I've got an ASUS Chromebook Flip C100, which comes with a 12v, 2A charger (i.e. 24W), using a custom jack.

I've also got an Anker PowerCore 13000, which doesn't provide enough power to charge the Chromebook (2 ports, max 3A, 5v; so 15W), even if the cable (and voltage) was compatible.

Does anyone make an external battery pack which would be able to do this?

There seem to be quite a few 18000mAh "jump starter" battery systems on Amazon. Those would have more capacity than the Anker, but dunno if it is enough.

A number of them include a reasonable complement of laptop/charging adapters etc.

Eg. DBPOWER 600A Peak 18000mAh Portable Car Jump Starter.

Several of the newer Chromebooks support USB-C charging, allowing a number of external USB-C PD (PD = power delivery) batteries to do what you want. You certainly get a monster like the Anker PowerHouse that supplies 120VAC, but that's probably not what you have in mind.

Otherwise? I did an Amazon search for "12v 2a battery" and actually found things. Here's one. Would you by a weird product from a no-name vendor if it had specs that sounds in the ballpark of your needs? Hypothetically you might only need to wire up a cable to suit.

Sure, but I'm not looking to get a newer Chromebook; I like this one, because it's tiny.

I realise that having a tiny Chromebook and then asking about an external battery pack might seem like a contradiction. Here's my use case:

When I'm on call (pager duty), and I want to go out, I usually just throw my phone and my Chromebook into a bag. However, sometimes, the Chromebook's got a flat battery (because either it isn't always going into standby properly; but that's another story, or I'm simply misunderstanding whether it is supposed to be going into standby).

At that point, I want to throw some kind of external battery in there, so that I've got juice. The Anker powerbank works fine with the phone (and with tablets, etc.), but not with the Chromebook.

Actually, thinking about it some more, I don't necessarily need the Chromebook at all. All I actually want is an SSH-capable terminal with a reasonable keyboard, and a battery life that I don't have to worry about. Maybe a smallish tablet with a proper keyboard stand would work better...

Many of the "jump starter" battery packs (per above) advertise 2nd generation "smart" chips for the USB outputs. They claim to provide up to 2.1amps to pretty much any device that supports/negotiates it. Plus the 19V output for direct connection to a notebook's charger input.

Dunno how well any of that works, but they do work well for starting impossibly large engines with dead primary batteries in the dead of winter here.

Or a largish phone with an external Bluetooth keyboard and a decent ssh client.

No.

I've got a Samsung Galaxy Tab S2, and a Celicious Keyboard Tri BK01 keyboard (which is superb, btw).

But they're separates, which is a faff. Hence the (attachable -- which I wasn't clear about) keyboard, rather than a separate keyboard.

It turns out that the answer to USB-C charging the C100 Chromebook is to... get a C101 Chromebook instead.

It's unclear to me whether my Anker 13000 (5v 3A) will charge it, though: the included power supply offers 15v 2.2A. Does USB-C allow that kind of thing?

I suspect (maybe) not, based on the specs for the Anker 20000 PD which says "some USB-C laptops are not supported as their input exceeds 22W", and the spec for the C101 says that the PSU outputs 33W, which implies that the Chromebook might need 33W. However, also on that page, it says "support power delivery".

I have posted in the past regarding the gifts of potential confusion that USB-C brings. I suspect it will be a long time before it becomes clear to the majority of ‘technical people’ how the USB-C pieces fit together, and how device capabilities and requirements need to be presented.

It is easy to be certain that it is unclear how a lot of these USB-C devices will interact, especially when from different brands and different kinds of things.

For the non-technical ‘majority’, will USB-C device and cable interconnectivity ever be anything other than generally unclear?

The simple use cases, like power brick and smartphone with a USB-C cable, eventually most of those will work for most people. Beyond that, seems rather lumpy.