[bapbap]

I am not a battery or electronics expert but my understanding is that it is not ideal to simultaneously charge and discharge a LiPO battery, so most don't support passthrough.

I use a RAVPower Xtreme (26800mAh - though I'm not sure if I believe that rating) to power a dashcam and had similar requirements to you. However, I've implemented a DPDT relay and a 12V boost regulator to switch between the battery pack and car power (ACC), depending on the ignition status of the car. So when the car is on, the battery is being charged only and the dashcam is getting power from the car. When the car is off, the battery is only being used to power the dashcam.

I've not tested it with a Pi but there is not an interruption in power long enough to disturb the dashcam (the relay spec is 8ms operate time). The battery pack is charged using a 2A 5V charger. I can't properly assess the charge status/history of the battery as it only has 4 LED's and I don't want to take it apart but so far my commute and other driving is long enough to keep it ticking over.

[disposeofme123]

Its definitely possible, think about the circuitry in your phone and how the batteries work with that.

As far as I'm aware, most modern phones can't turn on without a battery pack present in the phone, even when plugged in, which would suggest that the juice is actually coming from the phone battery at all times and not direct from the charger regardless of charging state.