Though the data sheet for the Boron says that the maximum Vin voltage for
the Boron is 6.2V, you can actually directly power it on the VUSB pin at
12V, or even as high as 16-17V. At higher voltages, you could run into heat
problems, and you may need to limit charging current to the LiPo battery to
500ma.
It is still recommended that you are not powering your device by battery, to
use a converter to power the device at 5V, either by the VUSB pin, or the
USB port. Also, you probably should not connect anything to the USB port if
you are supplying power on VUSB (though, if you supply power via the USB
port, you can use the VUSB pin to supply 5V power to other peripherals).
NOTE: Boron only. Other devices use a different charging circuit. I recommend only using Particle's stated maximums for each device, unless you confirm for yourself that you can safely exceed those limits.
https://community.particle.io/t/powering-boron-using-12v/46838
Many automotive projects can be powered by simply using a USB converter plugged into a power port (formerly known as the cigarette lighter). But if one of those ports isn't convenient, you may want to draw power from the car battery, either directly, or indirectly through a fuse, or other car wiring. However, the power environment in an automobile can be harsh -- there are power spikes/dropouts and noise to deal with. It is recommended that you use some sort of power conditioning when powering micro-controller projects. One such device is this step-down voltage regulator, found on Amazon.com:
https://www.amazon.com/gp/product/B07D4D4Z75
This device can take in up to 24V, protects against over/under voltage, reverse polarization, etc, and outputs clean 5V up to 3A (there is also a 10A version).
For more information, see this thread on the Arduino forums:
https://forum.arduino.cc/index.php?topic=213399.msg1649634#msg1649634
When powering Electron or Mesh devices via their onboard LiPo connectors,
be aware that the Vin pin will not supply any power. Vin will only
supply voltage when VUSB is powered.
https://community.particle.io/t/powering-a-5v-device-via-electrons-lipo/45605/2
The Electron and the Mesh devices (Argon, Boron, Xenon) can all be powered by a 3.7V LiPo battery. Often, it is useful for a device to know whether it is running on battery power or not, and what the charge status of the battery is. The Electron and Boron boards both have an advanced Power Management chip built in which can report details on the battery status, while the Argon and Xenon do not, and can only report back a simple battery voltage level. This can still be useful, as you can read that value, and have your device send alerts or shut itself down if the battery reaches a critically low level (general wisdom is don't go lower than about 3.2V).