Apple has previously filed patents for an auxiliary solar cell, that would work as a backup power source for mobile devices, as well as a method for covering a device in solar cells.
From the techcrunch.com
The fuel cell-centric auxiliary power unit is modular and can be reconfigured for a wide range of uses.
From the sciencedaily.com
He took us up a road that ran from the dock past several auxiliary buildings, many of them now in ruins, to the cell-block building on the summit.
From the sacbee.com
In a three-electrode cell, a counter electrode, also called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode.