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.

Steve

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.

Daox,

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 . ..

Steve







Steve

Use a Schottky diode for the lowest voltage drop.

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.

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.

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

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?

Steve

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

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

Thanks!

Steve

I agree that the panel is to small if the battery is being used at all.

I have a few 10 watt panels that I connect to batteries that have a slow discharge from vehicle clocks and such and they work perfect for that, but I would not expect them to charge it in the winter.

The $30-50 option is to buy a 2nd solar panel.

My neighbor has a Mule opener. The battery lasts 2 years. His issue was not enough charging capacity vs. times the gate was opened. I added a second 20 watt panel and a deep cycle battery. I also buried the battery in a battery box almost to the top. That stabilized the temperature.
The battery charger in his unit was made for bulk charging only. It never reached float. So, occasionally when he was waiting at the gate for the school bus, I had him apply a quick charge once a month. So far that battery has lasted 4 years.
BTW, a fully charged lead acid battery will not freeze in -50 deg F.

About diodes
 

The 1N4004 is a silicon diode, it's a member of the 1N00x series of diodes 1N4001 has a peak inverse voltage (PIV) of 50 volts, the top 1N4007 has a PIV of 1000V, silicon diodes have ~ a .7 volt forward voltage drop (insertion loss). I buy these diodes by the hundred for 7 cents each, I always buy the 1N4007 because they work in ANY application calling for a diode in the 1N00x series.

At one amp, which these diodes are rated at, the drop will be .7 volts across the diode.

Shottky diodes have much less insertion loss, a 1N5819 has about a .25 to .35 voltage drop under load but the schottky diode has higher leakage than silicon diodes. For PV's you must put a diode in series with the battery or the battery will discharge into the module. For low current applications you can use a 1N5817 or a 1N5819, the 1N5817 has a lower PIV, the leakage is low enough on these diodes that the module won't drain the battery at night.

Rob