I think the fact that I don't know anymore than what I stated should be pretty plain. "I have been told" is the limit of my knowledge. I really thought maybe one of the electrical geniuses could explain it so I was hoping for a little back up. But no, I crawled out on this limb and I can sit here. Since you need some more info for your files I will add, "I have been told this several times by different people."
You might look at the directions that come with with a battery charger. Do they say you can use their product as an electrical source?
I would not use a battery charger as a power source for something like a radio. It might be different with something like a head light but I gave up on this type of electrical experiment when I was 11 years old so I haven't tried that one either.
I'm sorry if it seems I'm telling you that it was a stupid move to try and test the radio with a battery charger but that may just be the case. If you would hook up a battery now you might find out if the radio does or does not still work.
If any body wants to say that they test radios and electrical components with a battery charger all the time then I apologize in advance but I don't think it is done.
Looks like a nice Windjammer you have there, good luck with the sale. Winter time now so maybe somebody can use it.
Never memorize something you can look up.