Apple Watch To Reportedly Offer 19 Hours Of Battery Life Under ‘Normal’ Conditions
This is coming from "people with knowledge of the Apple Watch's development" who spoke with 9To5Mac, presumably on the condition of anonymity since Apple itself hasn't disclosed any such details. According to those sources, Apple is aiming for a high-end experience by combining a high-quality screen with a relatively powerful processor, along with a stripped-down version of iOS codenamed SkiHill.
The downside to this approach is that battery life could be somewhat short when actively using the device. How short? Apple is aiming for 2.5 hours to 4 hours of active application use. As mentioned, a combination of the two could yield up to 19 hours, while pure standby time could keep the Apple Watch ticking 3 days (add an extra day if left in sleep mode).
Apparently Apple has been busy stress testing the battery life of its wearable with both pre-loaded and third-party applications. It's Apple's goal that its device will give up the ghost in as little as 2.5 hours only when running an intensive task non-stop, such as a processor intensive game. For standard tasks, non-stop use would yield 3.5 hours.