Well, I didn't get the result I was hoping for, but still, found a way of reliably running a Raspberry Pi for several hours without it being plugged in. Oh the possibilities!
Raspberry Pi UPS
The accessory is a BlackBerry ACC-50256-201 which is a spare battery pack for a BlackBerry Z10 smartphone. This portable power pack has a nice little feature which sets it aside from other smartphone spare battery packs – the ability to plug it into any micro USB device – including a Raspberry Pi.
The aim was of course, to see if it would work as a low-cost Raspberry Pi UPS (uninterrupted power supply). Did it work? The answer to this is, well, sort of…
After plugging it in, the Raspberry Pi began booting – a good sign to see it had the output to run the Raspberry Pi. Then I plugged in the charger to the battery pack (which was still plugged into the Raspberry Pi) and found that the Raspberry Pi went off for a split second, but quickly began booting again.
Once the battery pack was fully charged, it was time to see if pulling the plug would maintain the Raspberry Pi's on state. I pulled the plug, and bump! The Raspberry Pi went off again for a split second – fail!
Why didn't it work?
After doing some testing, I found that when the charger is plugged into the battery pack, it charges the battery and also supplies enough power to the attached device. By unplugging the charger, the battery pack's power manager switches the supply from the charger to the battery itself. In the process of doing this, the power is momentarily dropped. This causes the Raspberry Pi to turn off for a split second, and begin the boot procedure again.
Well, the bad news is, you can't use the BlackBerry ACC-50256-201 can't be used as a UPS, but the good news, it is a brilliant little bit of kit which will reliably run your Raspberry Pi for at least a couple of hours off-grid.