17 March 2018

Portable 45W Power for the Google Pixelbook!

I've been using a Google Pixelbook as my primary computing device for five months now. The Pixelbook has a ton of great qualities: it's unbelievably quick, has an incredible build quality, runs Android apps, packs a fantastic touchscreen that folds flat for tablet usage, and (with developer mode + crouton) can handle just about everything I need to do on a computer. I have also been pleased with the battery life; depending on what I'm doing, I tend to get 6-7 hours of use out of a charge.

Of course, even with great battery life it doesn't hurt to have backup power available - and the convenience of dual USB-C ports should make that pretty easy to accomplish, right? I should just be able to grab any power bank which advertises 30W or 45W USB Power Delivery and get started charging on the go, right? 

Unfortunately, it hasn't worked out to be that easy. I've been quietly and casually testing USB-C PD power banks for a month or two now, trying to find one which worked reliably with my Pixelbook. I don't have the skills, knowledge, or resources to do compliance testing like Nathan or Benson, but I used a Plugable USB-C power meter to monitor the current flow and make sure that it didn't drift wildly out of spec. I probably tested a half-dozen power banks before finding one that actually works to my satisfaction. 

I didn't encounter any that I'd consider dangerous; they just didn't work. Some would only draw power from the Pixelbook. One wouldn't pass any power when the Pixelbook was connected. One apparently provided enough power to activate the meter but the measured flow was 0V @ 0A. A few would constantly reboot; one only did so while the meter was connected and seemed to charge the Pixelbook okay without the pass-through meter in-line but that was still clearly incorrect behavior. Without a PD sniffer I don't know why these power banks didn't work but my hunch is negotiation problems. 

All of which brings me to the reason for this post: I found one that works! 


This is the ZMI PowerPack 20000, a 20000mAh battery pack with 45W USB PD output. I was a bit apprehensive since I hadn't heard of the brand before but this has succeeded where power banks from brands like Anker, RAVPower, and dodocool let me down.

While testing the ZMI power bank, I measured a pretty consistent 14.4V @ 2.7A - which exactly matches what my meter reports from the 45W wall charger that came with the Pixelbook. 


It charged the Pixelbook from 67%-87% in about 25 minutes. (The charging rate dropped from 45W (14.4V @ 2.7A) to 30W (14.4V @ 1.8A) somewhere around 85% so that did slow things a little bit. Again, I've observed the same behavior with the original charger). 

As a bonus, the ZMI power bank also has a USB 2.0 hub mode which works great for USB mice and USB storage drives - while still charging the Pixelbook at the appropriate rate.

In subsequent charging tests the ZMI power bank has continued to perform predictably. Again, I'm not able to comment on any of the PD negotiation traffic or USB spec compliance but it seems to me that it is doing exactly what it's supposed to. I haven't had any problems with the power bank trying to charge from the Pixelbook, failing to charge the Pixelbook, or trying to push an unexpected voltage/amperage combination. 

Consistent 45W charging in a small form factor with the added benefit of USB hub functionality at a reasonable $70 price makes the ZMI PowerPack 20000 an easy recommendation.