One day, when HDC came to town, I bought a Grip 12V battery tester. I didn't need it right then, it was cheap, so I bought it for future use. I ended up testing a car battery. The instructions say to hold the switch for 10 seconds. I let off the switch after 4 seconds because smoke started coming out the vents.
I'm not sure, maybe it has to burn oil off something or something like that, but I didn't think it was supposed to do that.
I ended up wishing I had purchased a "higher quality" Harbor Freight item.