I don't understand why this hasn't been included previously?

While the capacity is obviously different for every device model, it's not like the battery capacity is going to change for a device after it's built other than degradation. Just include that information in the device info file already reported to the system and have it calculated based on the designed capacity for the device.

I thought after Apple implemented it, Android was going to add it immediately since it shouldn't be difficult and is a huge consumer help. At least one of the OEMs, like Samsung if not Google directly.