Battery Tender Issues

[Archrms]

I have battery tenders on the Cobra, a 1966 Mustang, and a motorcycle. The tenders have been on the Mustang and motorcycle for years with no issues. Last year I noticed a bad egg smell in the garage one day and traced it to the trunk of the Cobra. The battery was fried. It was the original battery so I figured it had just gone bad.

Got a new battery, and hooked up the tender. A few months later, I noticed the smell again. Drove it to the store where I purchased the battery. It was red hot by the time I got there, where they confirmed that it was fried. They gave me lip about replacing the battery so soon under warranty, and said they would not replace it again.

I threw the battery tender on the Cobra out and hooked up the one that has been on the Mustang. Less than two months later the smell was coming from the trunk of the Cobra again. I have not had it tested, but it is likely fried again.

What could be causing the batteries to be fried one after the other while on the battery tenders? I have used two different battery tenders with the same results. I have replaced the Ammeter with a Voltmeter, which was showing normal voltage before being garaged for the winter.