Battery Tenders do have issues...
I had 1 that shorted out and melted the case...
I took a picture and sent a message back to their corporate headquarters, but they didn't want the unit returned but they did sent me a replacement.
But a week later, their head of engineering called me and requested I see back the defective unit for their inspection and failure analysis...
Too late it was already tossed into the dumpster.
Note that 1 drawback of a battery tender that it does not address sulphate buildup which adds resistance and can destroy a unit.
So I started using a battery minder product, that sends a high voltage spike every 5 seconds to burn off the sulphate buildup.
5 years later using the battery minder, to date no problems and unit remains cool....

Just my $0.02...