If you’ve got some 18650 Batteries then your going to need a charge, i opted for a cheap one that included 2 batteries from eBay. It charges them for around 4 hours in which time the charge light goes from cherry red to a more orange / red.
My batteries are only lasting around 1 week on full power for around 15/20 minutes of use. and towards the end of the week you can tell that they are starting to die off, as there is less output from the light.
If you find your light won’t cycle through modes properly then its more than likely a weak battery over a faulty light unit.