I only assumed (I know the old joke) because the problem started after a few engine starts and a few times charging with the motor running. Also a club member was of the opinion that as the car battery is being charged the voltage is rising and falling and that some chargers don't like that. (I did have a charger before this one that said not to run the car engine, I forget the brand name.)
Other than those things I can not say for sure why the charger died. I really did get my money's worth out of this charger. It has really been worked. I have charged hundreds if not a few thousand times in the past four years with it. Even with it's demise I would not hesitate to get another Hyperion charger. I am using Hyperion lipos that charge at a higher rate than a lot of other brands. Some of my larger lipos start charging at 20 amps. That is why I'm draining my car battery low enough to cause the lipo charger to say low voltage input. But that is after a few hours of flying. Maybe I'll get a better car battery in the future.
So, in the end I'm not totally sure why it died but I'll disconnect my charger when I run the car engine to top off the car battery just to be on the safe side.
Thanks for everyones opinions.