Computing stuff tied to the physical world

(9)50 days and counting

In AVR, Hardware, Software on Apr 18, 2013 at 00:01

The new JeeNode Micro has joined the ranks of the test nodes running here to determine battery life in real time. The oldest one has been running over 2 and a half years now:

dsc_1822

That LiPo battery has a capacity of 1300 mAh, and since it’s still running, this implies that the average current draw must be under 1300/24/950 = 57 µA, including self-discharge.

The other two battery tests now running here are based on the new JeeNode Micro v3, i.e. this one – using a boost converter:

DSC_4401

… and another one using a coin cell. Here’s a picture of a slightly older version:

DSC_2858.jpg

The boost version is running off a rechargeable AA battery, of type Eneloop, which I use all over the house now. These batteries hold 1900 mAh, but there’s a substantial penalty for running off one AA cell with a boost converter:

  • energy is not free, i.e. drawing 10 µA at 3.3V means we will need to draw 30 µA at 1.1V, even if we could use a 100% efficient boost converter
  • real-world efficiency is more like 70..80% at these levels, so make that 40..45 µA
  • the boost converter has some idle current consumption, probably 10..20 µA, so this means we’ll draw 50..65 µA even if the JeeNode Micro only uses up 10 µA at 3.3V (actually, it’s 3.0V in the latest JNµ)

This would translate to 1900/.065/24/365, i.e. ≈ 3-year life. Or perhaps 2, if we account for the Eneloop’s 85% per year self-discharge.

The coin cell option runs off a CR2032 battery, which is rated at about 225 mAh, i.e. considerably less than the above options. Still, since there are no boost converter losses, this translates to 225/0.010/24/365, i.e. ≈ 2.5 years of life if the JNµ draws only 10 µA.

These figures look excellent, but keep in mind that 10 µA average power consumption is a very very optimistic value, particularly when there are sensors hooked up and packets are being sent out. I’d be quite happy with a 6-month battery life for a single AA or a coin cell.

For reference, here is an earlier post about all these power calculations.

Here are the current reports I’m getting via HouseMon:

Screen Shot 2013-04-17 at 11.12.08

That’s just about 50 days off a coin cell. Let’s see how it holds up. The nice bit in these tests is that the new nodes now report several different voltage level measurements as well (this also consumes some energy!). They haven’t dropped much yet, but when they do, I hope that we’ll be able to use the drop as a predictor.

Onwards!