This pin is the output of the on board 3.3V step-down switching regulator (Torex XC9258A). The regulator is rated at 1000mA max. When using this pin to power other devices or peripherals remember to budget in the current requirement of the Xenon first. This pin can also be used to power the Xenon in absence of the USB or LiPo power. When powering over this pin, please connect the ENABLE pin to GND so that the on board regulator is disabled.
A coin cell? A CR2032 at 3V just barely makes the mimimum on the recommended operating conditions. If you really want to try, then you could hook up the CR2032 directly to 3.3V. A CR2032 is rated at about 225mAh so you might get up to 10 hours on it without any peripherals attached but if the battery gets weak and voltage dips, you are risking a lot. You might dip under 3V very fast depending on the current draw especially since you are below 3.3v. You can’t yet use low power modes so this is strictly an on or off proposition. I would rather suggest a small LiPo using the connector. You can get some fairly cheap LiPo batts on Digikey.
As the data sheet states, do not use 3.3 V pin for powering the boron. It just won’t work. Use a lithium ion battery.
This pin is the output of the on board 3.3V step-down switching regulator (Torex XC9258A). The regulator is rated at 1000mA max. When using this pin to power other devices or peripherals remember to budget in the current requirement of the Boron first. Unlike the Xenon or the Argon, this pin CANNOT be used to power the Boron.
I was basing my thoughts on the Nordic, (which I have been using with some success). As an example their DK has a button cell which we using to test range and power consumption. That being said if I understand you correctly true “BLE” is not supported on the Xenon. The logic that I have been taught is to write minimum code with battery consumption being your foremost thought in mind.
BLE functionality has not been exposed for us end users yet. This is too early into the firmware development to have that up and running. Wait till early to mid year, 2019 for BLE. That is outside the original question of how to power the Xenon board but there is no included button cell holder on the Particle boards. You’ll need some sort of external holder. Or, get an Adalogger Featherwing. That’s the beauty of choosing the feather form-factor… there’s already a rich offering of featherwings on the Adafruit website. Add a couple female headers to the Adalogger, and you’ll have a coin cell holder without a lot of hassle. You can also get the INA219 featherwing to do your power measurements. I picked one up on pre-order but haven’t done anything with it yet.