[EricH]

The rule on the subject of charging the backup battery:

<R53> The 7.2v backup battery may be charged on or off the robot. When off the robot, the battery is to be charged with the provided 7.2V backup battery charger. When mounted on the robot, the backup battery may be charged from the EX18-12 primary battery by using the custom charging circuit available from Innovation First Inc. (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)

It seems to say that you have to use one of the methods supplied by FIRST. However, it is not exactly clear. Try Q&A for a definitive answer.

And for the record, I don't think we've ever had any longevity problems. We could run most of the day on one backup battery, partly because it was rarely used.