I checked the voltage at the battery and it was getting 18 volts and climbing!
I've isolated the problem to the AC branch circuit #2. Whenever it is on the battery charging voltage goes thru the roof.
Today, I switched batteries and had the same problem.
To rule out any possible mis-wiring, I switched all AC wires around between the other 2 breakers. Same problem, when the #2 breaker was on my battery started to cook.

When that breaker isn't on, everything works just fine. I get good DC voltage where I should, and good AC on #1 and #3. The battery voltage stays around 13V, so the charger must be working, too.
I should mention that when I received the unit, it was damaged. The mounting flanges were bent down, and the case looked tweaked. The labels had become unglued and had fallen off. The box it came in was OK, no damage, and it was the original box that had come from Parallax.
Did I get a lemon, or is there something I'm overlooking? I'm about ready to see if I can trade it for a WFCO, since so many other people here use them and seem to have no problems.