Since tablets are designed for mobility and portability, one most desired feature in a carrier is an embedded battery pack. Tablet battery cases include a power supply included, so that no extra battery pack needs to be carried. The inset battery pack should be seamlessly integrated into a lightweight case, usually somewhere in the bottom half of the case.
By definition, all solar-powered cases qualify as battery cases due to their included voltaic power cell-type battery. Otherwise, the tablet battery pack will typically be found in the ranges of bags with just the slight bit of extra space available. Sheer-fitting sleeves and envelope cases, for example, simply cannot contain a solar-powering feature for space concerns and so some sort of external charger must be employed.
While tablet battery cases definitely help with the convenience of extra quality time with the tablet, most such cases (excepting solar-powered cases in general) will suffer from a noticeable increase in weight. And would-be owners should know their case: some tablet batteries are notorious for providing poor performance with heavy use of the tablet on the charge.