We had 1 Battery Tender short out internally and actually melted its case..
We called Battery Tender and reported this event, they sent me a new one no charge. Then 2 weeks later they called me back and said their engineers wanted the defective unit back for investigation.. Too late already thrown into the dumpster.

Anyway after that dangerous incident with Battery Tender I found a device called Battery Minder, works much better than the Battery Tender. Plus it has an instantaneous high-voltage spike charge to burn off the sulphate than can form on the cells and destroy a battery..

I highly recommend the Battery Minder, I use (3) of them no problems..

Here is the link if interested..
https://www.amazon.com/BatteryMINDer-Model-1500-SmarTECHnology-Motorcycles/dp/B00D7HZ6FC

Battery Minder works. I have used Battery Tender, Schumacher. And others. Then I found this one. The desulfator works. My last Battery Tender was junk. Car would sit for a month or 2 on the Tender and I still had to charge Battery. Found Battery Minder and no more problems. I threw out the junk Tender and started using the Minder, same Battery and never had a problem again. Going on 3 years same Battery.

Just my $0.02...