Simple solar charge design question

[BertKu]

Hi Lieau, This is what I have asked my friend.
"Does a solar panel have any long term consequences of being shortened for long periods of time. I.e. when a panel is transported, normally it is shortened. One can shorten the panel to read the short circuit current. However what happens to the increased heat, I assume, if a panel is shortened day in day out for months while the sun is burning on the panel." Bert

 

[philSweet]

The way I handle this is to use a battery switch to combine all batteries when I'm off the boat. I have a second battery switch for engine start and instruments. My charge controller's output is run to the battery switch. The issue now becomes "how do I wire two battery switches so that nothing bad happens and I know what is connected to what at all times." There are several reasonable ways to do this, depending on what you need to do and where the two switches are located (Mine are about 15 feet apart, one inside and one in a cockpit locker.

<edit> dug this attacment back out.

 

[L'eau.Life]

Thanks again everyone. I will update once I have it set up.
Cheers!

 

[BertKu]

Thanks again everyone. I will update once I have it set up.
Cheers!

I received a reply from a friend.
"Morning Bert
If it is under open circuit conditions, no damage will be done. At worst normal degradation will take place, around 0.5% per year (more during the first year of exposure, typically around 3%)
Why would you want to leave in short circuit condition for such a lengthy period though?"

From what I still remember, short circuiting is basically the same as continuous charging the battery and it degrade the lifetime of the panel. Enjoy your time in NZ, too far away for me to pop in and see your beautiful country. Bert

 

[L'eau.Life]

Confused!

I received a reply from a friend.
"Morning Bert

From what I still remember, short circuiting is basically the same as continuous charging the battery and it degrade the lifetime of the panel.

Thanks Bert but now I'm confused as I thought the purpose of a solar controller was to regulate the charge to prevent continuous and over-charging.
The unit I have on order charges and monitors Battery 1 until that is fully charged before switching that supply and monitoring to Battery 2 and [I assume] cutting all charge until either 1 or 2 needs topping off.

Have I misinterpreted how this is is meant to work?

Cheers, Alan

 

[CDK]

Solar panels can be short circuited indefinitely without any ill effects.
Power = volts x amps: with the short circuit the voltage is zero so there also is no power generated. In an open circuit, the current is zero so there also is no power, but the unloaded cells are easily damaged by electrical fields like lightning or strong transmitters.

For that reason the shunt principle is the preferred way to control solar power.
In a multiple output regulator the panel(s) can be short circuited only if both batteries are completely charged.

 

[BertKu]

Thanks Bert but now I'm confused as I thought the purpose of a solar controller was to regulate the charge to prevent continuous and over-charging.
The unit I have on order charges and monitors Battery 1 until that is fully charged before switching that supply and monitoring to Battery 2 and [I assume] cutting all charge until either 1 or 2 needs topping off.

Have I misinterpreted how this is is meant to work?

Cheers, Alan

No you have not misinterpreted your controller. All what he said. If a panel is not used for extended periods of time, it is better to have the solar panel in a kind of open circuit then to have it short circuit. Open circuit, you have the normal degrading of the panel by up to 0,5% per year, while in short circuit, it means "not used efficient", because you are away and don't use appliances, but just having the panel short circuited. I assume it is the same as having a car standing for 3 - 6 months in the garage for 10 years, the wear and tear is less than if you would have used your car continuous for 10 years. It maybe a stupid comparison, but it easier to understand. short circuiting of the panel, means the same as charging the battery while the appliances are drawing from the battery. But you are away and it is a waste in principle. What I try to get info on, is it 0,1% more than degrading or is it a 1/2 percent more, i.e. total 1% or more. If 1%, it means that after 25 years, the panel give still an output of 77%, while if only 0,6%, it will be in a better condition. Bert

 

[BertKu]

That does not mean you mustn't use your 20 watt panel, to keep the batteries from draining. But a simple floating circuit will basically keep your panels in an open circuit, while away and still keep your batteries topped up. The type of batteries you are using have probably a very low leakage current and a small solar panel used for keeping a car battery topped up while on holiday would be a good solution. Your battery may only lose a few watts per day, your 20 watt solar panel is good enough. Bert

 

[BertKu]

Solar panels can be short circuited indefinitely without any ill effects.
Power = volts x amps: with the short circuit the voltage is zero so there also is no power generated. In an open circuit, the current is zero so there also is no power, but the unloaded cells are easily damaged by electrical fields like lightning or strong transmitters.

For that reason the shunt principle is the preferred way to control solar power.
In a multiple output regulator the panel(s) can be short circuited only if both batteries are completely charged.

Please explain what happens to the electrons made free while being bombed by the sunlight photons at 30 - 40 degrees Celsius?. It may not have an effect on your wiring, other that it may gets a little hot when the wiring is underrated. Yes Voltage x current = zero because of the short circuit and the thick copper wiring. But what happens in the solar panel is another issue. I agree it is not a bad thing to have the solar panel short circuit, but the question is, does the panel degrade by 0.1% per year or 0,5 percent more than the normal degrading at open voltage. I cannot give that answer. I still haven't received a reply of my friend who was in the manufacturing of solar cells. Bert

 

[BertKu]

Pity that this report does not mention anything on open or short circuit.

http://www.nrel.gov/docs/fy12osti/51664.pdf

But only on total systems and where the failures are coming from.
Bert

 

[philSweet]

Try this one, Bert

http://www.nrel.gov/docs/fy12osti/53713.pdf

From what I can gather, Fill Factor and Isc degradation appear to be due to tech/mfg issues. Voc degradation appears to be a function of cumulative solar exposure, and I can't tease out anything that suggests that current per se has an effect. Needless to say, testing panels in the "off" position hasn't been a priority. Somehow, I feel that if this was an important issue, somebody would have noticed it by now.

 

[BertKu]

Try this one, Bert

http://www.nrel.gov/docs/fy12osti/53713.pdf

Hi philSweet, I tried that one and read through it. Nowhere it concluded about open voltage in bright sunlight. All of them concerns degrading in charging currents and panels in use. Both are very good articles. But my logic tells me, that when a panel is short circuit, the interlinking connections are carrying currents, while in open circuit only the junction is affected by the radiation. But nowhere open circuit or short circuit is mentioned, except if I have overlooked something.
Bert

 

[philSweet]

Okay, found a reference to another NREL document stating open circuit degradation is 0.5 to 0.7% per year. The source document is purchase only. http://ieeexplore.ieee.org/xpl/logi...re.ieee.org/xpls/abs_all.jsp?arnumber=1190879

but the results are extacted here http://etd.fcla.edu/CF/CFE0003326/Leyte-Vidal_Albert_201008_MS.pdf

Previous studies performed by Sandia have shown losses on open‐circuited modules of about 0.5% per year while NREL reports degradation of about 0.7% year [35].

If Isc is only about 25% more than Impp, there isn't much in it for decent panels based on extrapolation.

 

[BertKu]

Okay, found a reference to another NREL document stating open circuit degradation is 0.5 to 0.7% per year. The source document is purchase only. http://ieeexplore.ieee.org/xpl/logi...re.ieee.org/xpls/abs_all.jsp?arnumber=1190879

but the results are extacted here http://etd.fcla.edu/CF/CFE0003326/Leyte-Vidal_Albert_201008_MS.pdf



If Isc is only about 25% more than Impp, there isn't much in it for decent panels based on extrapolation.

Fully agree, it means that our OP has to become 150 years old before he notice that his panel is packing up. 0.2 % more degrading is to be neglected . However I think often different than other people. If he is 3 - 6 months away per year, I would not have a regulator or maybe 2 of them controlling my batteries while I am away. A 100 watt solar panel will fry the batteries when they fail for whatever reason. contact burning, component failure etc. His 20 watt panel with 13.1 maximum Volt regulation with a 2N3055 (as per my quick sketch) will be a safer solution. This old fashioned 100 watt/15 Ampere/60 Volt Vco robust NPN transistor will not be able to fry the batteries. Special if it is spread out over 2 batteries. But 0.2% longer pleasure from your solar panel is over 10 years only 2%. By that time new technology will be on the market. Leau.Life can now decide what he likes to do without any concern.
Bert

 

[L'eau.Life]

So from the above I take it my panel can be expected to degrade over time, yes? If so, that doesn't bother me too much as it would be several years before it became ineffective so I'm going to go ahead with the 20W panel & duo controller on the start batteries and worry about the house system further down the line.

My question now is, with a 12V 20W panel mounted (in terms of cable length) about 30ft from the controller and the second battery around 15ft from the controller, what size (AWG) cable should I use to connect it all to minimise losses please?

TIA