Question about small panels

cizzi · May 2008

I have 2 15W/1A panels in parallel on my roof (the 2 panels positives are together and the 2 panel negatives are together going to my charge controller) and with the full sun I never see more than 1.2A on my prostar-15M charge controller (even under a load cause in PWM regulation it tapers off). Is this normal behaviour? They are amorphous panels that I got for 100$ each. Should I just replace them with a multi-crystaline panel or something? Shouldn't I be seeing 2.4A? I know that in paralell the tension stays the same and the amps add up so why am I not seeing that in optimal sun conditions? Thanks.

BB. · May 2008

Re: Question about small panels

It may be several issues...

The first is what is the Vmp rating of your panels... Typically, the smaller panels are rated around 14 volts or so--to be used as a battery trickle charger. Normally a larger panel is rated around Vmp=17.5 volts or higher... This allows for the battery to be equalized (~15 volts) plus a 2 volt drop across the solar charge controller (17 volts minimum). If the Vmp rated output voltage is lower, you will get less current.

Another is that the typical amorphous panels can lose 30% of their output current in the first 6 months of sun--after six months, the decay in output is supposed to reduce to much less (2% per year?)--but I am not so sure.

It is possible that your solar charge controller is not letting the full current through (battery pretty much fully charged?), the controller is wired incorrectly (too much resistance from solar controller/panels to battery bank). Or, the readings are wrong (would not be the first meter that needs calibration).

Or lastly, poor quality or damaged panel(s). Try one panel at a time and see if one outputs near 1 amp and the other less than 1/2 an amp.

You should see near rated current in full sun and cool weather--but if it is within +/-10-20% of the rating--everything is probably working OK.

-Bill

cizzi · May 2008

Re: Question about small panels

This is the panel information

http://www.sunforceproducts.com/english/details.asp?id=50

Its my first panels so I didnt wanna spend much, eventually I'll get crystaline ones at higher output power.
My charge controller is a MorningStar prostar-15m with metrics and my battery is a deka soolar 8GU1 31AH

BB. · May 2008

Re: Question about small panels

Yes, that is a 15 volt panel... Really designed to connect directly to your battery for proper charging. Take an accurate volt meter and measure the voltage on your battery when charging. From here:

While our gel cell will accept a charge extremely well due to its low internal resistance, any battery will be damaged by continual under- or overcharging. Capacity is reduced and life is shortened.

Overcharging is especially harmful to gel cells because of their sealed design. Overcharging dries out the electrolyte by driving the oxygen and hydrogen out of the battery through the safety valves. Performance and life are reduced.

If a battery is continually undercharged, a power-robbing layer of sulfate will build up on the positive plate, which acts a s a barrier to electron flow. Premature plate shedding can also occur. Performance is reduced and life is shortened.

Therefore, it is critical that a charge be used that limits voltage to no more than 14.1 volts and no less than 13.8 volts at 68F. Batteries used in float service should be charged at 13.8 volts. For deep cycle service, a maximum voltage of 14.1 volts should be used. Most of the charge controllers that we offer are compatible with gel cell type of batteries.

The charger must be temperature corrected prevent under- or overcharging due to ambient temperature changes. Most charge controllers that Affordable Solar also offer an optional temperature sensor which is placed right on one of the batteries in your battery array. Here are the ones we offer for Trace, Outback, Morningstar and SolarBoost charge controllers:

However, without a charge controller, you would not want to leave it connected for storage (if these are one or two "car --It will probably overcharge the battery. Check the battery electrolyte often (once per month or so) and make sure that the plates are not exposed. If needed, refill with distilled water.

But your battery is a 31 AH gel cell? You cannot add water so if overcharged--it will fail.

To be honest, I am not how a gel cell would respond to a 15 or 30 watt 15 volt panel directly connected without a charge controller... Gel Cells tend to be one of the "more fragile wrt to misuse" battery types.

Perhaps somebody else here has the answer.

-Bill

cizzi · May 2008

Re: Question about small panels

The purpose of the charge controller is to allow proper current to and from my battery as I understand, so even if there's more or less being generated from the panels it shouldn't be an issue. But when I was on my roof I tested both panels in an open circuit and it was around 22V from each panel.

So when I go for the better panels like Shap/Kyocera they will have a higher output voltage than 15V like you said which will in turn give me more current. And also yes my battery is always topped off so perhaps its not letting the full amperage go through.

BB. · May 2008

Re: Question about small panels

Technically, the solar charge controller is ensuring proper voltage to your battery. Simpler controllers may have one output voltage, others may have two or three voltages plus other functions (like equalize). All, ideally, should have some form of temperature compensation (as the batteries get cold, voltage goes up, as they get hot, the voltage needs to be dropped).

The smaller controllers may not even have any current control--the maximum current is set by the battery voltage and solar panel voltage/current.

And, in the end, if the battery is nearly fully charged, you should see much lower current during charging (if the battery is fully charged and you are getting 1-2 amps--that is probably too high).

Your best bet (for battery health) is to check the battery voltage in the middle of the day and see what voltage and temperature it is.

Charging at 13.8 to 14.1 volts at room temperature (13.8 is best for long term storage, closer to 14.1 volts if you are using/cycling the battery) is a pretty tight range--and if you go out of it for long periods of time--the gel cells will be damaged.

-Bill

Question about small panels

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