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stevehull 06-09-14 03:40 PM

PV panel draining batteries at night?
This is a question for those of you that have made a PV panel or two out of wafers (made without a blocking diode).

Here is the experiment . . .

Hook up a panel to a storage battery with no controller. With light, there is current going into the battery. We all (hopefully!) agree.

What happens when you put the panel in the dark hooked up to the battery? I am not trying to reveal my bias, but can one or two of you do this for me and disclose that here?

The issue is, does a PV panel without a diode or controller, allow the battery to discharge into the PV panel at night?

Thanks in advance.


Daox 06-09-14 03:51 PM

Yes, if you don't use a diode you will drain your battery. I measured this with an ammeter while I was doing the testing on my solar alarm clock.

stevehull 06-09-14 04:39 PM


What was the back flow current (in the dark) as a resistive equivalent. In other words, was the backflow significant as in the PV panel "looked like" a low resistance shunt? Am trying to recall the Norton or Thevinen equivalent . . . .

In other words, how low was the equivalent resistance (50 ohms or less?).

I am discussing this with a friend that has a Apollo driveway gate opener (deep cell marine battery charged with a 10W PV panel). He complains that he has to replace the battery every couple of years. It fails in mid January every time. He is blaming the control board and to replace it Apollo wants $375.

The 10W panel is likely to small to keep up with his gate opening frequency in mid winter when we have little sun and LOTS of dark night. Finally the battery electrolyte is almost like water - and on a frigid cold night (down to 5 F or so) it freezes, shatters the partitions between cells and is useless as a 12 V battery.

Suggested a 1N004 diode ($0.20, e-Bay) in series with the positive lead to battery . . . but he doesn't believe that the battery will back feed at night.

$0.20 vs $375 . ..



NiHaoMike 06-09-14 07:33 PM

Use a Schottky diode for the lowest voltage drop.

where2 06-09-14 08:42 PM

I would second the consideration of insulating the battery from its cold surroundings as much as possible. An inexpensive used cooler from a yard sale might work well. Give the cooler a custom paint job and create a small opening at the highest point in the lid to vent the hydrogen. An inverted "P-trap" made out of 1/2" PVC would keep rain from running in the vent hole. If there is "extra" space in the insulated box, I'd add a few jugs mixed with an anti-freeze solution to trap more BTU in the space and buffer the temperature swings.

verdigo 06-13-14 06:45 AM

Assuming a 12 volt 80 ah battery 10w /12v = .8 amp of charging capability in good full sun. Waaaaay under charging. That little solar panel is barely a battery maintainer, much less a charger. Using the 5% minimum charging rule he needs 4 amps.

Daox 06-13-14 08:13 AM

Sorry, I don't know or remember how much power the panel consumed in the dark.

stevehull 06-13-14 11:08 AM

I agree that 10 W is too small, but that is what came with the Apollo system. I have already recommended a 20W panel in addition to the 10 W (total of 30 W).

Do small 20 W panels (e Bay) come with a blocking diode in there?


Daox 06-13-14 11:12 AM

Unless it specifically says it does, I would assume not. Most don't.

stevehull 06-13-14 04:45 PM


Originally Posted by NiHaoMike (Post 38579)
Use a Schottky diode for the lowest voltage drop.

Mike, I already have about 25 of the 1N004s but when I run out I will get some SR5100 type.



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