Battery life estimation

Exciting… what will be the outcome of the calculations based on monitoring 2 motion sensors during a period of 24 hours and extrapolating this calculated power usage? OK, let’s start with writing down some ‘facts’, taking the worst case of the 2 sensors for each individual measurement:

  • The JeeNodes active time during a period of 24h is 102.4 seconds, with a current of 6.5 mA;
  • Power down current is 20 µA;
  • The XBee sends an average of 120 packets per hour, requiring the XBee to be on for a duration of 34 ms for each packet, using 40 mA current (from the datasheet);
  • XBee power down current is 1 µA (datasheet);
  • The PIR uses 170 µA (datasheet) while in standby mode and 270 µA (datasheet) during motion detection; motion detection duration is estimated at 400 seconds.

With these numbers it should be possible to calculate the average power consumption of each component:

  • JeeNode: 27,68 µA;
  • XBee: 2,88 µA;
  • PIR: 170,46 µA.

This adds up to a total of 201,02 µA. That’s the average current the motion sensor is using, based on a period of 24 hours.  With 2000 mAh batteries this results in 9950 hours = 414 days = 59 weeks… cool! That will do, for now, lol

However… how about the battery self-discharge?? How much will this influence the time this motion sensor will last on 1 set of batteries? I really don’t know; so, still, time will tell how well this sensor performs in terms of long lasting batteries.

Fortunately, I still have enough options to reduce power usage even more, like

  1. Using the energy saving version of the PIR;
  2. Disabling stuff on the ATmega like brown out detection (how do i change fuses? As i mentioned before, where will this end? 🙂 );
  3. Running the ATmega on a lower frequency.

And now that i have it all in a spreadsheet, i can manually adjust some values and see how it effects battery life. The smartest thing would be to go for the 1st option, cause that would reduce the standby current of the PIR down to 46 µA instead of 170… meaning 3 years !!

I’m going to order some extra PIR’s now…

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