The voltage depends upon the design of the battery pack. I have battery packs from 3.7V (single 18650 Li-ion battery) to 12V (lithium polymer battery intended for CCTV backup). A lot of the newer lights are designed to run on 5V so you can use the USB phone charger but there is no combination of Li-ion batteries that achieves that voltage so they have to incorporate electronics to get 5V. I have bike headlights that run on 8.4 volts and use an external battery pack with 4 18650 batteries. I'd never go back to throw-away batteries as the Li-ion batteries pack a lot more energy for the same battery weight. It's also so much cheaper to use a battery that can be recharged 500 times versus the good quality but expensive ones like Energizer or Duracell. It's a good idea to charge your Li-ion batteries on a non-flammable surface and always use the appropriate charger that comes with the batteries. If you do that, they are safe. It's easy and cheap enough to carry a spare 8.4 volt battery pack or buy one designed to switch individual 18650 batteries. A single 18650 battery only weighs 36 grams so it's not a lot to carry.