MPPT PV Watts not closely matching Open Voltage
 

Hi,

I am very new to solar and I'm seeing something on my MPPT's monitoring display which I am not quite understanding.

When I conduct an Open Voltage test of my panels near the MPPT, I see under so-so sun, 32+ VDC/ 6.8 amps Let's say roughly 217.6 watts.

When I immediately connect the PV to the MPPT and I create a draw of more than 62 watts, I'll start to draw from the battery.

I would have thought that I would be able to draw 200+ watts from the PV's before tapping into the batteries.

As a newbie, do I have a fundamental misunderstanding? :p

Details

• Two Costco Golf Cart batteries connected in series for 12V/220 AH
• 100' of 12g stranded cable between the PV's and the MPPT - more on this shortly.
• A 40 amp controller[1]
• Two 280 watt solar panels[2]. Due to minimal sun, they're in parallel.

100' run

This location is our cabin. We only use it during the summer time. It points north and is shrouded by trees. Perfect for summer and horrible for solar! :)

I have measured the DC loss and it is not significant.

References

1. Controller - EPSolar Tracer 4215BN MPPT Charge Controller - EP Solar - Brands | HES PV
2. Canadian Solar panels - CS6K-M

ps: as I am new to the forums I am not allowed to use URL links in my posts yet.

Yep, you have a misunderstanding. :) As a rough example, a solar PV panel is like a battery. Resting voltage of a 12V lead acid battery is around 12.7V. However, as soon as you start drawing power from it, the voltage sags down a bit. The more amperage you pull, the farther it sags. The same goes for solar PV panels. The more you try to draw from them, the farther the voltage sags. That is why we have MPPT controllers. The MPPT controller trys to balance the load on the panel to get the most wattage out of the panel.

http://ecorenovator.org/forum/attach...1&d=1506006771


That being said, you're probably better off with your panels in series than parallel. Higher voltage will increase the efficiency of the system, and your MPPT controller can handle the higher voltage.

Thank you for the quick response and the image.

I really like the image as it clarifies how MPPT works. I annotated it to show how without MPPT the power is not maximized along different points. I thought someone else might like to literally see how MPPT works (well, at least on virtual paper).

Given there's sag when there's a load on a PV system, how does one determine the expected output under load?

If it helps any, below are the specs on my panels:

• Module Watts STC: 280 Watts
• Max Power Voltage (Vmpp): 31.50 Volts
• Max Power Current (Impp): 8.89 Amps
• Open Circuit Voltage (Voc): 38.50 Volts
• Short Circuit Current (Isc): 9.43 Amps
• Max System Voltage: 1000.00 Volts
• Series Fuse Rating: 20 Amps
• Module Efficiency: 17.11%
  [*[ Temp Coefficient (Pmpp): -0.4100%
• Temp Coefficient (Voc): -0.3100%

Thanks a lot!
-pablo