# Calculating energy capacity of a battery

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?

• There's a catch: The battery's open-circuit voltage is not likely to be constant throughout the charge/discharge cycle. For most battery types, the voltage will decline as the battery discharges. The shape of the curve varies with battery chemistry, but it always monotonically decreases. – Solomon Slow Apr 8 '19 at 21:09
• There's another catch: Two different applications using the same battery might have different minimum voltage requirements. So, the total amp hours and total energy used by one application before it considers the battery to be "dead", could be different from the amp hours and energy used by another application. – Solomon Slow Apr 8 '19 at 21:11
• The capacity in mAh is determined by the amount of electrolyte in the battery; each Lithium atom can hold 1 electron, and 1 Ah =3600 Coulomb = 2* $10^{23}$ electrons. more or less! – patta Apr 9 '19 at 12:17
• As written by Solomon, mAh capacity is not directly related to available energy – patta Apr 9 '19 at 12:19