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Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5107

Posted: 01:05am 21 May 2006

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I bought a 2nd hand solar panel last year. Its a 70 watt BP panel for 12volt charging. The panel had not been mounted very well by the original owner, and this lead to cracking through several cells. Remember if you buy a solar panel to mount it on a very secure frame, as any flexing in the panel caused by wind will cause cracking in the cells.

The panel's output was way down, only a couple of amps shorted, but its output voltage was still 19 volts, so it would still be ok to trickle charge a battery. The panel was good for about 20 watts, and I bought it for $50.

It worked good for a few months, but it started to get intermittent joints. The output current would jump around, one minute it was 1 amp, the next nothing. As the panel was heating up, the cracks were causing bad connections, and it slowly got worse untill the output was only a few milliamps. So the panel was useless, unless I could fix it.

You can see the cell on the right is full of cracks, while the cell on the left is in perfect condition.

These BP panels are well made and impossible to pull apart, as the whole panel is silliconed together. The backing plate is a couple of sheets of white plastic about 1mm thick. To access the connections between the cells I had to gently cut through the backing plate. I needed to bypass each faulty cell, so I had to break the connection to the faulty cell to effectively open circuit it. I didn't want to simply short the cell as each cell did occasionally work, and a shorted cell would get hot. I soldered in some short jumper leads across the isolated cell, been carefull to not overheat the good cells.

This was a very delicate operation, and took an hour to bypass 6 cells in total. Once completed I took the panel into the sun to test. 15 volts open circuit, 5 amps shorted. I connected up a 12 volt battery to test charging current and measured 2 amps. And there was no intermittent operation, the panel was now stable and reliable. So I took the panel back inside and smeared silicon on all the new connections to keep moisture out, and then mounted it back on the shed roof.

So I'm pretty happy. I have a 24 watt panel for $50. That 2 amps will keep the battery charged when the winds not blowing.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

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