10,5V to 13,8V DC is within the secure range. Would not recommend to use more than 14,5V…
Same question: I would like to use Shelly 1 in my car... But when it chargibg from generator (in working engine) it is above 14 VDC (14.3, 14.4 Volt)...
Could I use it for? Or is there any (other) solution?
Would not recommend to use more than 14,5V…
Hence, with 14,4V you’re still in the secure region… 
No kidding - you‘ll find some (not very much, but some) examples in the forum where a Shellies were used in a car. Give it a try!
You may use a diode (1N400x; x from 1 to 7) to add 0,7V more electrical strength. Arrange the diode in the lead for power to the Shelly 1 - cathode (ring) towards terminal N, Anode tied to plus pole of car battery.
