Since tablets are designed for mobility and portability, one desired feature in a is an embedded battery pack. Tablet battery cases include a power supply built into a case. The insert battery pack would be integrated into the case, usually somewhere in the bottom half of the case.
All solar-powered cases qualify as battery cases. Otherwise, the tablet battery pack will typically be found in the ranges of bags with just a 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.