Solar Panel Max Amps

[cwyckham]

Originally Posted by zboss

Take the following solar panel:

Peak Power (+/- 5%) - Pmax 137 W
Rated Voltage - Vmp 24.0 V
Rated Current - Imp 5.7 A
Open Circuit Voltage - Voc 29.1 V
Short circuit Current - Isc 6 A

Let assume an MPPT genasun controller.

The panel spec show a rated current of 5.7 AMPS.

If my batteries require a charge of 14.8 volts peak what is the max amperage you can expect to see from the panel?

5.7 amps or 9.25 amps (137 watts / 14.8 volts)?

Although I agree with everyone about SOC of batteries, etc. I think that your key question is why your calculations show you that you will see more then the Imp of the panel.

I see you've labeled Imp as the rated current. I don't know where you got that, but Imp is the current for maximum power, not some sort of maximum current.

Have a look at this typical solar panel curve:


The maximum power point of the panel is reached when it's running at Vmp and Imp. However, the maximum current is actually at zero voltage (short circuit). For your panel, that's 6 Amps.

A lot of people are telling you that you'll get twice the current at half the voltage (this is a 24V panel). This isn't true. As you drop the voltage, the current will max out at Isc, the short circuit current.

So if you hook the panel up directly to batteries that are at 12 V you'll only get 6x12 = 72W from it.

Now comes the magic of MPPT (as Ocean Planet so eloquently explained). Your MPPT will keep your panel at the max power point. Your panel will see 24V and will produce 5.7A.

The MPPT will then take that 137W worth of power and downconvert it to the 12V for your battery (that is all kinds of wrong technically, I'm sure, but it's the easiest way to think about it. The real answer has more to do with impedence, etc). So your batteries will get 137/12 = 11.4A from the panel which is getting 5.7A from the panel. This magic is possible because the power's the same on either side.

This is why MPPT controllers are really required if you're going to use high voltage panels. Otherwise, you throw away half the power.

[hellosailor]

zboss-
The panel manufacturer lists the PANEL output because they are just selling you the PANEL. And there are standard ways to test and list that.
The fact that other equipment exists, like an MPPT controller, and that other equipment might change the output from the SYSTEM to something else, has got nothing to do with the specs for the PANEL.
Similar to the way that a rose garden transforms horse manure into something that smells nothing like horse manure. If I plant a rose garden, I'm still not going to say my horse's patootie smells sweet.(G)

[Henk Koornstra]

"The gist of my question is - if a panel manufacturer states a maximum rating (above example is 5.7 amps) is that the highest amperage you can expect to get out of the panel, regardless of higher voltages available? Or can you exceed that amp output under the right conditions,"

I assume that you are talking about the output of the panel, not the MPPT controller. My understanding is that the panel will never be able to provide you more than 5.7 amp, which is the max rating of the panel. This is due to the sunrays energizing your panel. When the sun starts hitting your panel, the voltage of the panel slowly increase together with the amps. The voltage of the panel does not stop increasing while only the amps increases. There is a relationship between an increase in the voltage and an increase in the amps. The sun during the middle of the day will only provide maximum output voltage and amp wise.