Faulty Solar Panels

[StuartO]

Our solar panels are five or six years old and sometime over the winter storage period they stopped generating.  I thought it was probably the lack of sunshine but it became clear that the zero solar output registered on our MH's Schaudt control panel was for real.

 

I rang the suppliers for advice and they told me what to check:  disconnect the power lead from the panels at the regulator and check the open circuit voltage, which is normally 15-21 volts.  I got nothing, so something on the roof was the likely problem.  It could also have been simply a blown fuse and if it had been the regulator they would have sent me one to fit myself but it wasn't -  and I was only 80 miles from them, so we arranged for me to bring the MH in for investigation.

 

They confirmed my findings and set about the panels  The panels themselves are not repairable but in a weatherproof junction box underneath are the connections and the set of bridging and blocking diodes, which are normally very reliable but can fail.  The technique for repair was therefore going to be to replace all the diodes, in this case four, then re-install the panels and test again.  Fortunately for me this did the trick.

 

The diodes are normal, low cost electronic components (available from places like Maplins) so if you can cope with un-bolting the panels on the roof to get them down and use a soldering iron to change the diodes, taking care to fit them the right way around, this could be a DIY job.  My gymnastic days are over and I was content to pay them for the couple of hours they spent doing this job.

 

Why did three of my four diodes diodes fail after five or six years of valuable service?  That's a mystery and will remain so.  Only a tiny failure rate (much less than five percent) occurs.  Inside the junction box all was snug and dry so maybe it was just a bad batch.

 

Is it worth carrying spare diodes?  Maybe, because solar panels are so useful these days that they are almost mission critical.  Diodes are also small and light, so why not?

 

But solar panels are all made in China, all made the same way and are installed using blocking and bridging diodes in the same way by most caravan and motorhome dealers these days, so you should be able to get a repair without travelling too far.  Knowing that the diodes can be changed to make a cheap repair should prevent you from being sold replacement panels unnecessarily.

 

My Dorset-based supplier/repairer (www.solarsolutions.co.uk) could not have been more helpful and I heartily recommend them.  They also do camera, satellite TV and other gadgety things.

 

[Charles]

I wonder if they were undersized and therefore had a short life. What is the component number on the diodes? Also what wattage is the panel?

Big diodes are called stud diodes and will handle much bigger currents than the standard black plastic ones.

[StuartO]

I rang them again this morning to ask them to send me some spares and they are getting back to me - because their stock is low at present. 

 

The diodes are cylindrical shaped components with a wire at each end.  Black with a grey bandat one end.  The technician said he thought they were rated at 9 amps but I can't see anything larger than 6 amps on the Maplins website.

 

[Charles]

A 100w panel will produce 8.3 amps in full sun so they might have been getting hot.

 

Look at these bad lads:

 

Look at this on eBay http://www.ebay.co.uk/itm/201685442913

[Charles]

Actually it will be less than 8.3 amps but even at 6amps they'll get hot.

[StuartO]

My panels failed around October/November and so the current would have been modest at that time of year, even in bright sun - I would guess no more than 3 or 4 amps max.  We have experienced high generation in bright summer sun in France but even so I've never seen more than 6 amps from the panels reported on my Schaudt control panel.

 

The Maplins website product specification for their black/grey 6 amp diodes cites a maximum (peak) forward current of 400 amps (!) with an average current tolerance of 6 amps, so I would guess they would do the job on my MH.

 

 

[plwsm2000]

Usually CPC (http://cpc.farnell.com) has better stock of electronic parts than maplin and are usually cheaper.

Do you know if these are schottky diodes or standard rectifier diodes? (you need to look up the part number to tell). Basically schottky diodes are much more efficient in certain applications as they drop less volts than standard rectifier diodes (typically 0.5V instead of about 1V when passing several amps). This may not seem alot, but the reason they get hot is due to their power dissipation (power = volts x amps) so for the same amps, you will almost half the power dissipation.

The main issue with schottky diodes is that they usually take less volts across them in the reverse bias condition. This should not be a problem in solar panels when combining the outputs of the banks of cells because they should be conducting (forward bias) unless one of the banks has failed or it is heavily shaded.