https://www.pi-supply.com/product/pi-ups-uninterrupted-power-supply-raspberry-pi/

Pi UPS

Pros: 6AA batteries, so the raspi can run in a powerless scenario for a long time.

Pending: does this device charge the batteries when power returns?

do it yourself:

http://www.repairhub.co.uk/content/resources/raspberry-pi-battery-backup