Ah, so you were running rectified ac rather than dc. We had one clown at bae who hooked an electronic board onto the 28V line. We got sent a whole lot of boards back from the field with exploded capacitors. The poor little things had been trying to smooth the entire generator output all on their own To give the guy his due he was ex-Concorde so he couldn't really be expected to know.
Your pal using the Ground Power Unit was quite right to keep the battery on line. It acts both as a zener (of sorts) and as a large capacitor. Very necessary.