+Russell Holly, actually what you are describing is a common problem but it is actually a flaw in the simple Type-C battery pack and not necessarily the phone.

The battery pack, if it was implemented with USB Power Delivery and is connected to a USB PD phone like the Pixel, could identify that the other party is a dual-role device with a small battery like a phone and not a fixed power source like a wall charger and hold-off on draining from the phone.

If i were to design the pack, i'd allow the battery pack to just draw enough power to boot itself up until it can recognize the other device is a dual-role device without its own external power source using PD, and then stop charging. That way neither phone or the battery pack charge.

We could make the phone behave in the way you want... such that it only ever acts as a power sink until you explicitly ask to turn on power source and host mode, but that would mean that if you wanted to plug in a USB thumbdrive, or a USB accessory like, oh I don't know... USB-C digital headphones, they WOULDN'T work unless you explicitly change some setting first. You'd not even get any notification when you plugged in your USB-C headset because the port is fixed in sink-only mode, and it needs to supply power to the headset before the OS can even identify it as a headset.

These corner cases are hard to solve, but simplistic solutions like "phone always charges itself, never supply power" have pitfalls too!

There are really clever things that we can do if both phone and battery pack support Power Delivery.
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