Here you will find useful information about the unboxing of a PowerShield which is compatible with the Core/Photon (focused for international users).
What is the PowerShield? Main thing the device gets the "necessary juice".
"The Power Shield, as the name implies, allows the Particle device to be powered from different types of power sources. The shield has an intelligent battery charger and power management unit along with a wide input voltage regulator and an I2C based fuel-gauge. You can power a Particle device with either a USB plug or a DC supply of anywhere from 7 to 20VDC and charge a 3.7V LiPo battery all at the same time." - Particle Documentation
Where and why you want to buy a Power Shield?
Especially the question of where can be answered simply in the times of constantly available onlineshops easy, but especially international users - outside the United States - should be well before the tax disadvantages well. Before an import fee is payable by the customs, I advise to purchase from a regional (online) dealer. In this case, questions regarding the guarantee in the run-up to regional legislation are also relevant.
Another note about the battery: also some shipping service providers are not included because it is not allowed to transport batteries in airplanes according to international regulations.
I myself have purchased the article in the Adafruit shop and was served with more than 25 Euro fees at the end of my purchase of several articles more than.
The questions about the application area I have not answered myself: a project with a photon on the back could be realized where I could still provide the Photon with WiFi, but no power is nearby. Here is the garden area (I have no power supply available in the garden). It would also be conceivable within the mailbox to signal urgently expected mailings. The best: you can combine this shield with solar-panels.
Here I refer to the excellent documentation in the online shop of Particle itself and its documentation. Of course, I am in the greasy nap right at the beginning stumbled: everything blinked wild and came to rest, as I pulled off the attached battery again and again, check the note under the documentation part.
The best: I2C support is given and you will find a libary here for the Particle WebIDE.