[time-nuts] power requirements to run a wristwatch over time

[Erik E. Fair]

	Date: Mon, 08 Aug 2022 01:17:34 -0700
	From: Hal Murray via time-nuts <time-nuts at lists.febo.com>

	It would be fun to see a plot of the power needed to run a watch over the years.

I don't have such a plot, but a proxy for it would be a look at Citizen & Casio solar-powered quartz watches reserve power specifications: many of them have a "sleep" or "power save" mode which greatly reduces power consumption, and extends the rechargeable battery charge lifetime considerably: high numbers of months, low numbers of years. When exposed to light once more, they "wake up" and display current time (modulo drift, but there are radio controlled models which deal with that too, both terrestrial (e.g., WWVB, DCF77, JJY) & GNSS).

Fundamentally, what they do is turn off the display when the watch is not in use. Their most common model of "watch not in use" is being in the dark long enough, which they can determine because they're clocks with a light sensor (e.g., Citizen F150 solar GPS watch movement - in "power save" mode from full battery charge, the manual claims this movement will keep time for up to 7 years with +/- 5 second per month maximum drift without time signal sync).

Some models also have accelerometers, so if they're still and in the dark long enough, they go to sleep. The accelerometer allows them to wake up even in the dark (e.g., Casio GWM-5610 solar powered, radio-controlled LCD digital watch).

For the analog display models: sleep mode means the stepper motors which drive watch hands are run less often (e.g., stop the second hand entirely, only move minute/hour hands every 60 seconds), or stop the display entirely.

For the digital LCD models: turn off the LCD. The ana-digi combination models do some combination of both.

In all examples I've seen, these watches continue to run their oscillators to keep time (the watchmakers are very proud of this - it's prominently mentioned in the manuals), and the terrestrial radio controlled models continue to attempt reception of time signal every night (typically between midnight & 4am, when reception of such signals is likely to be best).

Technology development in this area continues: Casio has begun shipping watch models with memory in pixel (MIP) displays - much lower power consumption for static display, accomplished by having one SRAM bit per LCD pixel integrated into the display.

https://casiofanmag.com/g-shock/mip-lcd/

https://www.mouser.com/new/kyocera-display/kyocera-mip-display-modules/

	just FYI,

	Erik