A Word or Two About Power Usage

A project log for People Counter

Count people entering and leaving a space through a portal with a low-cost, battery-powered, connected device

Kris WinerKris Winer 02/01/2020 at 01:240 Comments

January 31, 2020

One of the design criteria is that the People Counter should last two or more years on two AAA batteries. This is about 1200 mAH capacity at 3.0 V, so two year battery life would mean an average current usage of:

1200 mAH/(2 Years x 365 days/Year x 24 H /day) ~ 69 uA,

which isn't easy to achieve.

I measured the power usage of the People Counter under conditions known to minimize power usage including 4.2 MHz clock speed, "fastest" Arduino compile optimization, and 10 minute LoRaWAN duty cycle. 

The variable was the period of threshold adjustment, either once every 10 seconds or once every 20 seconds. The former is about the fastest one needs to adjust the threshold since the Calipile sensor drift is <1 count/second.  Adjusting the threshold every 20 seconds saves a significant amount of power but runs the risk of the threshold lagging the sensor baseline and introducing inaccuracy in the transit detection. For most applications, I would expect the "sweet spot" to lie between 10 and 20 seconds.

I measured:

<166 uA at 10 second threshold adjustment period

<106uA at 20 second threshold adjustment period

These currents are the average current after one hour of operation and are upper limits, since the average current will continue to decrease with time until it asymptotes. What this means is that the threshold adjustment uses on average ~120 uA of current at 10 second period, and the rest of the ~46 uA is the cost of the LoRaWAN, sleep current of the STM32L082 and the current to power the sensors. The latter (~40 uA) is consistent with my experience measuring the power usage of similar devices (like the Bin Level Monitor).

So the threshold adjustment is using the bulk of the current in this application and increasing the period to more than 10 seconds is the way to eke out more battery life. But this will eventually come at a cost of accuracy, just at what period and how much loss of accuracy we do not know yet. I would guess that a 20 second period would probably be OK in most cases but I don't know what might happen with longer periods. More testing will tell.

So at 10 second threshold period, two AAA batteries would last ~300 days and at 20 second interval, 472 days. So we are getting close to the power usage goal; at 20 sec period two AAA batteries could last ~1.3 years.

But these are baseline numbers, and battery lifetime will be less depending on the number of people "counted" each day. At 10 second period, the threshold adjusts 360 times an hour, which costs 0.12 mAH. Assuming counting a person uses as much power as a threshold adjustment (a good assumption), then counting one person costs 120 uAH/360 ~ 0.33 uAH/person.

So counting 100 people per day will cost an additional 0.033 mAH per day and reduce the battery life from ~300 days to ~298 days, barely noticeable.

Counting 1000 people per day will cost an additional 0.33 mAH per day and reduce the battery life from ~300 days to ~278 days, about ten percent.

Lastly, counting 10,000 people per day or 1250 people per hour during an eight-hour business day (pretty heavy traffic for, let's say, a bathroom) will cost an additional 3.33 mAH per day and reduce battery life from ~300 days to ~164 days, about half.

Of course, proof is in the testing and while we can measure the power usage of individual units and make projections like those above all day long, we really need some real world results to better judge accuracy and battery life.