FYI, I have had a few Battery Tenders for 10 years, always hooked up.
When the BT's were (2) yrs old, there was a lightening strike near my house which wiped out
A single BT and a 4 channel BT. (It also wiped out all low voltage chargers and my vintage stereo receiver).
I sent the (2) units back to the BT factory and said I would pay to have them fixed since they were off warranty.
They sent me (2) brand new units, no charge. I don't know of many companies these days that will stand behind their product like that..........!