your ride. The latter is probably more likely to happen.
Use quality protected cells and charger and you should be fine. Use cheap UltraFire/TrustFire cells and chargers and you need to take more precautions. I have UltraFIre cells and the WF139 (newer) charger and I don't leave them alone while charging. Even when I use my Pila charger and AW or Pila cells, I don't leave them alone to charge. Also, I never use more than 1 x 18650 cell in a flashlight. This reduces the chance of an explosion significantly. Having dis-similar charge states in LiIon cells is dangerous.
I helmet mount my Naibender SST-90 LED 3-mode drop-in module in a Surfire 6P host bored to accept 1x18650. 600lumens OTF (out the front).