Solar panel ratings

[amiklic1]

I got some specs of my PV panel, it is 150 W rated, but what I need someone to explain are some other specs.

It says:

Voltage open circuit (Voc) = 23,00 V

Voltage on max. power (Vmp) = 17,80 V

Current short circuit (Isc) = 9,9 A

Current of max power (Imp) = 8,42 A

How do I get 150 W?

Vmp x Imp = 17,80 x 8,42 = 149,87 Watts

I guess that's it.

Is "open circuit voltage" on PV panel similar with the one in wind generator, when it's not connected to batteries?

Also, do I need to connect PV cells to get open circuit voltage, if I want to build something like this at home? Or to get voltage on max. power?

The specs also rates cells to be 165x165mm, and 0,61 V each.

I don't know the manufacturer.

Thanks!

[ghurd]

Yes, V at MP x I at MP = W at MP. That is the maximum power.

Voc is the maximum voltage in good sun, when it is not connected to the batteries.

Use 36 cells in series.  

Test each cell too, a single 'dud' cell can hold back the output considerably.

G-

[richhagen]

When building a panel, you want to match the cells based upon the current, and then have enough in series to charge your batteries under adverse conditions such as higher temperature.  Think of the current as being equivelant to the amount of water flowing through a pipe.  Think of the voltage as the water pressure.  Each solar cell is like a pump.  If you put a pump in that can handle less water than the other pumps connected to it in series, it will limit the amount of water, or in the case of solar cells, the current, of the whole system.  You want to connect enough cells in series to generate a charging voltage under the most adverse of conditions.  One likely reason that most manufacturers use 36 cells in series for 12 volt charging is that the voltage of the individual cells goes down with increasing temperature, and the panels get very hot in the sun.  They have to plan for charging under the most adverse conditions, and the cost of a few extra cells is likely less than that of having a bunch of panels returned because they aren't working at charging batteries.  Rich

[finnsawyer]

Take three six volt batteries, connect them in series, discharge them down to 17.8 volts, and connect them to the panel in full noonday sun.  Or get two panels and three twelve volt batteries.  Not very practical I suppose. If you connect the panel to a 12 volt battery the current will be somewhere between Imp and Isc, so you lose 30 to 40 watts of power.  The maximum power occurs for very specific conditions, which will be hard to maintain.  The manufacturer had to make the panel usable over a range of lighting conditions, so that's probably why the high (17.8 volt) value for maximum power.  The panel is obviously non linear in its response to light, so the 23 volt Voc probably doesn't mean much except you get no current out at that point.  Of more interest would be how the current or maximum power curve drops with increasing voltage.

One other point.  Under full sun the panel will put out about 9 amps no matter what the voltage of the 12 volt battery is.  This could be a problem.  I wonder how people regulate these panels.

 

[ghurd]

The amps stay about the same until around 16V, where there is a pretty steep drop off.  Charging amps under great conditions is a little more than Imp.

The 16 or 17V designed operating condition is to allow for line losses, blocking diode drop, charge controller drop, etc., but still maintain the amps.

The less intense the sun, watts per square meter, the lower the amps. It is very linear.

It is easier to think about solar module output in Amps instead of watts.

Solar charge controllers are designed to handle the open circuit voltages.

G-

[finnsawyer]

Actually I miswrote.  I meant to say the I-V relationship of the panel is clearly non-linear for a constant illumination.  That is, you can't find a Thevenin equivalent circuit that describes the behavior of the panel for all voltages and currents.  Sorry for the confusion.

As far as regulation is concerned, I envisioned a circuit with a transistor in series with the battery.  The battery voltage is sensed and as it rises the current through the transistor is decreased and eventually cut off. The sum of battery voltage and transistor Vce then approaches Voc.  Is this what is done?

[ghurd]

There are 4 common controller styles.

Some are not much different than a big zener diode accross the battery. (Shunt control) PV amps keep flowing, just not to the battery.

Some just shut off at, say, 13.85V then turn on at 13.75V. (Series control) PV amps stop.

Some are a combination of series and shunt.

Then the PWM control that has a lot of ways they do it.

PWM is best, IMHO.  NASA uses it!

G-

[Dreadstar]

You forgot MPPT. Outback MX60. Picked up one and am very pleased with it. closer match to that open circuit voltage out on the panels and a fair amount of actual AH into the battery bank. went from 50ah to 75ah 12 volt bank with 220watts of panels. 4 x 55 watt shells. Had a Xantrex c-40 unit in place prior. 440ah bank