This battery holder connects 2 AA batteries together in series for powering all kinds of projects. We spec'd these out because the box is fairly slim, and 2 AA's add up to about 3V for many low voltage electronics projects, plus it has a nifty on-off switch. Fits any standard AA battery. When using rechargeable NiMH the output voltage will range from about 2.7V with charged batteries to 1.8V at the end of life with a nominal voltage of 2.4V. When using alkalines, the output will range from 3.2V with new batteries to 2.2V at the end of life with a nominal voltage of about 3V.The polarity matches that of our 2-pin JST cable and Li-Ion/Poly batteries. Uses a genuine JST connector so it won't 'catch and tear' in JST connectors.