The current Room Node sketch only gets 1..2 months of life out of a 3x alkaline AA battery pack. Not enough – by far – for practical use around the house.
In fact, I’d like to push it a bit further and see how long one could run a Room Node off a single rechargable AA battery using the AA Power board. If it’s not that long, I could still opt for the 3x AA pack.
Let’s estimate the idle current draw of a JeeNode first.
The big consumer is the PIR sensor. I recently switched to a new one, because it only needs 3.3V and because of its low current draw. Note that the PIR motion sensor is the only component in the Room Node which should not be disabled during sleep: the whole point is to be able to detect motion at all times, after all. Also, a PIR sensor needs time to stabilize, in the order of a minute, so turning it on briefly is no option.
Here’s the good news:
That’s 41 µA idle current.
The JeeNode draws ≈ 3.3 µa, let’s round it up to 4 µA.
The SHT11 draws 2..5 µW, let’s round it up to 2 µA.
So total idle draw is 47 µA.
As it turns out, transmissions consume a neglegible amount of current – if the frequency of transmission is 5 minutes, for example (i.e. 288x per day). The motion detection is different, this needs to go out as soon as there is motion, but this probably won’t happen more than a dozen times a day, on average).
For a transmission, the RFM12B needs to be on for about 2 ms, at which point it draws around 25 mA. With 2 ms once every 300 seconds, that’s a duty cycle of 1:150000, i.e. under 2 µA when averaged out.
So let’s round up and assume that the average current draw of a Room Node is 50 µA @ 3.3V.
A 3x AA battery pack of 2500 mAh would last 50,000 hours, i.e. over 5 years if we ignore the self-discharge rate … assuming that the low-power code is perfect, that is!
Now for the AA Power Board lifetime estimate. At very low current levels, the efficiency of the LTC3525 regulator is 75..80%, according to the datasheet. But first, we need to estimate the current draw. Keep in mind that the power source will be 1.2 .. 1.3V, so we basically need a 3-fold step up. Let’s calculate the input current draw, assuming 1.2V in, 3.3 V out @ 50 µA, and 75% efficiency:
input current = 50 x 3.3 / 1.2 / 0.75 = 183 µA
With an Eneloop battery, I’ll assume it has 1900 mAh capacity, and loses 15% of its charge in two years. Let’s assume that it’ll be used no more than 2 years, and that it lost that 15% charge right at the beginning. Then this is effectively nothing more than a 1600 mAh battery.
Ok, how long can it power a JeeNode plus Room Board?
lifetime = 1600 / 0.183 = 8743 hours = 364 days
So all in all this setup should be able to run just about one year off a single rechargable AA battery. Note that there was some rounding in several places in these calculations, so these estimates are probably a bit conservative.
One year battery life… that’s in fact exactly what I was hoping for!
Now the “only” remaining challenge is to optimize the “rooms.pde” sketch enough to make this happen.