Resistance of dark solar panel

[flippineck]

I have two identical Chinese solar panels, which both claim to have "inbuilt diode prevent the reverse of charging situation".

Open circuit voltage quoted as around 19V, quoted current around 5A.

Let's say I connected the two in series, and illuminated both with bright sunshine sufficiently strong to achieve the maximum output as per spec sheet.

If I then connected a voltmeter across the two, I would expect to see something approaching 38V. And if I connected an ammeter, something like 10A.

If I then blacked out one panel completely so it was in 100% darkness, leaving the other in the sunshine, would I get 19V and 5A?

Or, would the dark panel cause a significant dimunition of the power flow?

panels are similar to:

http://www.ebay.co.uk/itm/Biard-100...119699?pt=UK_Gadgets&var=&hash=item232d13ad93

 

[Anon_LG]

When you put two batteries or solar panels in series you add together the voltage not the amperage, you must put them in parallel to add together the amperage output. I expect you would get lower than 5A if you did what you said, the photovoltaic panels will have internal resistance.

 

[KrisBlueNZ]

Right. Currents don't add if you connect things in series. You can put a reverse-connected diode (rated for the maximum current) across each panel so that if either panel is in darkness, the voltage across it is limited to about -1V.

Edit: I would be interested to see the results of any testing you do on these panels. I'm curious to know the "exaggeration factor" used in their specifications

 

[flippineck]

Thanks guys. I'm building a string of these panels to feed an MPPT charge controller. It's rated as being able to accept 30A and 150V from the panels, it then does it's MPPT magic and charges a battery bank. The controller says it's auto-sense 12/24V for the battery bank; I have two 110Ah lead acid leisure batteries in parallel.

I understand one way to look at an MPPT controller is to say it can convert excess voltage from the solar panels, into current to charge a battery bank.

I have 3 panels on one side of the building facing southeast that gets full sun in the morning, but which is in shadow in the afternoon. I have 3 more panels on the adjacent wall facing southwest which is at 90 degrees to the first set (square building). These get sun in the afternoon but are in shadow in the morning.

I'm wondering how best to connect everything together. I'm in england so, thinking about winter, I was going to arrange each group of 3 panels in series configuration to try and get the best chance of a voltage level which would be sufficient to charge the battery bank i.e. greater than about say, 13V.

Easily feasable in summer (more like 60V out of each string on the best days) but in winter the light is so low - I've been trying 3 panels in parallel with an old PWM controller but I'm not getting above the 12V level often in the dark season.

So maybe in winter, 3 panels in series will at least sometimes, build up sufficient voltage to begin to charge. Then, if I have an exceptionally sunny winter day, or in summer, with the new 'proper' MPPT controller I've bought to replace the old PWM controller (which claimed to be MPPT but wasn't), if the voltage gets up to high levels it won't either blow the controller or be wasted.

Seems I'll have 150V to go up to, so if I connect 2 60V strings in series at the controller, on midsummer's day I might pump 120V into the charge controller with all 6 panels firing on all cylinders, and things'll be ok.

But if I have an average winter day, with one set of panels in virtual darkness and the other string of 3 struggling to produce let's say 15V between them, will I get most of that 15V across the controller input despite the 3 non-contributing panels being present in the circuit?

Not too bothered if the 3 non-generating panels only cause a drop of a volt or two; but worried if they would have a very significant effect in negating the power generated by the 3 sunny panels...

Would it be better to run the batteries as a parallel pair, set to charge to say 13.8V as a final float level, or in series as a '24V battery' with a final float charge level of say 27.6V?

I'm thinking a parallel configuration with a lower float voltage would allow to make more use of dim winter days?

Here's the type of controller I've ordered:

http://www.ebay.co.uk/itm/MPPT-30A-...21267044990?pt=UK_Gadgets&hash=item4acd02127e

 

[(*steve*)]

There are 2 ways to connect diodes to solar panels.

The first way is a parallel diode. This means that if the panel is unable to supply the current that the other panels do, it is effectively shunted by the diode.

The second way is a series diode. This is used to prevent batteries from discharging back through the panel when the battery voltage exceeds the panel voltage.

This situation sounds like the latter. In the latter case, placing one panel in the dark is going to limit the forward current of the array to the lower of the two panels. Placing a diode across the entire panel may achieve something like what you want. This will then limit the array current to the maximum of any panel. Note that the voltage will drop significantly and this is really only an option if some panels can be significantly shaded.

 

[(*steve*)]

In general, connecting panels having different orientation together into an array is bad juju. It would be better if your controller has multiple channels so it can do MPPT on each array separately. Depending on how you connect panels to make an array, you could end up with either the best or the worst determining the power available (the worst is the worst!).

Connecting the two arrays in parallel with protective diodes to prevent one array feeding back into the other is one option. Connecting them in series with diodes across each panel is another.

I've considered the option to connect each separate array to a boost SMPS designed to raise the panel voltage for the lowest voltage panel up to that of the highest. I'm not sure how practical that might be especially considering the voltages and currents involved.

 

[flippineck]

Seeing as I'm going to have two pairs of leads sprouting out the wall next to the controller box, one pair from each set of 3 series panels, each providing a variable voltage from 0 to say 60 VDC, 'connecting the two arrays in parallel with protective diodes to prevent one array feeding back into the other' sounds like an option that would be the easiest to put into practice.

I could feed all 4 wires into a box on the wall, have the diodes in there, and feed 2 wires out into the single channel available on the controller.

Which diode part numbers would you recommend and how should I connect them?

 

[(*steve*)]

Have a close look at the panels (or measure them) to try to find out if the existing diode is in series or parallel. Then you can make an informed decision

 

[flippineck]

They're all fixed in place on the building, and if I could get one down, the box on the back that houses the terminations is a sealed affair - rather not break the seal if at all possible. So measuring would be preferable.

I do have one string in series with the ends of the array easily accessible to a multimeter, and one set wired in parallel again with easy access to the output wires. What measurements should I look for on the meter?

Have emailed the panel manufacturers to ask. I could try researching the panels on t'internet I guess, too

 

[flippineck]

got a response from manufacturer and they said;

The 100W solar panel is fitted with two 50V-10A bypass diodes in series

We do not fit blocking diodes to the 100W solar panels as we always advise
using a charge controller which would have the diodes fitted.

I sent them a schematic of my understanding of that, to check if I've understood right. file attached.

 

[KrisBlueNZ]

The bypass diodes would be connected in the opposite direction from your schematic. If you connect diodes that way, they will conduct when the panel is trying to generate voltage, and would short out the panel! (Or nearly short it out.)

I have no experience with solar panels, but... If you have two banks of panels, and only one input to your MPPT circuit, and if there could be a situation where there is sunlight on both banks, I think I would connect them all in series, with a reverse diode across each bank. That way, if there is sun on both banks, you would be able to make use of the power from both banks. If you OR the two banks together using diodes, only the bank that generates the highest voltage will contribute. Unless I'm missing something, which is quite possible...

 

[flippineck]

Conventional flow versus electron flow.. I always get it wrong. I must write out 1000 times "The arrow does NOT show the way the electrons go". Thanks

Would I actually need a seperate reverse diode across each bank? If, at the controller, I simply solder together + from one string and - from the other, then feed the remaining + & - into the controller, would I be ok with the bypass diodes already present in the panels?

I consciously went for the 150V rather than the 100V controller just in case I ended up using six 20V panels all in series, so there should be plenty of headroom.

 

[KrisBlueNZ]

Right. If each panel includes a diode reverse-connected across it, then yes, you can just connect them all in series.

 

[flippineck]

Here we go.. the actual info from the manufacturer. See image attachment:

 

[KrisBlueNZ]

Right, that's good. Yes, you can just connect them all in series.

 

[flippineck]

Did just that & got 80V out of 6 panels, which rapidly fell cause it was dusk. Pitch black outside now & the MPPT & my multimeter agree the panels are providing 0.1V from what.. scattered light.. moonlight.. stars..

Fingers crossed for sun-up hope the wires don't melt