PV panel draining batteries at night?

stevehull · 06-09-14, 04:40 PM

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

Daox · 06-09-14, 04:51 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, 05:39 PM

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

NiHaoMike · 06-09-14, 08:33 PM

Use a Schottky diode for the lowest voltage drop.

where2 · 06-09-14, 09:42 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, 07:45 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, 09:13 AM

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

stevehull · 06-13-14, 12:08 PM

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

Daox · 06-13-14, 12:12 PM

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

stevehull · 06-13-14, 05:45 PM

Quote:

Originally Posted by NiHaoMike

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.

Thanks!

Steve

06-09-14, 04:40 PM	#1
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	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 __________________ consulting on geothermal heating/cooling & rational energy use since 1990

06-09-14, 04:51 PM	#2
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	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. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

06-09-14, 05:39 PM	#3
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	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 __________________ consulting on geothermal heating/cooling & rational energy use since 1990

06-09-14, 08:33 PM	#4
NiHaoMike Supreme EcoRenovator Join Date: Oct 2008 Location: Austin, TX Posts: 1,154 Thanks: 14 Thanked 257 Times in 241 Posts	Use a Schottky diode for the lowest voltage drop. __________________ To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...

06-13-14, 09:13 AM	#7
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Sorry, I don't know or remember how much power the panel consumed in the dark. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

06-09-14, 09:42 PM	#5
where2 DIY Geek Join Date: Apr 2013 Location: Sunny Florida Posts: 401 Thanks: 74 Thanked 83 Times in 73 Posts	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.

06-13-14, 07:45 AM	#6
verdigo Helper EcoRenovator Join Date: Dec 2012 Location: Cave City Kentucky Posts: 45 Thanks: 7 Thanked 7 Times in 4 Posts	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.

06-13-14, 12:08 PM	#8
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	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 __________________ consulting on geothermal heating/cooling & rational energy use since 1990

06-13-14, 12:12 PM	#9
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Unless it specifically says it does, I would assume not. Most don't. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.