Detecting power loss (here: Pixl.js)

Something like that gets you to safety (even while recharging when plugged in):

`
                                     .---------.
                    .-----------+----| LOW R   |------- 12.5..14.5V
                    |           |    '---------'
                   .-.          |     100..120 R
                   | | 75..100  |
                   | | k        |
                   '-'         ---
analog .-------.    |           /   13.8 - 15.0          
in ----+ opt.  +----+          /_\  V Zener 3W
put    | some  |    |           |   (Good Quality)
       | fil-  |   .-.          |
       | ter   |   | | 22       |
       | (RLC) |   | | k        |
       '---+---'   '-'          |
           |        |           |
GND -------+--------+-----------+-----------­----------- GND

`

You could add additional safety paralleling the zener w/ a fast transient suppression device / circuitry (TVS; google for 'transient voltage suppressor circuit'). 'After' that you can then also use the power to feed a regulator that runs you digital electronics, including Espruino.