DIY USB solar charger

Hungarian version.

I would like to show you my solar USB charger what I built in 2009. I have ordered from Ebay from China 6 solar panels for 64 USD. It would cost 72 USD today if we calculate inflation. I have put the panels into a DVD case, which is getting a bit worn in the past 7 years. I want to build a new case for them. By the way, who watches DVDs these days?

The Diode is hidden behind the duct tape.

These are 6V 133mA panels, so 0,8W per panel. So the six pieces I bought together is 4,8W for 72 USD. Today I could buy more than 35W power rated panels for the same price. In 7 years their price dropped to seventh of the original price!

I was thinking that 6V would be good to charge USB devices, because the USB specification suggests 5V. As it can be seen on the picture I used three panels and I connected the in parallel, so theoretically they will give out 400mA current. I have also put a diode in front of the USB connector to prevent the current flow to the panel direction.

The solar panels supply the specification ratings at ideal conditions and without any load on them. I was curious how much power they give under load. I have used my magic KEWEISI usb capacity meter to measure the voltage and current. It seems the three solar panel supply around 4.6V with 0,2A current. It is interesting that they supply around 10% less power if I used them in a horizontal position, not in an ideal angle. In horizontal angle they supply 4.5V with 0,15-0,17A.

As it charges very slowly, I barely used this charger. I usually use it if I go to a bicycle excursion and I am charging my PDA.

The funny thing is that, I have sold the three leftover solar panels on an auction site for double the price than I have paid for them, so the three I am using were for free. :)

Convert your calculator to run on rechargeable batteries

Hungarian version.

I have a solar powered calculator, which is cool, but it stopped working recently that the light bulbs are changed either to leds or fluorescent bulbs. After disassembly it turned out, that it has a 1,5V button battery which helps powering the device together with the solar cell. But the battery has drained, so it worked only from solar power. However as the light from the new light sources is not enough, it only works if I put it so close to the light. It does not work on the table. I decided to add an AA battery case which would power it instead of the button cell and it would also last longer and rechargeable.

I had some solar powered garden lamps which do not work, I took the battery case from this. I have cut the unneeded plastic from the battery case. I can also use the NI-CD 600 mAh battery from it. If it would drain too fast, I can switch to normal AA alkalines.

Strangely there is a green led on the circuit board, it can be seen on the upper part. It seemed that it is there unnecessarily, only draining power without purpose. I thought if I take it out, the solar cell will give enough power to run the calculator from the desk. However once I removed it, it still does not worked from the desk, if I moved it closer to the light, it turned on, but the display get too much power and it became dark. So the led is there simply to regulate the current. It seems to be ridiculous.
And it turned out, that the calculator only turns on if the voltage is sufficient. So there is no dim display, you can only use it if it gets a certain amount of voltage. I do not think this is a good design. So my only chance left is to connect the battery to the button cell case on the circuit board.

I used a two sided glue tape to stick the battery case to the calculator case. Luckily the display can be lift up so the extra battery case has space behind it. If it would not bend, I would have to stick it under the frame. The setup works great, the solar cell helps to the AA battery to power the calculator.