robust design
Decades age I did design engineering. Down the hall was David Sun who became a zillionaire as Kingston Memory. Good for David. But I had the better real estate in the building though, a glassed in cubicle overlooking the lab area. Lots of talent there. Maybe it was part company culture, or specs or instinct, but the products I did, I designed to be overloaded, over voltaged , over temp'd and maybe take a lightning zap or two. It is part of good design
The FAA is of like mind with their TSO's . Critical to flight electronic accessories must be able to survive several kinds of serious electrical insults without going toes up. A buzz box charger across a starter battery should not cause any serious transients. Starter batteries are excellent protection for anything on the line. So long as the battery is on line in parallel with, in this case, an Emag, transient damage should be unlikely.
One important exception is that using any unregulated charger on a very dead battery with little load will likely get you 18 volts or more as soon as it is connected. Maybe that is where the problem is. The Emag may not be able to handle a continuous overvoltage. Can it?.
An over volt event in the air is not unheard of. It shouldn't turn your airplane into a glider. I would think that a design goal for electronic ignition should be operation at a very minimum of 150% over voltage. The thought of a runaway alternator taking out your ignition systems - cascading failure is not good.
I judge from reports here that the E/Pmag is a pretty solid product. However, this thread is about how delicate the device is. I am concerned for a product that should be reliable and robust to the point of being nearly impregnable to damage from something like a charger
The good news is that everyone can replace their shabby HF charger with a regulated power supplies that can be used as bench power supply, battery charger or as stand-in for an alternator while building/testing. They are available for about $30 on Ebay, etc. I got one about 3 years ago and use it as a bench power source. It is rated at 15 volts 27 amps and ~100 to 250 VAC input. A tiny pot in the corner will adjust the voltage ~11 to 18 V. It has the good stuff like OV and foldback overload/short protection and puts out near pure DC. Recommended.
Several sources for similar. Here is what they look:
https://www.meanwell.com/webapp/product/search.aspx?prod=LRS-350&mws=731CEC2D6F
Ron