By Dan Drown in bluetooth —

Bluetooth Low Energy sensor, part 2

To continue from part 1: I want a wireless sensor that will last more than a month on 2x rechargeable AA or AAA batteries.

Some ideas I have for what I could do with such a sensor:

  • indoor/outdoor temperature/humidity sensor
  • garage door open sensor
  • washing machine finished sensor

Collecting Power Usage Data

I setup influxdb and started logging the battery voltage measurements to it.  I've updated https://github.com/ddrown/bluetoothadc with that code.

I also measured the battery voltage by hand with a multimeter and logged that to influxdb as well.

I setup Grafana to produce some nice graphs with that data:

Battery Voltage: multimeter vs ADC

These two data sources match pretty well, and adding +8mV to my ADC reading looks spot on:

ADC +8mV

From Energizer's AAA battery datasheet, the 100 mA curve is the lowest that they published.

Typical NiMH battery voltage over time

At 2.518V, that's 1.259V per battery.  On that graph, that's around 50% left. So maybe it'll last another week?  It looks like the voltage should start dropping faster now.