Computing stuff tied to the physical world

First solar results

In AVR, Hardware on May 16, 2012 at 00:01

Some first results from trying to run a JeeNode off a 24 x 32 mm indoor solar cell…

In each of the cases described below, I’m using a JeeNode without regulator and with 100 µF cap hooked up, with fuses and settings as described in this post. The cap should have enough energy to cushion the dip from a small packet transmission. I’m using the latest radioBlip2 sketch, which now sends out the following 7-byte payload:

Screen Shot 2012 05 14 at 13 33 17

The benefit of this version, is that the sketch reports not just the battery level but also how far the battery level drops after sending out a packet once a minute. That value is sent out in the next packet, so it always lags.

To get started, I connect the JeeNode to a BUB, which charges the 100 µF cap to 5V (and runs the RFM12B slightly above spec). Then I disconnect and hook it up to the solar setup. This way I don’t have to deal with startup problems yet – which is an entirely different (and tricky) problem.

Yesterday’s elaborate setup didn’t get very far, unfortunately. Two different runs gave me just a few packets:

    L 09:16:01.571 usb-A600dVPp OK 17 1 0 0 0 1 209 0
    L 09:18:07.445 usb-A600dVPp OK 17 3 0 0 0 1 86 51
    L 09:19:10.308 usb-A600dVPp OK 17 4 0 0 0 1 86 50

    L 09:24:12.477 usb-A600dVPp OK 17 1 0 0 0 1 206 0
    L 09:25:15.707 usb-A600dVPp OK 17 2 0 0 0 1 86 210

Values are 20 mV steps, offset by 1V – the actual battery voltage is: 1 + 0.02 * X (where X is the reported value).

In the above runs, the battery is 86 (2.72V) before sending, and 50 (2.00V) after. That’s pretty close to the edge, I’m not sure why the drop is so large.

Another test with a 0.47 Farad supercap, charged for about 3 days to get the charge “deep” into the supercap, seems to fare a little better:

    L 09:43:06.943 usb-A600dVPp OK 17 19 0 0 0 1 210 52
    L 09:44:10.771 usb-A600dVPp OK 17 20 0 0 0 1 175 210
    L 09:45:14.549 usb-A600dVPp OK 17 21 0 0 0 1 175 146
    L 09:46:18.339 usb-A600dVPp OK 17 22 0 0 0 1 175 147
    L 09:47:22.100 usb-A600dVPp OK 17 23 0 0 0 1 175 147

That’s 4.50V and 3.94V before and after transmission, respectively. But a 0.47F supercap has a lot less energy in it than that 3.4 mAh Lithium cell used in the first tests above, so it’ll probably run down a lot faster.

After one hour, voltages drop to 4.28V and 3.72V. Two hours: 4.14V and 3.60V. Five hours: 3.92V and 3.36V. I’m not sure this will work, unless the node sends less at night perhaps or always restarts reliably the next day.

To be continued…

  1. If you charged the cap to 5V with yesterday’s setup, the main content of the cap would have gone via the resistor to the battery, wouldn’t it? Did you start with an empty battery and just the cap charged? Otherwise I would have expected a much longer life from the battery and not such a quick drop from the initial 5V.

    • The cap was charged via the solar cell, which seems to go up to about 4.50V.

      As for the battery, I don’t understand yet what is going on. I’ll charge it via a power supply and try again.

  2. Could it be that during the trial there was no sun on the solar cell to produce sufficient power and that the BAT43 (I think the schematics lists BAT34 but the picture shows BAT43) leaked through the regulator into the solar cell? I found a data sheet which mentions 0.5uA leakage current for BAT43.

  3. Another thought: Since you replaced the regulator on the Jeenode with just a cap the battery will also discharge through this cap thanks to leakage currents?

  4. As far as I can tell, leakage currents of both diode and cap are far under 1 µA at these voltage levels. I’ll be doing some tests to make sure this is indeed the case, but that’s what I’ve seen so far.

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