I have added more features to my tablet. The batteries are in parallel, so the load is shared from the screen and the RPi. A switching 12VDC to 5VDC regulator replaced a very warm LM338 5A linear regulator. The switcher is from a cigarette lighter to USB car charger. It runs very cool in the case. A battery charger was added using a wall wart 12VDC @500mA. I connected an extension cord to the wall wart and fed it outside the case, because I don`t want to carry another charger around like cell phones and iPads have. Just a cord and a wall outlet.
The cigarette lighter plug is active and has a 7.5A fuse just in case. The lighter port can be used in two ways, power supply and 12VDC charger for charging the internal batteries.
Switches have been added for the master power from the batteries to the rest of the tablet, screen power, RPi and USB hub, and internal battery charger. The switches get the user full control of th tablet.
A kick stand.
A cord reel like a vacuum cleaner.
A pair of audio speakers with amplifier.