Any updates to this whole idea of using a battery pack?
It's tricky - and the short answer is "There's no canonical answer."
I've tested several battery packs to try to get one that works (where by "works", I mean that you can both charge and run it at the same time, while the Pi is running - and, most importantly, that when power is removed from the charging side, the Pi does not crash.)
Almost by accident, I found one (one of all those I tested) that works (but see below). The on that works is called a "3G Wi-Fi Router" and cost about $15 on Amazon. Yes, it is billed as a 3G router, but a) I was never able to get it working as a router and b) it also serves as a battery backup unit.
However, the glitch is that it worked - for about 6 months (of 24x7 use). Then it stopped working (and could not be revived). It basically "wore out" - which is something that electronics are not supposed to do, but it did.
I never had any urge to replace it.