It's been rumored that Apple's iWatch might be packing some solar charging capabilities, but it's now being reported that they will be using lithium-ion stepped batteries distributed by LG Chem, Samsung SDI and Tianjin Lishen Battery. A stepped battery is much like a traditional lithium-ion battery, however it carries and extra "step" on the back that provides more capacity — about 16% more.
Apple is said to be struggling with battery life on prototype models of their smartwatch and currently only seeing 2-3 days on a charge. It's not far off from some other watches we've used but Apple is striving for even more battery life.
The rumors around Apple's smartwatch are starting to come in hot and heavy, so we'll certainly be seeing more and more as we get closer to the estimated launch this fall.