Battery Life Calculation???

[manumenzella]

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

[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...

[manumenzella]

Hey. Thanks for your answer. I have just checked the Energizer datasheet for the con cell CR2032. It is rated at 240 mAh, and with a pulsed drain (2 seconds, 12 times/day, 6.8 mAh), it should give about 225 mAh.
Thats very little difference for such a big current (for a coin cell, I mean).
I think I would perhaps not be able to get the full 137 days, but perhaps about 3 months??? Please correct me if im wrong!!! Thanks again.
Manuel

[skimask]

Hey. Thanks for your answer. I have just checked the Energizer datasheet for the con cell CR2032. It is rated at 240 mAh, and with a pulsed drain (2 seconds, 12 times/day, 6.8 mAh), it should give about 225 mAh.
Thats very little difference for such a big current (for a coin cell, I mean).
I think I would perhaps not be able to get the full 137 days, but perhaps about 3 months??? Please correct me if im wrong!!! Thanks again.
Manuel

As far as I can tell, probably not the full 137 days, probably not even 3 months. Surge currents kill coin cells. My experience says you might get a couple months at most. Maybe my coin cells were old, maybe my calculations weren't corrent. Who knows. These types of things are crap shoots, you never know until you try it...so...

Try it out and get back to me in April

[manumenzella]

Hey. Thanks for all your help.
I dont have any experience with this tiny lithium con cells, so i hink that you must be right!
Anyway, this project is supposed to beep in the classes of a teacher (she is really rude, and everybody hates her!!!) I ehink I saw another post on a similar project, but i cant seem to fin it. If you know the addres, can you post it please???
Thanks again, and sorry for my english!!!
Manuel