Before we go any further, we'll disconnect and remove the battery, to avoid shorting out and damaging any of the internal circuitry.
This is an AG4 alkaline battery cell, with a typical capacity of 12-18 mAh. At 1.5 V, that's around 0.0225 watt hours.
Alkaline? What's up with that? Button cell batteries are typically available in alkaline or silver oxide chemistry. Silver oxide batteries are better in every way—recycleable, slower voltage drop during discharge, less leakage, and typically 50% higher capacity. Maybe we caught Apple trying to save a buck?