The Apple Watch & Batteries

One of the things not mentioned at today’s Apple Keynote was the battery life of the APPLE WATCH. It was implied that it would be recharged every night, there was nothing specific about the hardware itself.

Battery life is determined by three things – size of battery, power intensity of the CPU/SOC and the power consumption of the display.

We will assume the display is using the most efficient display possible – IGZO or aSi. This will minimize the power draw from the display as much as possible.

The CPU/SOC is likely minimized as much as it can for the first generation product. This is where Apple will need to innovate – integrating more and more functions from discrete chips in the APPLE WATCH package into one piece of silicon that can be fabricated at a small process (which as of right now is 20nm from TSMC).

Batteries, however, don’t progress as quickly as everything else. They improve at about 8% per year, and thats in a good year. It’s why we’re not all driving electric cars right now. Electrochemistry is difficult.

Specifically, the issue with the batteries for the APPLE WATCH is the volumetric energy density. Apple needs a battery that has a high volumetric energy density (measured in Wh per liter) so that they can cram as much watt-hours as they can inside a specific volume. This is different that EV companies, which are typically looking for high gravimetric energy density (Wh per kg).

Right now, some of the best batteries are about 700 Wh/l (for this application, NCM Li-Polymer from SK Innovation at 200Wh/kg and ~700Wh/l). If we assume the battery inside the WATCH is 30mm x 25mm x 5mm, or 3750 cubic millimeters, thats 0.00375 liters. If Apple used the best battery technology possible, they would have about 2.7Wh of storage (730mAh at 3.6V). That figure seems high, so I’m guessing they’re using something less substantial (probably in the 500 Wh/l range) around 1.9Wh, or 520mAh. 

The difficulty Apple faces when it comes to battery development is that it is largely on its own – most battery companies are working hard to increase the gravimetric energy density and the cycle life of batteries. For example. Li-S batteries, while the gravimetric energy density may increase to from 200 to 400Wh/kg, the volumetric energy density is only in the range of 425Wh/l. Lighter batteries do Apple no good – they need more space-efficient batteries.

This is why my hopes are dim for amazing battery life on the APPLE WATCH anytime soon. It will take several generations of hardware integration and software optimization before the product matures. But this is no different than the original iPhone, with its slow 2G data speeds, adequate battery life and no apps.

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