This battery holder connects 3 AAA batteries together in series for powering all kinds of projects. We spec'd these out because the box is slim, and 3 AAA's add up to about 3.3-4.5V, a very similar range to Lithium Ion/polymer (Li-Ion) batteries and have an on-off switch. That makes them ideal for use with 3.3V projects that have a 2-pin JST connector meant for one of our Li-Ion/Poly batteries. (Of course, you can't recharge them like Li-Ion/Polys, so don't try to plug this into one of our Li-Ion/Poly charger boards!)