I understand that you can easily calculate the energy capacity of a battery simply by multiplying the mAh rating by the Voltage. However, my question is how do the manufacturers calculate the mAh rating (and therefore energy capacity) in the first place?
Likewise, certain sporting categories like Formula E use batteries. If (more likely when) they move away from standardised batteries towards allowing teams to build their own, I guess there likely will be regulations requiring that the batteries must have a maximum energy capacity of no more than X MJ. In this example, what would be the simplest way for the officials to calculate the maximum energy capacity of the batteries, to ensure the teams aren't in breach of the regulations?