During pandemic, I am allowed to work from home 5 days a week. I haven't used my car a lot like I used to. So I bought 2 battery tenders from Harbor Freight in 2020 to keep my batteries full charge. This morning I needed to use the car so I removed one of the charger, and used it for 1/2 hour then parked in the garage. Fast forward to this evening I proceeded to rehook up the charger to the car, parking in garage, the car was not used for 10 hours so it was as cold as my garage temp, 20 degrees F. After reconnecting all leads, red clamp to positive post, black clamp to negative post, I plugged the charger adapter into an extension cable (about 6 feet), and then felt the need to clean up some trash in the garage. This impromptu helped me avert the disaster. When I was done with the trash outside, I went back into the garage to find out my garage was filled with smoke. Running to my car, lifted it hood, the charge was on fires and flame shooting out. I quickly ripped the charger, threw it on ground and stamped it my foot to extinguish the fire. It was shooting flames and chars everywhere. Had I not cleaned my trash, it would have burnt my car and garage down. I was shaken to my core. For now, I have unplugged the second unit. This unit I believe is still being sold in Harbor Freight. I hope my incident report would help people to be aware of the fire hazard and to not facing into same situation like mine.