You could solder tabs on the 18650’s so they better mimic a typical alkaline battery cell ... he used a few strips of cardboard from a case of beer — again, this is just making use of ...

You can salvage lithium 18650 cells ... In this case [DIY GUY Chris] has gone with the ATmega328P, with a pair of addressable WS2812B LED bars to show the battery’s health and charge levels.