Trying to make a tablet but stumped on power supply

So currently I’m trying to make a tablet using a Raspberry Pi model B (though will probably get the latest version before its finished)

Rather than go with the official touchscreen I went for a much larger and in my opinion nicer touchscreen.!/14-HDMI-interface-LCD-with-capacitive-touchscreen/p/36248806/category=3094861

Problem I am having right now though is powering the whole thing. As obviously it’d be nice if it was mobile but I’m unsure how to approach the power issue.
As not only must I provide the pi with power but also provide a 5.5volt output for the touchscreen seperate from the pi. While the touchscreen came with the means to power the pi from the same linein, the board its all on gets disturbingly hot when in use (need to check what the actual temp is) but this could possibly be resolved with a heatsink/fan combo.
And if possible provide a means for the pi to know how much charge is left in the battery pack.

Any input or direction to useful information would be much appreciated here.

How much current are we talking about? There is this, that will give you 5v at up to 2A. You’ll need some beefy batteries to feed it though. And how much run time do you want or need?
Looking at the link you posted it looks like that display wants 6V? That’s throws a wrinkle into things.

According to the manufacturer its fine with anything between 5.3 to 6 volts. But yeah 6 volts is obviously preferred.
As I noted though it came with a nice little gadget for running the pi off the same power in as the screen. But the gadget gets a tad hot.
As for how much runtime. Well I’d prefer it to be measured in hours, but more I look the less realisable that sounds. Though with the relative size of the screen to the pi, I should have plenty of space for batteries in the case.

I am in general though getting the impression I’m going to have to figure out enough to design something to do the job.

Just a heads up but a Pi 3B+ will likely draw more current than a B, or did you mean a 2B or 3B?
Have a look see here,
All the Pi related battery power supplies I have seen like the Adafruit PowerBoost are 5V. I have a PowrBoost 1000c driving a Pi A+ with Sense Hat and other odds and ends. I went with a 6600 MAH battery. It’s on the big side, 3 round cells. I have a large rectangular box case so its not a problem. There are flat LIPO’s but they top out around 2500 MAH.
I’m not trying to dissuade you from your project, just pointing out it may be a bit of work getting it done.
This buck converter says it will work with as low as a 6V input, Assuming you can find a 6V battery supply solution.

So its looking like I’ll have to make something then. And its a 3B. Though thinking of grabbing a 3B+

I guess the main question then is how do I get the pi to read how much charge is left in a battery?
How do battery powered devices typically do this?

That I don’t know, measuring the actual battery voltage I guess? My PowerBoost gives a low battery indication when the LIPO battery discharges down to a set point. There isn’t any indication on how long that will take though.