As a company who investigate a load of battery powered devices, we were curious to the capacity of Apple Watch’s battery. There has been many teardowns of Apple Watch, most notably by iFixit. They revealed that the 38mm Apple Watch had a battery capacity of 205mAh. For comparison, your iPhone 6 carries a 1,810mAh battery.
There are very few teardowns of the 42mm Apple Watch, I guess most people don’t want to open up the more expensive version. A website called TechRepublic did have 42mm Apple Watch teardown. Their photography was no where near the quality of iFixit making it difficult to read the numbers on the battery.
Luckily, the battery also has the watt-hour (Wh) number written on the back in a larger font. The 42mm Apple Watch battery has a capacity of 0.94Wh compare to 0.78Wh of the 38mm battery. But, what we really want is the milliampere-hour (mAh) number. For this we need the batteries voltage.
After some sharping up in photoshop, you can just make out that the battery is running at 3.76 volts.
To convert watt-hour to milliampere-hour we must times the watt-hour (Wh) by a 1000, divided by the voltage in volts (V).
mAh = Wh x 1000 / V
250mAh = 0.94 (Wh) x 1000 / 3.76
So the 42mm Apple Watch battery has a capacity of 250mAh compared to the 205mAh of the 38mm Apple Watch. This means the 42mm version has a 22% bigger battery then the 38mm.