This battery holder connects 3 AA batteries together in series for powering all kinds of projects. We spec'd these out because the box is compact, and 3 AA's add up to about 3.3-4.5V, a very similar range to Lithium Ion/polymer (Li-Ion) batteries, plus it has a nifty 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!). It also features an ergonomic belt clip for taking your power on the go!