If only an "18650" was always the same! Despite the name, they're never exactly 18.0mm diameter by 65mm long. Some have protection circuits built in, some don't. Some have solder tabs, some have wired connections (2-pin or 3) and some fit into holders the way your garden variety AA battery does. Then there are a variety of capacities and charge curves. 18650 Li-Ion is not a standard the way that a AA alkaline is. The core can may be "standard" but by the time that gets packaged into a cell in a product, there are an infinite variety of options.