DIY Tip: Check for Faulty Solar Panel Blocking Diodes

Solar panels are guaranteed for life, but over the years small problems may surface.  If you experience a drop in the power output or your batteries are not charging properly, something is likely wrong with your solar power system.

You can trace the problem with the use of a voltmeter, but do this only if you’re knowledgeable about what you’re doing.  Remember that you’re dealing with a live system, so the danger of electrocution exists.  Call a professional if you have doubts that you can handle the job.

In cases where power drops, the main culprits are either corrosion of connectors or a faulty blocking diode.  The blocking diode prevents current from flowing back from the batteries to the solar panels.  Every blocking diode has one to prevent battery discharge, so if your solar panel has a faulty blocking diode it will not work properly.

If corrosion is not the problem, then it’s time to check the solar panels.  You could disconnect the solar panels or disconnect the batteries from the system and check the output of each solar panel.  If it’s below the expected output, it can be due to a problem solar cell or a faulty blocking diode.  A problem cell will be hotter than the rest of the solar cells.  If you can’t find any problem cells, then your problem is almost certainly a blocking diode problem.

Check whether the blocking diode is installed with the silver ring away from the solar panel.  Some diodes have an arrow to indicate the current flow.  With the multi-meter set to OHMS, place the common probe on the end away from the diode’s ring end and the other probe at the ring end.  When functioning normally the reading should be zero or at a very low level.  If the result is high, then there’s a problem with the blocking diode.  You should replace the blocking diode for your system to work properly again.

Always use a single, properly rated blocking diode. Don’t connect blocking diodes in parallel to get the right rating, as this can lead to problems in the future.  Diodes often don’t match, and the weakest diode in the connection will take the entire load and subsequently fail.  This will continue until all your diodes fail, preventing the current from flowing into your batteries.

You should conduct periodic maintenance checks on your DIY solar panel system to prevent small problems from becoming huge headaches.  Make sure you follow the original manual when installing a system to avoid damaging it.

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