Building a solar charger

It's July and this has special meaning for Swedes. They disappear. Where once the town was vibrant with the comings and goings of students, it's now quiet with a preponderance of people who are, well, over 40. Many shops have shut and signs of "sommarstängt" abound. At work, the doors that are normally open now require a passcard and code to enter. Many co-workers left on Friday with wishings of "trevlig sommar". So what about the rest of us who chose to stay on and continue working in the silent halls of the lab? While I don't know what everyone else is up to, I can describe Uwe's and my just-for-fun project: building solar chargers.

Previously I've described a solar module. I've made many modules during my years at Ångström, but almost all have been used for some very destructive tests to see what they can tolerate. Last fall, I decided to make a batch of 20 modules that I would keep for fun. I ended up destroying 5 or so while trying to sandblast the edges to remove areas where the modules would short-circuit themselves and while trying to glue protective cover glass on them. Some modules were duds, but I think there are about 10 left for fun projects.

To get started, Uwe and I needed to buy a couple of things. I headed to a store called "Gallerix" where I bought a picture frame. This will house the module and it also has a stand for support. Uwe went off to "PC City" to buy a cell phone/ipod charger kit designed to be powered by the 12 volt outlet in cars aka cigarette lighter. The output of the charger needs to be a regulated 5 or so volts. Uwe guesses the charger to be about 85% effective - in other words, it will sap 15% of the incoming power. Our financial outlay: 50 crowns for the frame, 100 for the charger kit and one stolen solar module .

Next, Uwe gutted the charger to get at the electronics inside. We measured the module under the solar simulator to see what kind of ouput it has: 11.8% efficient, with 12.3 volts and 76 milliamps at maximum power output. Uwe said we needed a larger capacitor on the output side to prevent the charger from going into short-circuit mode. I included a picture of the I-V curve with the charger electronics and the new capacitor to be installed for the benefit of my geekier friends.

With all the parts in place, the rest was assembly. Uwe replaced the capacitor, soldered a couple of wires from the solar module to the charger and soldered the connection cable that came with the kit on the output side. To house the electronics, I cut the soft plastic packaging that came with the kit and we glued it with the electronics inside to the back of the picture frame. We also stuck on a wire holder so that the cable can be neatly coiled when not being used. A quick test under sunlight revealed that, yep, the thing works. The voltmeter reads 5.4 volts and the little led charger light is lit.

The next day, today that is, I was eager to see whether the module was worth the effort. A quick check of the connectors that came with the kit revealed that I didn't have the one needed to charge my digital camera which takes a USB connector. Nevermind, I did get the connector to charge my cell phone, a Sony Ericsson K220i. The cell phone indicates battery life in fourths. Great - only one fourth left! I headed to my covered veranda to point the solar module into the sun while keeping the phone itself out of the sun and, possibly, out of the rain. The immediate indication from the phone was good - when I plugged it in, the display showed "charging". I spent the afternoon checking progress and repositioning the module while writing this post. The weather varied with a couple of hours of sun and then clouding up. I finally brought it in a half hour ago. It's completely cloudy now and the phone no longer indicated it was charging even though the little led on the back of the module was still lit. Result: the battery's 3/4 charged now.

I'm pleased.