Simply put, 50Amp fuses. The first backup I installed right away had some quality issues. The metal end caps were unsecured so they could twist and essentially break/damage the fuse inside when I removed them to work with my stereo. Although I didn't check the others, the 2nd one turned out to be ok. Even though they were cheap, I still expected them to work as intended without falling apart in my hands, so I had to re-evaluate those "2 stars". Maybe you are trying to find a better bang for your buck I think.