29Apr 2015

Apple just released its Apple Watch joining a growing list of manufacturers of new smart watches, including Samsung, LG, Motorola and others. These watches include, in addition to timekeeping, features such as messaging, basic email, navigation, in some cases voice calls, health & sports monitoring, as well as a slew of growing apps that seem dedicated for this tiny screen on the wrist. For the most part, they seem so far to be an extension of your mobile device, with the Apple Watch extending the reach of the iOS ecosystem, and the other watches performing the same for the Android ecosystem. Let’s examine in today’s blog the batteries used in these smart watches as well as batteries in “older and dumb” watches.

One of my favorite and practical watches is the Casio Pathfinder family. It was first introduced ca. 2001 and was the first to integrate a series of useful microsensors for the outdoorsman (or woman). Incorporating an altimeter and digital compass, it also was able to synchronize its time to NIST’s universal clock broadcast radio signal out of Colorado. And it never ran out of battery juice — it had a little lithium-ion battery made by Panasonic (CTL 1616) and was recharged by ambient sunlight. Made of a lithium cobalt titanate chemistry, this little cell had a terminal voltage of 2.3 V and a charge capacity of 18 mAh, equivalent to 0.041 Wh — plenty to power the sensors and gray-scale LCD display.

Now, let’s look at the new generation of smart watches in a family portrait of their respective batteries, courtesy of various teardowns from ifixit.

These batteries have a charge capacity that is about 15X larger than the one used in the Casio watch. This is clearly not surprising since these new smartwatches consume significantly more power to operate the AMOLED display and all the radios (e.g., WiFi, NFC, Bluetooth, and in some cases, LTE).  Consequently, the batteries are physically larger and thicker, and use half or more of the smart watch volume. The CTL 1616 is a mere 1.6 mm thick whereas the cells in smart watches are twice as thick or more. Among these smart watches, the LG G Watch has the highest capacity at 400 mAh and the Apple Watch the least at 205 mAh.

But this 15X-increase in charge capacity does not yield a longer use time relative to my decade-old Casio. I never have to worry about charging my Pathfinder but these new smartwatches seem to have a run time between 3 hours (for the Apple Watch) and 4 hours (for the Android flavors) when operated constantly, with the display and radios on the entire time. Simple math gives us a quick estimate of the power usage: approximately 250 mW for the Apple Watch going up to about  350 mW for the Android watches. Naturally, aggressive power management, fancy parlance for frequently shutting down the display and the radios and CPU, preserves the battery and extends its life to an estimated full day of use. Additionally, there is probably room to bring these power consumption figures down with time as watch designs get optimized — time will tell.

Apple does report a charge time of 2.5 hours corresponding to a rate of approximately 0.6 C, or about 120 mA of charge current into the battery.  Measured charge times for the Samsung and LG watches appear to be in line, with the Moto 360 being a smidgen faster at about 2 hours (equivalent to 0.7 C or a charge current of 200 mA). This means that the charging power into the watch, whether it is wired or wireless, varies between 0.5 W and 1 W, considerably less than the charging power into a smartphone (which may reach up to 18 W).

So let’s see if we can synthesize a coherent picture of the challenges that batteries in these smart watches face if this product category will become mainstream. First, it’s imperative that the battery capacity is increased significantly past 300 mAh, the level that seems to be the norm for now. In other words, unless consumers want thick and large watches, the energy density of the cells will have to rise above the already-large figures in present batteries. This is not an easy task. Second, these batteries had better be thin. But thin and high energy density don’t go well together. Third, I really don’t like a 2-hour charge time. Consumers will want to see these watches off their wrists for no more than 15 – 30 minutes. So expect to see fast charging soon. Yes, that’s doable. And lastly, time will tell how often consumers will replace their watches, but I bet that at $350 ea., they will need to last way more than 2 years…so has anyone yet screamed foul on cycle life.