Large Backup Battery

[Vikesrock]

According to <R73> any commercially supplied 7.2V NiCad may be substituted for the supplied backup battery. Your question appears to asking about making a custom backup battery of some sort which I would say is not legal. Purchasing a 7.2V NiCad with a higher capacity does appear to be legal and would likely be the best solution to pursue.

<R73> The 7.2V Robot Control backup battery shall be connected to the Robot Controller as
described in the Robot Controller manual. The 7.2v “backup” battery is considered an
integral part of the Robot Controller, and shall not be used for any other purpose. The 7.2v
battery should be charged to at least 7.0v before entering a MATCH. As a replacement for
the FIRST-supplied battery, any other commercially available 7.2V NiCad battery pack may
be used.

emphasis mine