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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@tage
I found a good web page that explains the 18650 protection.
http://batterybro.com/blogs/18650-wholesale-battery-reviews/18306003-battery-safety-101-anatomy-ptc-vs-pcb-vs-cid
It is good to know that even with built in circuit board, the voltage limitation remains. the MOSFET used to disconnect the cell is a low voltage one, about 20V rating. so it is better to use an external circuit board that monitors all cells and contains one MOSFET disconnect switch instead of one per cell. (actually the MOSFET is a double MOSFET that makes it able to block current in both directions. a MOSFET always has a body diode that conducts current from source to drain even if gate-source voltage is zero).
the 18650 cells are still in heavy use, for example the Tesla battery uses them. when working with your own battery pack designs, make sure you use cells that have a metal tab spotwelded to the terminals, never try to solder wires directly to the cells!
I worked with very large batteries, some configurations up to 700Vdc and many hundreds Amperes, all using 18650 cells. the type of cell was LiFePO4 which is a lot safer but has only 3.2V nominal voltage instead of 3.7V. as the Dreamliner designers found out, this cell type is a lot safer. (I heard that after some terrible battery fires, their Li-Ion battery had to be enclosed in a fireproof container that vented the smoke and fire to the outside of the plane. this would not have been necessary if the cells were LiFePO4 type. they may burst and the fumes can catch fire, but the destruction usually is contained to one cell, not the entire battery pack).