Battery Charger

I did not know that you can actually buy 18650 cells that has built in electronics for protection. The technological development must have sneaked up on me again. 8-)
When I was working with battery electronics ten years ago, the 18650 was a popular choice for battery packs because production machinery was available for this type of cell, it was cheap so it was used in huge quantities even in large batteries. Every cell had (has) a small PTC disc inside, at the positive terminal. When there is overcurrent this PTC resistor heats up and becomes resistive, and this limits the current. For high power cells that allow more han 5A current, the PTC disc had to be replaced by a metal disc, to avoid tripping the overcurrent protection of the PTC disc. The next line of defense was the connections to the cell, they would fuse open. If overpressure built up, the positive cap usually would rip open and that disconnected the cell but unfortunately the spark then ignited the fumes. One weakness of the PTC disc was that it only has 20V rating so when 18650 cells is used in battery packs you have to be aware that when one PTC trips, the entire pack voltage appears across that PTC disc. this can cause arcing inside the cell and is a hazard, so all bettery packs need electronics with overcurrent protection etc. and many other safety functions such as cell balancing and disconnect MOSFET to protect against deep discharge and cell overvoltage.