cHIP
I hate to see that.
A few stories of building fires in the last year. Now I am going to buy a bigger fire extinguisher for my garage/shop.

There may not be a way to know, but I doubt the battery tender alone caused the fire. Perhaps the wind or an animal knocked the clips off and gasoline fumes caught fire, then oil, etc.

1993 Daytona, 5.50 at 130mph (1/8) 1.19 sixty ft (PG). Link to 572 B1 - Part 1