Not much of a schematic, as it is just a 12V power station.
So, each unit would be a specific diagram, depending on what one wants to do.

The circuits one would want to consider are:
A charging circuit to keep the 12v battery charged automatically.
A few Bannana outlets to let one have different sources
A Reverse polarity sw , [DPDT wired X]
And an Ampmeter than can be switched on for checking load draw .
And a couple of fuses for the unit ....one for 12v draw and one for charger 110 V supply line

As I mmentioned , most of these features are already in those Jumper Paks and they are also a handy item to have anyway, so that wouyd be the simpliest way for the average DIYer. i ACTUALLY SEE THESE GET THROWN OUT B/C THE BATTERY CRAPS..SO THEY ARE THE PERFECT STARTING CHASSIS FOR A pOWER STATION B/C THEY HAVE THE BAT CHARGER CIRCUIT ALREADY IN THEM, ALOMG W/BAT COND led ..SO A COUPLE OF WIRING CHANGES AND YOU ARE ALL SET.
Wooops Cap lock again and I'm aware of it OK??????????????????????

Also , to simplify the circuits, one could use different 12v out bannanas for polarity and Anp draw leads ..that will eliminate any switching/switches...you just label each outlet +/- or Amp 12V Out.

__________________
A Dalton