That would be a power supply to recharge the battery. It is not on the bill of materials and not mentioned at all in the manual.
Once the battery drains the fun stops.
I guess that makes our Spirit Rovers just like the real Mars Spirit Rovers.
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I do not mind getting a power supply but we really need to know what size (5 volts 2 amps ?) and what plug size (N, M, ??).
I am presuming that it would plug in on the main board with the obvious DC type power connector.