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

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:

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.