Each Battery Cubelet contains two of these RCR123A size batteries. It's important to use these batteries instead of anything else because some RCR123A batteries are different voltages and could cause problems with the robots you build. These batteries are pretty cool. Lithium Iron Phosphate (LiFePO4) technology is much more stable than Lithium Ion, so these batteries are much safer and more robust. Remember the stories of exploding laptops and cell phones? Not with LiFePO4!