How To Live Forever (almost, if you're a watch)

Posted on September 25, 2014 by Gary Carruthers

Trinity Church Square, London, 25th September 2014

We've spent the last few months agonizing about timekeeping. Time for a change.

Let's agonize about power instead.

Much of a timepiece is taken up by the power source. From 4000BC, there was a big bowl of water at the top of the water clock. In 500AD, the world-beating time source had its power provided by a tower of bee's wax. In mechanicals even today, longcase weights and mainsprings providing the heft, taking up a fair part of the movement.

We work with coin cells. Do it right, and you can make them last years instead of the days that other timepieces can achieve. (When you finally have to replace a cell, remember they are a chemical power source that should be recycled responsibly. Here in the EU, most supermarkets will recycle them for you.)

But we really want to push those limits. A big coin cell helps - the 250mAh of our CR2330 cells pack a punch. But in contrast to smart watches needing recharging every night, we're trying to work out how to break the ten-year barrier, or even the twenty year barrier, if the cell chemistry is up to it. 

A lot goes on in our watch movements. Microcontrollers switch in and out of sleep states. Bluetooth radios emit radio beacons. Stepper motors pump hands around. We measured it all and built up a power budget spreadsheet to analyze how we could cut it back. 

The "killer app" was to cut back on non-essential services when power was getting low. Turn off everything else, but still show the time.  When we'd pared back all we thought we could, one line-item on the power budget stuck out like a sore thumb - the seconds hand, taking up over 90% of the power budget. So when power starts to get low, we turn it off. Exactly at the 12 o'clock position, as an indication it's time to change the battery. The hours and minutes will still tell the right time for years. Exactly how long, we don't know. Nobody's tested lithium chemistry quite this way yet. Ask me in a decade or two.

We filed a patent on it today. 

We worry about this kind of stuff so you don't have to.  Watches should just work, without you having to care how.  Pictured, The Seven Ages of Man engraving by Charles Knight. Live long and prosper!

Posted in