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Calculating power consumption and battery duration

 
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Gabriel Caffese



Joined: 09 Oct 2003
Posts: 39
Location: LA PLATA, ARGENTINA

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Calculating power consumption and battery duration
PostPosted: Sun Jun 13, 2004 8:47 am     Reply with quote
Hello everyone,

I´m involved in a project, in wich there is a 16LF88, and two I2C perpheralls. Timer 1 is being used along with a 32,768Khz crystal oscilator, and Pic micro configured to count and wake up every 15 seconds. There´s also an infrared module, for sending data to a PC at 57,6kb, and reconfigure everything.
I want to measure power consumption, and estimate battery duration (after choosing the right battery).

This is what I measured for the complete circuit, @ 3,6V:

During 15 seconds, current is 5uA
During .1 seconds, current is 3ma
Twice a week, during 90 seconds, current is 4ma.

The cycle repeats for ever, and once started, will NEVER be turned off.

Now, how do I calculate "final" power consumption, and select the apropiate battery ?

Hope someone can help. Thanks in advance.

Gabriel.-
Ttelmah
Guest
Re: Calculating power consumption and battery duration
PostPosted: Sun Jun 13, 2004 2:55 pm     Reply with quote
Gabriel Caffese wrote:
Hello everyone,

I´m involved in a project, in wich there is a 16LF88, and two I2C perpheralls. Timer 1 is being used along with a 32,768Khz crystal oscilator, and Pic micro configured to count and wake up every 15 seconds. There´s also an infrared module, for sending data to a PC at 57,6kb, and reconfigure everything.
I want to measure power consumption, and estimate battery duration (after choosing the right battery).

This is what I measured for the complete circuit, @ 3,6V:

During 15 seconds, current is 5uA
During .1 seconds, current is 3ma
Twice a week, during 90 seconds, current is 4ma.

The cycle repeats for ever, and once started, will NEVER be turned off.

Now, how do I calculate "final" power consumption, and select the apropiate battery ?

Hope someone can help. Thanks in advance.

Gabriel.-

Basically, you need to sum the current over a 'standard' time period (normally 1 hour). So in the case of the 'twice a week' event, only 1/84th of this occurs in the average sampling period (given that there are 168 hours in a week), and the mAHr figure for this is:
(4*90)/(3600*84) = 0.0012
Then for the 0.1 second 'event', this occupies 0.66% of the time (0.1/15.1), giving:
3*0.00663 = 0.020
Then the 15 second event, occupies the remainder of the time (15/15.1) giving:
0.005*0.994 = 0.00497

Round these up, and add them, giving an average Ma rate at:
0.02617 => 0.03mA
So to run this for a year, you need:
365.25*24*0.03 MaHr useable capacity in the battery = 263mAHr.
Remember then that battery capacities are quoted at particular discharge rates/enviroment conditions, that may well not be met (in your case, the battery self discharge will reduce the available capacity, and depending on the enviroment, the full capacity may not b available - if cold, most batteries give a lot less than their quoted 'room temperature' rating).

Best Wishes
Gabriel Caffese



Joined: 09 Oct 2003
Posts: 39
Location: LA PLATA, ARGENTINA

View user's profile Send private message Visit poster's website MSN Messenger

Thanks a lot....
PostPosted: Mon Jun 14, 2004 9:27 am     Reply with quote
Ttelmah,

Thank you very much for this explanation.
I bought ER14505 batteries, wich are 3.6v, 2400mah and would have expected at least 2 years of duration.
For what you explain, I think there is a quite a bit more duration Wink
Ttelmah
Guest
Re: Thanks a lot....
PostPosted: Mon Jun 14, 2004 10:28 am     Reply with quote
Gabriel Caffese wrote:
Ttelmah,

Thank you very much for this explanation.
I bought ER14505 batteries, wich are 3.6v, 2400mah and would have expected at least 2 years of duration.
For what you explain, I think there is a quite a bit more duration Wink

Yes. The capacity on these drops to about 1.6Ah, at 0C, and the self discharge will perhaps halve the available capacity, but you should get four or five years relatively easily. :-)

Best Wishes
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