Computing stuff tied to the physical world

Interesting battery option

In Hardware on Jan 19, 2010 at 00:01

Found an interesting (one-time, i.e. disposable) battery cell the other day at Farnell:

Screen shot 2010-01-17 at 21.03.13.png

These have the size of an AA cell, but they deliver 3.6V, so they can effectively replace 3 AA cells at once. The voltage is just right – this battery could be used with or without the JeeNode’s 3.3V voltage regulator.

The discharge graph is as follows:

Screen shot 2010-01-19 at 16.47.29.png

I’ve ordered a few with wires attached to them, so they could be tied directly to the PWR & GND pins of the FTDI connector. Will report back once some “field tests” have been done.

Update – better discharge graph, see comments below.

  1. Price seems to be about 6x the price of an Alkaline (this depends much on the brand though), but with comparible Ah and 3x as much volts. That would be about twice as much per energy unit for getting the 3x higher energy density. Looks like an interesting option for reducing space requirements while maintainig duration.

  2. Hmmm, isn’t the graph a bit odd for 20mA? Ten hours of 20mA doesn’t add up to 2Ah.

    • Indeed – I’ll need to test it. Room nodes will draw under 100 µA most of the time, with one or two very brief 15..30 mA spikes per minute. One benefit is that this battery keeps its voltage, but that will also make it impossible to predict when it runs out.

  3. Hmm, I prefer something thats widely available and cheap. Don’t want to order from farnell everytime my battery runs out.

    Have you thought about using a step-up regulator with two AA cells? You could scale down the clock of the Atmega and run it direct from the 2-2.4 V during sleep. On wakeup enable the step-up, scale up the clock again and work.

    This way you could run a jeenode with just two regular AAs or even AAAs and still wouldn’t waste energy from running the step-up all the time.

    Downside is you’d have to change the board layout to add such a step-up configuration.

    • That 3.6V battery is an option, not a requirement :)

      Yes, in fact I’ve been looking into just the approach you mention, using an MCP1257. One idea, if I can get the average draw well under 100 µA, is to combine this with 2x AAA, and either mount the whole thing on the back of the board, or on the FTDI connector. By shorting out PWR/+3V the voltage regulator can easily be disabled. Such a combination happens to be exactly the same width as a JeeNode.

  4. There is a newer version of the ER14505 datasheet available where the 20mA curve looks much more reasonable:

  5. Very similar to these ones, which may be cheaper:

    • Ah, nice! I won’t be re-selling from them, but for people want to get their own cells, that might indeed be quite a bargain.

  6. I’ve removed a comment from “moi@mailinator,com” (if that’s you: please comment in a less anonymous way).

  7. I have noticed that rechargable batteries are not always suitable for devices which sip power or are infrequently used. Their self discharge rate sometimes exceeds the rate of usage. Will be interested to see the performance of this sealed unit. It has impressive specs for its size.

    • The self-discharge highly depends on the type of battery used. Some models are optimized for capacity and reach up to 3000mAh per AA cell, but have a high self-discharge. And other models are “only” about 2100mAh but have very low self-discharge. One brand with low self-discharge is eneloop. I switched nearly every rechargeable in my household to eneloop about a year ago and don’t have problems with self-discharge anymore.

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