Computing stuff tied to the physical world

Room Node battery life

In AVR, Hardware on Oct 6, 2010 at 00:01

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.

The good news is that it’s almost entirely a software issue. There have been JeeNodes running for many months now, some up to 10 months, so there is no hardware limitation to achieve such lifetimes.

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:

Dsc 2077

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.

  1. I have a comercial wireless alarm system installed, which used PIR detectors in the rooms. I remember reading in the manual, that in order to save power in the emiters, these are put into power saving for 3 or 4 minutes, hence not detecting movement, after movement has been detected, i.e., they are always on, except after detecting and sending the report. This would lead to some extra savings during the hours when there is someone in the room. If, due to the stabilization time, the sensor would be on for 1 minute in, say, every 3 minutes after movement has been detected and while movement is detected, only 1/3 of the power would be consumed during what is hopefully a good chunk of the day, and in the worst case the calculation above would apply.

    It would also mean that the house would be (and mine is) vulnerable in up to 3 minutes after setting the alarm!

  2. 364 days… It’s a fix!

    /me goes to fetch pen and paper…


    Pretty impressive though :-)

  3. There is one serious error in your assumptions. NiMH batteries self discharge just sitting on the shelf let alone with a current draw regardless of how small. I recall reading that it was as high as 1% per day when they were first introduced. I refer you to the following reference.

    What is the shelf life of a NiMH battery?

    You probably mean to ask: What is the self discharge rate of a NiMH battery? The rate of self discharge for any battery depends on the temperature at which it’s stored. Stored at 70 degrees F (20 C) NiMH batteries will lose up to 40% of their charge within a month. If they are stored at a higher temperature, they will self discharge at an even higher rate. Stored at a lower temperature they self discharge at a lower rate (Self discharge chart available soon)

    • Ah, but these are not ordinary NiMh batteries. The ones I was referring to are based on the Sanyo Eneloop batteries. From the charger webpage:

      Unlike many other reusable batteries, these batteries have an incredibly long service life — up to 10 years.

      I see that my assumption of 15% loss after 2 years, is actually 15% loss after one year and 20% after 2, but that doesn’t change the assumptions I made substantially.

  4. You can still achieve near infinite runtimes when you (partially) recharge the accu, using e.g. a small solar panel. or maybe a coil to harvest the 50Hz hum ? you consume 183 uA times 24 hrs in a day. if you want to recoop that in 6 hours of daylight you will need 0,8 mA from the solar panel at just over 1,5 volt. If my math is correct. the accu is just a big buffer (which can, on it’s own, survive the dark winter if it lasts 365 days unaided..)

    • Totally agree. One reason to aim for a 1-year runtime, was to see whether we can then later find some energy source which – however intermittent – can supply enough energy to top up the charge. If solar, then the only requirement would be that the battery is full again at the end of summer. It seems like this should be feasible, even indoor!

  5. What’s happened to Stef? Last time I saw him he was about to try a solar panel and 1F supercap.

  6. I believe service life is referring to how long you can expect your batteries to be serviceable, not how long they will hold a charge.

    An occasional trickle charge as mentioned by RvA will lengthen your run time tremendously.

    For the longest run times neither NiMH, nor NiCad are your best choice. A quality alkaline battery will hold its charge far longer, and the newer Li AA cells designed for high current draw devices such as cameras should last the longest.

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