maybe I ran into the same problem Mike had when I first tried the stabilized regulated 24 volt source…see below…
I like to sit in the 'plane with the battery Switch ON and play with the avionics - it’s a great way to learn - more real than the PC sims, and less stressed/hurried than in actual flight (especially IMC).
"I bought the Schauer Four Seasons Fully Automatic 24 Volt 20 Ampere battery Charger through Aircraft Spruce & Specialty Company . It cost about $200 (there’s a 10 Amp version, certainly ample, for $124). It’s in the Electrical Section of my printed catalog, on page 350. I also bought a Lexan plug - AN2551 - from the same source. This is the plug that fits into the external 24 volt source on the side of the airplane cowling.
I connected the charger to the airplane, switched the battery Switch to ON… and nothing happened.
The problem is that both devices (the charger and the airplane) are too smart. The airplane has an “External Power” relay that won’t connect the outside power source until it sees voltage is present; and the charger won’t provide power until it senses at least SOME battery voltage. In effect, both devices stand there saying “After you” - “No, After YOU”… etc.
An obvious solution is to use the charger the way it was intended - i.e. hook it up to a 24 volt battery (or 2x12 volt batteries in series), and connect those batteries in turn to the airplane. This arrangement has some advantages - for example, you could connect the battery to the airplane without the charger, provided it is charged; this would be very useful where there is no AC available.
However, I didn’t want the extra hardware (battery), so I modified the charger to add a pushbutton “trigger” to feed just enough voltage to the sense circuitry to convince it that it’s OK to start charging.
Now, after hooking the charger to the airplane, I switch the battery Switch to ON, and press my Trigger pushbutton, and off it goes.
The actual modification is very simple. If you’re interested, send an email, and I’ll fill you in on the details.
From what I can see in the wiring diagram, the relay that switches in the Ground Power Connector is a standard 24 volt (not 27/28) relay. In any case, ANY charger that hopes to charge a 24 volt battery has to be able to provide some “head voltage” - they typically do provide 27 to 28 volts with no load. What is true, though, is that many of them provide pretty dirty DC - very noisy and unregulated.