Computing stuff tied to the physical world

Meet the AA Power board

In Hardware on Sep 16, 2010 at 00:01

Remember the recent post about running a JeeNode off a single battery?

Dsc 1897

That was 9 days ago, and that LED shown above is still lit. It’s not a huge accomplishment, because the LED draws only about 1.6 mA and is very dimly lit, but still. Over two hundred hours of operation time, and the rechargeable battery I used (this one) is still going. It started off at 1.35V and is now at a decent 1.22V level.

Assuming a 70% efficiency (at such low voltages, it probably won’t be much higher than that), and an average of 1.3V getting boosted to 3.3V, the power consumption over this period is … scribble, scribble … 1.6 * 3.3 / 1.3 / 0.7 * 216 = 1253 mAh, i.e. about two thirds of the total capacity of this one-cell AA battery.

Not too shabby. Even without power-down tricks, just by entering idle mode in all the busy loops, an ATmega328P will draw less than 1.6 mA when running at 16 MHz on 3.3V (which is slightly overclocked, but it works just fine). Idle mode is very nice, because you don’t have to play any serious low-power tricks, you just have to tell the ATmega: I have nothing to do right now, take it easy until the next interrupt occurs.

In other words: using only the simplest of all the low power tricks, a JeeNode can run about two weeks off a single AA battery (less, if you have power-hungry peripheral chips connected, of course).

Try running an Arduino Duemilanove off a 9V battery, or even a 4x AA battery pack. Fine board, but it sure ain’t low power…

Anyway, I’ve decided to design an AA Power board for this little power magician:

Screen Shot 2010 09 15 at 21.20.21

It’s not specific to the JeeNode, but it was of course designed to be a particularly good match for it:

  • the width of the board is exactly the same as JeeNodes and all the JeePlugs
  • it can be attached in-line, i.e. like an FTDI board/cable
  • it can be used in piggy-back mode, i.e. attached to the back of a JeeNode
  • or use it in shield mode, i.e. on top of a JeeNode with stacking headers
  • it has mounting holes (since there is plenty of room on this thing)
  • the board can be fitted with either AA battery clips, or AAA battery clips
  • or attach external batteries to it: one, two, three (even four, if NiMh) … you decide!

Whatever the power source is, this board will either boost or step-down the voltage as needed. The efficiency will be up to 95%, depending on load and supply voltage. But in any case, it’ll be way more efficient than any linear voltage regulator, including the one on the JeeNode.

One AA is smaller than 3 or 4, obviously, so a JeeNode with this AA Power board will also fit into smaller places.

In case you insist on using 3x or 4x AA’s, then you’lll still benefit from a much higher conversion efficiency, and the ability to run the batteries down until they are completely empty. In other words: they will last longer!

Now it’s just a matter of waiting to find out whether I did all my homework properly, and got the connections, traces, and physical dimensions right.

  1. When you wrote the original post I was interested, and an image of how my ideal single AA voltage boosting board formed in my mind…

    And now I see it in front of me, and I didn’t even have to anything!

    Now where do I order?

  2. That should have been “I didn’t even have to do anything!”… To excited to type you see :)

  3. Too excited to type “too” correctly too! Aaaaah!

  4. Hehe.. Mounting holes! :D

    I guess this isn’t the right place, but I have been wondering what’s the preferred way to mount the JeeNodes, beyond foam board and hotglue :P

    Anyway. Very cool stuff as usual. These remarks feels somewhat obvious and superfluous now though.

  5. Cute! But there is still plenty of empty space on that board. What about the idea to add recharging the rechargeable battery pack as an assembly option? That will allow for the option to build a boxed mobile Node (i.e Sensors & data logger in a closed Box to go). Yee! It just crawled to my mind: What about a wireless recharge option? That would allow for a completely sealed (i.e. waterproof) node for uses in the wild!

  6. If it could also charge the AA / AAA from an attached solar cell…

  7. W.r.t. charging: one option is to use “trickle-charging”, i.e. a current which is so low that it won’t damage the battery when maintained indefinitely. That can be as simple as adding a resistor between the charging source (could be the PWR pin) and the battery “+”.

    I’ll talk some more about this in an upcoming post.

  8. I found an interesting trickle charging circuit called the “solar piston” at another source of inspiration is the BEAM robotics sites.

    • Interesting indeed – thanks! – I wonder why they did it that way, though. Why not use just the S-808, and use it to connect when the solar panel is above 3.5V? No diode, no cap, no resistor, no MOSFET. Would charge when it can and stop charging when it can’t. Maybe keep the cap in, but I’m not convinced that the diode is essential. But then, I’ve never tried this. Could be fun.

  9. Got mine got mine! skips round room throwing 66% of his AA battery collection about like confetti

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