I don't believe any rule supports your self-imposed position.
I don't beleve the GDC supports your position, otherwise, they would say so.
I believe only one battery can be used during a match.
A match begins with the field sound and ends with the refs all clear flag.
(it doesn't end with a field sound this season).
I don't accept the stored energy argument (even without the evidence of a rule), otherwise, springs, elastic tubing and other potential energy storage is also suspect. Thereby excluding human muscle power from the robot, as well, if the battery is the sole source of energy.
My beliefs don't constrain the GDC, but are what I will advise any team that asks.
I'm rooting for your students.
