Don't know about that portable power supply, but "Battery Minder" now has one (110 volt) that plugs into your ground power plug and will give you 25 amps for playing with your avionics. It is only portable to the extent of the length of your extension cord though. Pretty pricey too. Think it is several hundred bucks. On my 79 Skyhawk, when you plug in the ground power plug, the ground power relay turns on, supplying power to the battery side of the master solenoid and battery, but you still need to turn on the master switch to get power to the buss and in the plane. Pretty sure all single Cessnas are like that, even the new ones. Cessna twins will power up as soon as you put power on the ground power plug like all Pipers. Your idea of a couple batteries, and 24 volt charger and ground power plug sounds like a good economical one !!