Amps out of panel higher than Imp---is this right?

Plowman

Been futzing around with my little off-grid system today. Saw the amperage coming out of the charge controller was ~10.6, which seemed awfully high. My dim understanding of Ohm's law made me think the CC was stepping down the voltage and raising current, but I put a clamp meter on one of the wires coming out of the panel and amperage was indeed in the 10.6 range.

I have one Dmsolar 158W panel, specs from Dmsolar's site:
* Pmax: 158W
* Vmp: 18.9V
* Imp: 8.47A
* Voc: 22.7V
* Isc: 8.76A

What's going on? I thought panels couldn't produce more than their Isc, but this is not the first time I've seen the amps higher that that.

Weather today is cloudy with some sun breaks, sun was shining directly when I made these measurements but just barely out of the clouds. I have a Watts Up meter coming out from the charge controller, clamp meter is an Extech. There's no shade whatsoever on the panel.

mike95490

Re: Amps out of panel higher than Imp---is this right?

sometimes, panels are a bit "underspec'd" so that they meet the spec in 15 years as they age.

Cariboocoot

Re: Amps out of panel higher than Imp---is this right?

Yes it is possible. It's just unusual.

Certain circumstances can cause higher than normal insolation resulting in increased current from the panel. This can included edge-of-cloud events where moisture acts as a lens to concentrate sunlight on the panel (usually very short-lived event) or reflective surfaces such as snow or even water or white concrete putting more light on the panel than it would receive from direct sun alone.

It is also possible the panels have higher than rated output due to variances in cell quality.

Plowman

Re: Amps out of panel higher than Imp---is this right?

Cariboocoot wrote: »

Yes it is possible. It's just unusual.

Certain circumstances can cause higher than normal insolation resulting in increased current from the panel. This can included edge-of-cloud events where moisture acts as a lens to concentrate sunlight on the panel (usually very short-lived event) or reflective surfaces such as snow or even water or white concrete putting more light on the panel than it would receive from direct sun alone.

That might explain it. Like I said, the sun was just at the edge of the clouds, plus it's been raining today. Panel is mounted on a white metal cargo trailer that probably has water on the roof now.

Amps have been fairly high today, but voltage is low. Measured the voltage of the panel wires going into the charge controller earlier today and it was in the 13.5V range, but with amps ~5 (sorry, didn't write it down, just going on memory). What determines voltage coming from the panel?

Cariboocoot

Re: Amps out of panel higher than Imp---is this right?

Plowman wrote: »

That might explain it. Like I said, the sun was just at the edge of the clouds, plus it's been raining today. Panel is mounted on a white metal cargo trailer that probably has water on the roof now.

Amps have been fairly high today, but voltage is low. Measured the voltage of the panel wires going into the charge controller earlier today and it was in the 13.5V range, but with amps ~5 (sorry, didn't write it down, just going on memory). What determines voltage coming from the panel?

Panel Voltage is a function of insolation and load (as well as design of course). Under full insolation it can put out Imp at anywhere from near zero Volts up to Vmp. They are current sources so they try to put out maximum current 'first' as it were. If you were to 'fix' the light falling on a PV and then adjust the resistance of the load you would see this effect of rising Voltage at constant current - up to the Imp * Vmp max. When the resistance allows the V to go above that point then the current starts to drop off rapidly until you reach zero current @ Voc.