Battery Life Calculation???

[skimask]

Hi. My name is Manuel, and I am building a project around the PIC16F628A and a piezo. It is supposed to make 10 one second BEEPs everyday at a specified hour (spaced around 10 mins). I am using TMR1 as a RTC with a 32 kHz osc, and a 4MHz main oscillator. The PIC sleepts between TMR1 interrupts, so as to save power, so the main oscillator would only be active for about:
10 beeps * 1 sec each = 10 sec. Lets add 10 more sec to process the RTC routine (serious overkill!! i guess) so thats leaves us 20 seconds per day.
The piezo draws about one mAh, and the PIC about 800 - 1000 uAh.
The TMR1 oscillator will be on all day long. I want to calculate how long can a small 100 or 200 mAh lithium battery would last. This is what I have already done:


2 mAh * 20 secs/day = 0.0111 mAh per day
60 uAh * 24 HS = 1.4400 mAh per day
---------------------
1.4511 mAh per day



With 200 mAh battery = around 137 days.

Is 60 uAh correct for TMR1 on with OSC???

By the way, the pic will be powered from about 5 volts. (Two batteries in series and one diode so as to keep the voltage below 5.5V)

Can you guys tell me if this calculation is ok please???? Thanks for your work!!!!!!!!!!!!!!!!!!!!!!!! Sorry for my english!!!

Manuel

As I've found out, I'd doubt you'll get the full '137 days' from a set of coin cell batteries.
If you look at the datasheets for the batteries, you'll see that their mAh rating is based on a steady current much lower than 1ma, more like .1mA.
A draw of 1mA from a coin cell is more like a 'surge current' far above a steady draw. It's a lot like the current draw from an engine starter from a car battery, sure you can do it a lot, but you'll get a lot longer run time just running a dome light.
In short, the more you draw, the less you'll pull...