Solar panel power drain question

[HiddenMountain]

After looking around the site, it looks like this is the place to put silly questions. 

My wife and I have been having a bit of a disagreement regarding solar panels.

She says that they will drain the batteries if they aren't in the sunlight. I say that they don't. I really don't know much about them so we were hoping someone could prove that I'm right.    I am willing to admit if I'm wrong too though! Just not to her!

We don't use solar power except as a novelty. We live on the North side of an 8500ft mountain and the sun is basically gone for 3 1/2 months of the year, so solar just isn't practical here. 

[Opera House]

That's what a blocking diode is for.  Solar panels are just a series of diodes, but they are rather poor and most leak slightly.  I've used a lot of panels in parallel without blocking diodes at times.  Leakage would add up for days without sun.

[HiddenMountain]

Thanks for the quick reply Opera House.

Even though your in cahoots with the missus.......

I knew about diodes inside the panels, what about between each other and the battery bank?

Sorry if these are redundant questions........

Jon

[Flux]

Small panels usually have one series diode that blocks any drain during darkness. As you add more panels in parallel each will normally have its own blocking diode.

If you have panels without them you can add one diode per panel in the parallel string.

For larger installations at higher voltage you will find additional diodes which are normally blocking and these are connected across groups of cells. In normal use they do nothing but if part of the panel is shaded they provide a path for the remainder to continue producing current but at a reduced level. Without these the whole shaded string will produce nothing. They are not useful on small 12v systems so are not generally found in the lower cost panels.

Don't confuse these with the series blocking diode that is generally fitted in the connection box of most panels .

Flux

[Nil]

I may be wrong but I believe that most charge controllers keep the flow of electrons going one way, so there is no need for a blocking diode for nighttime or full shade conditions.

[DamonHD]

Most but not all controllers do this.

For example, the built-in BMS of my new LiFePO4 battery pack will allow at least 1/4A backflow at night.  I've made sure that all strings connected to is have blocking diodes built in or I add them.

Rgds

Damon

[HiddenMountain]

Most but not all controllers do this.

For example, the built-in BMS of my new LiFePO4 battery pack will allow at least 1/4A backflow at night.  I've made sure that all strings connected to is have blocking diodes built in or I add them.

Rgds

Damon

So, I'm guessing that these diodes are going to be pretty substantial then?

My wife had a solar array at her place before we got married and she didn't have charge controller. She said she'd "boil the batteries" if she didn't have a good load on the system. She's convinced that's why her batteries outlasted everyone else's in her neighborhood....

[ghurd]

They allow some back flow at night, but not much.
Decent panels are designed with extra voltage, and not be effected by the diode forward voltage drop, so there is no reason not to use a diode if Not using a controller.

She said she'd "boil the batteries" if she didn't have a good load on the system. She's convinced that's why her batteries outlasted everyone else's in her neighborhood....

It sounds like the batteries were not run down very far every night.  Less DoD means they last more years.

It could be other things too, like the batteries were mostly worn out but still enough to cover her use.

Decent solar controllers have a diode (or mosfet) suitable for the rated amps built in.
G-

[DamonHD]

Hi HM,

Well, not as much as you might expect, because I tend to use a separate one from each panel to the feed-in point to the controller/battery, so therefore each diode need only be big enough for that panel's maximum output.  All the diodes that I'm currently using (a mixture of ordinary silicon and Schottky) are plastic-package axial without so much as a heat sink.  They can get quite warm when the sun is bright, but never alarmingly so.

(I also do this so if damp or squirrels or whatever get into each individual panel then that can't suck away the energy that other panels are pumping in.)

Rgds

Damon

[ghurd]

(I also do this so if squirrels get into...)

Gotta watch out for those squirrels.

[HiddenMountain]

Hi HM,

Well, not as much as you might expect, because I tend to use a separate one from each panel to the feed-in point to the controller/battery, so therefore each diode need only be big enough for that panel's maximum output.  All the diodes that I'm currently using (a mixture of ordinary silicon and Schottky) are plastic-package axial without so much as a heat sink.  They can get quite warm when the sun is bright, but never alarmingly so.

(I also do this so if damp or squirrels or whatever get into each individual panel then that can't suck away the energy that other panels are pumping in.)

Rgds

Damon

Well, that's interesting. Thanks for the info.

We have our 4 solar panels tied into the 12V system, and I just have a switch to turn them on and off. I'll have to do some homework to find out each panels output and get some diodes inline.

(I also do this so if squirrels get into...)

Gotta watch out for those squirrels.

Funny, the  squirrels around here don't look like Chevy Chase....

[ghurd]

"We don't use solar power except as a novelty".

The panel should have a sticker on the back listing Isc = 'X' A

Get a diode rated for at least Twice that much current.
Get a diode rated for at least Twice the system voltage.
More rating does not cost much extra, and bigger is better.
Do Not parallel diodes.

Then you can get rid of that switch.   
G-

[rossw]

Most but not all controllers do this.

For example, the built-in BMS of my new LiFePO4 battery pack will allow at least 1/4A backflow at night.  I've made sure that all strings connected to is have blocking diodes built in or I add them.

Not wishing to be argumentative, but BMS != Charge Controller.

Unless I've very misinformed, the BMS on the LiFePO4 packs are there to balance the voltage across individual cells. As such, your power is supplied pretty much straight across the series pack and the BMS just tries to "partly bypass" high-voltage cells so the others get a fair charge.

As such, discarge would be entirely possible, nay inevitable.

"Charge Controllers" like the FlexMax 80 I use are substantial devices (in electronic complexity) performing DC-DC conversion, MPPT, logging etc and most certainly DO prevent backfeed to the panels.

[Ungrounded Lightning Rod]

"Charge Controllers" like the FlexMax 80 I use are substantial devices (in electronic complexity) performing DC-DC conversion, MPPT, logging etc and most certainly DO prevent backfeed to the panels.

If they're doing MPPT they're a switching regulator, which also serves as a blocking switch/diode.  So (as rossw said) you don't need the diodes to prevent backfeed at night.

Blocking diodes (even schottky) drop a nontrivial amount of voltage, and thus power if you have an MPPT controller.  So you might want to forego them if you have one of those.

However:  If you have a system with high-voltage panels or where panels are wired in series (typical with MPPT controllers), the voltage placed across a mostly-shaded panel from a fully-illuminated one might be enough to cause reverse breakdown.  Blocking diodes also protect against this, so you'll want them again despite the power cost.  If you're wired for nominal 24V or more from the panels, be sure each panel/series string has an individual blocking diode.

[DamonHD]

The MPPT solar controller for my SLA does indeed prevent backfeeding at night without any extra diodes.

I do however have some to isolate the parallel strings from one another.

The LiFePO4 battery + BMS was bought hot off the supplier's test bench in their office with the suggestion that that unit was designed to manage without a separate solar controller.  They connect a solar panel to its charge inputs and the load directly to the discharge outputs and the BMS keeps everything safe including LVD on the outputs for example.  The BMS is basically doing the job of a simple bang-bang controller plus cell balancer plus LVD.

I have suggested to them that they might want to include a diode on the input or modify their BMS to protect panels that don't have a built-in blocking diode.

Rgds

Damon

[rossw]

Blocking diodes (even schottky) drop a nontrivial amount of voltage, and thus power if you have an MPPT controller.  So you might want to forego them if you have one of those.

However:  If you have a system with high-voltage panels or where panels are wired in series (typical with MPPT controllers), the voltage placed across a mostly-shaded panel from a fully-illuminated one might be enough to cause reverse breakdown.  Blocking diodes also protect against this, so you'll want them again despite the power cost.  If you're wired for nominal 24V or more from the panels, be sure each panel/series string has an individual blocking diode.

All reasonable advice too.

For what (little) it's worth... I have 6 seperate arrays. 5 are 600 watts, one is a little under 500 watts. I chose TO220-style schottky diodes which are bolted on a copper busbar as both the + rail, and heatsink. Each diode is passing nominally 6 amps but I see up to 8.5 or 9A in particularly strong sun (cloud edge). Even as schottky diodes, they're dropping between 0.57V and 0.61V under load, or about 4 watts each. Getting close to 1% of my total PV input. 24W may not sound like much, but it's enough to get that copper bar quite hot.

I certainly don't need them from the batteries point of view - however as I'm running 120V OC (around 100V MPP) I really like the safety they afford my panels.

[wooferhound]

You should use diodes to combine your wind and solar power like this . . .



Here is my diode combining panel . . .
http://www.fieldlines.com/board/index.php/topic,129705.html

[HiddenMountain]

You should use diodes to combine your wind and solar power like this . . .



Here is my diode combining panel . . .
http://www.fieldlines.com/board/index.php/topic,129705.html

Gentlemen,

WOW, just... WOW.....

I am truly humbled by your knowledge of things electrical. I have a lot to learn.

That wiring diagram is a work of art too! At least I can understand it!

I cannibalized some old electronic stuff I found at the dump and I'm sure I have the right diodes in my junque box.....

Now, about the input for the perpetual motion....
 
Thanks again,

Jon