Annual clock project, 5+ years battery life possible?

As this project only makes sense when it works for years, I'm trying to figure out if it's possible to use such extreme power saving so it could run for 5+ years.

Controller:
I have a couple of pro micros laying around I could use, but they're all 5V versions, and using attinys would make more sense as they are advertised as low power I guess?

Timekeeping: an RTC module, there are a couple of them, it doesn't have to be that precise as even a minute a month doesn't add up for a significant difference in this project. I guess an RTC with GPS time is out of the question because of the power draw.

Motor:
I've read somewhere that there is a kind of stepper motor that stays stationary when powered off. This would be crucial for power saving.
Am I missing something? Is there a lower power way to move the hand?

Battery:
I have plenty of space, I could even use four AA batteries. That's 3600 to 5400 mAhs on 3 Volts. Or even 2 D batteries for 12000 mAhs.
Or would using PV to charge a smaller battery make more sense? Could it charge more than this system uses a day with interior lights?

Function:
It only has to turn on once a day to move the hand 1/365 degree at a time. The rest could be spent in ultra low power mode.

I think calibrating after a power loss is not necessary as I could just calibrate it once, then write the hand position into eeprom. This depends on wether the motor can be moved by hand. Or I could just put an acryl top on it so nobody can touch it.

I found this thread Arduino to last years on battery. - Project Guidance - Arduino Forum and it has a couple of good ideas here and there but no solution.

This project would be so trivial with a power adapter, and there are so many questions I don't seem to find the answer to, I might just go the power adapter way.
There's also the problem of how do you know the batteries are depeleted? It might take months after you realize the clock hand didn't move for a while, and you probably won't remember to change the batteries every 5 years.
On the other hand, a cable running from a wall clock is not very nice.

yoshileon:
There's also the problem of how do you know the batteries are depeleted?

There is the same problem with smoke alarms, think how they warn you the battery is going flat .............

srnet:
There is the same problem with smoke alarms, think how they warn you the battery is going flat .............

Ours have a led light that blinks once a minute. I guess they already have a low power circuitry so it's not a problem, but for my project, waking up the controller to blink a light every minute sounds like a lot of additional power draw, but obviously I could be wrong as I never had to worry about power draw.

I think steppers with permanent magnets have an effective "detent" which might be enough for a perfectly balanced day hand. And you could include some gearing, which should be pretty effective in keeping things in place with the power off.

You should be able to get a processor's sleep current down to well under 1uA. Or you could have the RTC switch on the power once a day. The RTC battery current should be down in the 1uA range, and I think they run several years on a CR2032. You could use several of those batteries in parallel, and they have very good shelf life, but it would all depend on the current draw.

What are you going to do about leap years?

Do the DS3231 (I2C) and DS3234 (SPI) handle leap years?

Complete Clock Calendar Functionality Including Seconds, Minutes, Hours, Day, Date, Month, and Year, with Leap-Year Compensation Up to Year 2100

Real-Time Clock Counts Seconds, Minutes, Hours, Day, Date, Month, and Year, with Leap Year Compensation Valid Up to 2099

There you go - 80 year clock.

How long does the RTC backup battery last? You could run the RTC off the system battery.

One minute a month will be off by an hour after five years, for me that would be a bit of a problem.

Put a little solar cell and recharged battery power system on in there.

Have a GPS unit that you fire up briefly once and again to set the clock.

a7

yoshileon:
Ours have a led light that blinks once a minute. I guess they already have a low power circuitry so it's not a problem, but for my project, waking up the controller to blink a light every minute sounds like a lot of additional power draw, but obviously I could be wrong as I never had to worry about power draw.

So working out the probable battery life might be an idea ?

Sleep current of a 'bare bones Arduino', which could be just a Pro Mini with the regulator removed and an RTC would be maybe 5uA tops.

So how long will your chosen battery last if the Arduino come on for say 250mS every minute, using maybe 10mA to flash an LED ?

"You gota do the numbers"

It only has to turn on once a day to move the hand 1/365 degree at a time.

A small geared stepper motor is fine for this, and gear backlash is not a problem since it moves in one direction only. You need only power the motor up once a day, and the gears will hold the position the rest of the time.

Follow this tutorial to learn how to make a "bare bones" Arduino, which will run for years on 2xAA batteries.

Or, buy an Arduino Pro Mini and remove the voltage regulator and power LED to get the same circuit.

An

ShermanP:
What are you going to do about leap years?

I suppose that's the job of the RTC, as CrossRoads provided some examples.
I expect to read the real time from the RTC once it wakes up, then move the day hand according to the day. Well, if the year can be divided by four, then it's 1/366 degree not 1\365. Depending on the stepper motor this can be easily calculated how many steps do these take.
Thanks for your input on the stepper motors.

alto777:
How long does the RTC backup battery last? You could run the RTC off the system battery.

One minute a month will be off by an hour after five years, for me that would be a bit of a problem.

Put a little solar cell and recharged battery power system on in there.

Have a GPS unit that you fire up briefly once and again to set the clock.

This would be the best solution, but I think I'll have to put the whole circuitry together first, then take some measurements to decide wether a solar panel and a rechargeable battery system would be viable.
An hour after five years is fine, there would be no day numbering so the hand only shows approximately how a year is passing, a couple of hours here and there is fine. It's more philosophical than useful.

jremington:
Follow this tutorial to learn how to make a "bare bones" Arduino, which will run for years on 2xAA batteries.

This is awesome, so with an rtc interrupt you can reach 150 nA, if I calculate it correctly, that means it runs 700+ years per 1Ah, so completely negligible. I'd only have to worry about RTC power draw, but those run for years with a 2032, so that couldn't be much.
Then the battery life will be determined by how much power the stepper motor draws.

After reading this tutorial I feel like it might be possible to run it for 5+ years.

I found other extreme low power mcus in the 100 nanoamp sleep range with built in RTC, I might dig into those later too.

Once sleep currents are below 10uA or so they have a negligable affect on AA type battery life, so searching for microcontrollers that have sub microamp sleep currents is only worth the effort if your using very low capacity batteries.

At 10uA a set of AAs would last around 32 years (ignoring self dischage)

At 5uA a set of AAs would last around 64 years (ignoring self dischage)

In practice the self discharge of the battery will mean they are flat in around 5 years for alkalines and 20 years for lithiums.

The PCF8563 RTC can be used without a additional battery, just power it from the same supply as the Arduino. When the I2C interface is not active it has a backup current of 0.25uA. It has a wakeup alarm function.

A GPS is one solution to updating the clock, fire it up, wait for time sync and update the RTC.

Be aware that unless the GPS has received the leap seconds update, which is in a navigation message only sent every 12.5 minutes (and which you may miss in low signal situations) the time indicated could be some seconds out.