SolarSlug - A Solar Powered Slug

In early 2006 I fitted a small solar panel to the side of my house. The original idea was to power my birdbox cam from it, but the bluetits never moved in and it has been idle. So I thought, why not try to power my slug from it?

The panel nominally generates 4.8 W at 12 V. Even half of that could be enough to run a Slug [aka Linksys NSLU2, a 266 MHz XScale Linux box]. All that's needed is an efficient power supply.

Then I saw a couple of 12 V to 5.1 V DC-DC converter modules on Ebay. They claim to be 76% efficient, which is far from impressive, but even so I couldn't resist, and have assembled one into this neat little converter box:

First I did some bench experiments, with a direct 5 V supply and a 12 V supply via the converter, and with the Slug alone and with the Slug plus a 512 Mbyte flash stick:

5 V 12 V
Slug 350 mA, 1.75 W 200 mA, 2.4 W
Slug + Flash 500 mA, 2.5 W 280 mA, 3.36 W

So the converter is about 73% efficient in practice, and I'll need to get 70% of the rated power from the panel to drive the Slug with its flash drive.

Before and after work the sun is at too oblique an angle to the panel to get much power out of it, so my experiments have had to wait for the weekend. Here is the test rig on the bathroom windowsill:

The weather this morning was sunny but not yet shining directly onto the panel, and the Slug failed to start. This afternoon the sun was more direct but the sky was slightly hazy; when it was at its brightest it was possible to boot the Slug alone (pictured), but it was never bright enough to boot the Slug with its flash stick.

In the picture you can see that I tried adding a 1000 uF capacitor across the input; this probably didn't make much difference. When the panel was unable to meet the demand from the Slug the regulator would get into a "100% duty cycle" condition from which it cannot recover without disconnecting; a better behaviour in solar applications would be to shutdown for a few seconds. I plan to investigate this further.

I will try again, but my conclusion so far is that you need a panel rated considerably above 4.8 W to run a Slug from.