This is a really nice little device. It actually works extremely well, and I have been very proud of the job it has done. But it takes a single 9v battery which drains REALLY fast, even when it is turned off. I did an experiment with it where I put in a brand new Energizer battery, used it once to make sure it was working properly, set it aside (it turns off on its own). A couple weeks later it was dead. Had to put a new battery in. That is the biggest bummer. The best testers I have found to work well don't use the 9v batteries.