This question is in regards to a 100 Watt solar panel that is less than a year old.

The solar panel was working fine but recently something happened.

Now when I hook the solar panel up to my batteries, the solar panel starts drawing energy from the battery. The panel used to be charging my batteries but now it's pulling energy from them.

There's no broken glass and it doesn't appear damaged at all. Still in nice condition.

What could have happened?


closed as off-topic by Brian Carlton, Leon Heller, uint128_t, Voltage Spike, Bimpelrekkie Dec 7 '16 at 9:43

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    \$\begingroup\$ There's supposed to be a diode to prevent that, which may have shorted out. \$\endgroup\$ – pjc50 Dec 6 '16 at 18:35
  • \$\begingroup\$ Use an MPPT tracker \$\endgroup\$ – Voltage Spike Dec 6 '16 at 23:24

As pjc50 mentioned in his comment, most solar panels have a blocking diode in series with one of the output leads.

In the solar panels that I have, this diode is located in the junction box on the back side of the panel.

The diode needs to be able to handle the full current rating of the panel.

Most likely cause of the problem that you are seeing is that this diode is shorted.


You shouldn't connect raw solar panels directly to a battery you don't want destroyed. Solar panels, unregulated, will overcharge a battery and boil out its electrolyte or blow it up (if it's lithium).

Use literally any charge controller and you will eliminate this problem and many others. I'm fond of the Morningstar SunGuard for small panels, because it's a top-tier manufacturer but a sensible price.

Some solar panels packaged as a commercial product have battery charger/controllers built into them. Maybe that's what you have.

If the panel is large enough for efficient charging to be worth about $150, consider an MPPT controller. It has a neat feature: it will dynamically adjust the current it draws from the panel, scanning to maximize voltage x current = power. Like any controller it converts solar output voltage to battery voltage.


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