Stopwatch running for for 100 years: 7-segs, LCD and battery life

Hello,

I'm total EE newbie, apologies in advance for stupid questions....

I would like to build a simple stopwatch which counts from zero upwards. I found several ways how to do it and now I'm considering two:

- 32kHz crystal -> 4060 + 4040 -> 1Hz, and then some 7-segment LED displays driven by 4026.

- 32kHz crystal, some ATmega with clock divider set so it ticks at 1Hz, which in turns drives some LCD display (like HD44780)

Now, my questions are:

- which path should I take?

- the largest problem I have is I would the stopwatch to run for several years uninterrupted. That means, it might be a problem to power it from grid, so I tought I would run it on batteries as I saw some simple digital clocks to run for years on this small batteries. But when I did the calculations (here's when my lack of education kicks in) I realized batteries wont last enough. Please, do you have any suggestion for me?

The calculations were like this: one "D" battery has 12000mAh and if my LCD circuit draws 2mA (how does mA relates to mAh? what does it mean a ciruit draws 1mA? for what time?), it will run for 250 days. I would like it to run for 25000 days ( i.e. 100x). Not to mention LCDs (1st option) consume far more power.

(side node: 2mA * 60 sec = 120mAs/min = 7200 mAs/hour = 2mAh -- I found this on net too, not sure if it is correct. I never saw mAs/min before....)

Also, I was thinking about using solar power and super-capacitor but that might be too complicated for me..

Also, in 1st option, is it even possible to run that many LEDs off 4026s off batteries?

Anyway, thanks a lot for any help!

You calculations are right for 2mA. The D battery is rated at 12000 mAh which means that it can supply 1 mA for 12000 hours, 12 mA for 1000 hours etc. 2 mA should give you 6000 hours or 250 days.

In practice, the more current, the fewer mAh, so you might get a bit more than 12000 hours at 1 mA, and you won't get 1 hour at 12000 mA.

Little LCDs don't take as much as 2mA. I know that because my watch has a much smaller battery and it has lasted much longer than 250 days. You will get several years from a D battery connected to an ordinary LCD watch electronics.

You need to look at battery shelf life. Applications like that may be best suited to Lithium Thionyl Chloride batteries. They are expensive, can't supply big currents, but they have huge capacity and excellent shelf life.

To get the longest possible battery life, I would use a PIC microcontroller as the timekeeper and the LCD driver. It has the ability to go into a very low power-down mode called "sleep", during which it is waiting for a counter to reach a predetermined count value. Using a 32kHz watch crystal as the time base, the PIC could wake up only once per second, increment a binary "seconds-since-start" value, convert that to BCD, change some port pins to update the display, and go to sleep for the rest of the next second...

The PIC power drain while awake while running code at 32kHz is a few hundred uA; while asleep, the power drain is ~2uA. The time awake would be a few% of a second; you do the math...

btw- a static LCD has only nA of leakage, so consumes no power while displaying a fixed number. It is some capacitance, so it takes a tiny amount of power to turn on or turn off a segement.