I have this one from ZMI https://www.amazon.com/ZMI-Pixelbook-Nintendo-External-Powerbank/dp/B072BD98CM/ref=sr_1_3?ie=UTF8&qid=1537989136&sr=8-3&keywords=ZMI+USB+PD+Backup+Battery It can charge the Pixelbook at the same rate as the charger it comes with which is nice. It gets one full charge from dead from it.

​

Some of the Anker and Aukey ones can't charge the Pixelbook if it is dead for some reason. You have to change a setting that pops up on the screen to use them. To me that defeats the purpose of them then so I passed on them. I don't think the Omnicharge one has that problem either but it is like 3x the price of the ZMI one which is why I went with it instead.

My company makes a 45W PD enabled backup battery that works well with Spectre x360 13. We specifically tuned our battery for Spectre. Our battery offers the following voltage/amperage profiles: 5V/3A, 9V/3A, 15V/3A, 20V/2A.

Disclaimer: I am definitely biased, as I'm associated with the company. Please see for yourself: https://www.amazon.com/ZMI-Pixelbook-Nintendo-External-Portable/dp/B072BD98CM?ref=ast_p_ei