As always, be careful with 18650 secondary lithiums. Always use a pack protector, and always use appropriate chargers. Since weight and volume isn't usually a big concern, it can be cheaper to just use sealed / AGM style lead acid batteries, plus they're a lot safer and charging is a lot easier (can be floated). Its also easy to interface 1-2 12V cells in series for most DC power heads, where a 12-24V series string of 18650s will require a more advanced setup and cell balancing.