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Under the custom circuitry rule, you can use a power supply to convert the battery voltage to a regulated voltage suitable for the end device provided doing so does not violate any other rules (e.g. R41, 24V limit)

The Kangaroo requires a 12V 3A input. I do not recommend a direct battery connection as the battery voltage can fluctuate wildly during a match. What you can do is acquire a 6-18V to 12V voltage regulator and use that for powering the Kangaroo.

On a related note, I have seen people add laptops to the robot powered by their internal battery under rule R31).