Ask HN: Battery power for Raspberry Pi two way

I am wondering if anyone has found a good USB based battery backup for a Raspberry Pi board. It's important to me that it both can receive charge (to charge the battery) while simultaneously supplying power to the board. I assume this this a common question but I can't find the wordage to google it correctly.

Ideally I'd like something high capacity but small enough to fit within a keyboard sized area. I tried looking for micro UPS, USB APC, etc etc... I know these things must exist. Any advice on where to look?