[dtengineering]

R73 : "As a replacement for the FIRST-supplied battery, any other commercially available 7.2V NiCad battery pack may be used."

R49 : "When off the ROBOT, the battery is to be charged with a 7.2V backup battery charger. When mounted on the ROBOT, the backup battery may be charged from the 12VDC primary battery by using the custom charging circuit available from Innovation First Inc. or any similar charging circuit (note: IFI
will provide the design for this circuit on the IFI website, however teams must obtain the parts for this circuit and assemble it themselves). The use of this circuit is strongly encouraged."


Now, these quotes are from Rev A of the rule book, so you may want to double-check the Q&A forums and go re-read the rules yourself, but unless these rules have changed, or I have missed something:

A) The first and best option is to build/buy the on-robot charger that tops up your backup battery from the main battery. You will never run out of power for your servos unless you run out of power for the whole robot... which just doesn't happen in a two minute match.

B) "any other commercially available 7.2V NiCad battery pack may be used" seems pretty clear... it makes absolutely no mention of a MAh capacity, and FIRST is quite good in supporting the rules as they are written (and updated in the Q&A)

C) If you are using any kind of fast charger for a NiCad, I highly recommend you use a peak detection charger which will determine when the batteries are charged. If you are worried about charge RATE, which depends on both the battery capacity AND charging speed (some NiCads can be fast charged, cheaper ones not so much) then I suggest you contact IFI directly. IFI Robotics also maintains forums where you can ask the technical experts who build this equipment these quesitions.

But check out that answer "A" option... it seems your best bet.

Jason