This tester couldn't be better. In just seconds you'll know if that battery you found in the junk drawer, basement or garage still has a charge or needs to be tossed. No outside power source needed simply touch either the positive or negative side of battery to the corresponding contact point on the tester face and then touch the correct test lead to opposite side of battery. The scale indicates a weak battery when the needle drops with an easy-to-read selector switch and color coded good/replace indicator. Extra-features include positive and negative contact posts on the front of the tester making 9 volt battery checks quick and easy and a button cell retainer with press-down test button that holds the cell in place while testing. Because this unit measures the Voltage by applying an appropriate load, rather than just testing the open cell Voltage you get a more accurate result.