While I’ve seen plenty of swollen and deformed phone batteries, I’ve never personally seen one that has burned. Obviously it’s happened in the past with certain phone/battery models, but I’d imagine that it’s actually very rare now days?
On the other hand, I have seen cheap 18650s spontaneously start smoking even when they weren’t plugged in to anything…
I would have hooked the smartphone to a small solar panel. The natural daylight cycle would have made sure that the smartphone kept having charging and discharging cycles.
I doubt the traffic hitting it would be sufficient to drain the battery overnight.
So the Fairphone 2 runs on just a USB cable with no battery inside?
Why not just have the charge controller "unplug" it if the battery is full?