Solar Panel Voc-MPPT?

BFL13 wrote:
Should be interesting tests!

When orienting, I don't have to run back and forth. The DMM has alligator clips so I can watch the dial while moving the panel--can do it with two hands, don't need more thanks to the clips.

Problem is if Isc is over 10amps--blows fuse in meter. When I had the two 100s at 6.2a each, I had to check just one, not both in parallel.

Once I then connect back up, now I have to run to the Trimetric to see what it says compared with the Isc I just saw at the panel.

Only have to do that once in a while. Once set up and you see you get rated Isc when you should, you can assume the lower amps you get at other times will be the most you could be getting just then.

Now I am wondering if I could just double the Isc and keep using the same marker. My panel Isc is 8.3. 230w/13.5 = 17 so that's going to be my expected amps? Twice 8.3 is 16.6

So ignoring the variable battery voltage, I might just say if I am not getting twice whatever Isc is , then something is wrong? Say I got 5a for Isc, then I should see 10a on the Trimetric? If not, then something is wrong?

you really need to compare MPPT controller input power to it's output power to the battery.

With MPPT controller and 24V nominal panel your going to look for "about" double Imp. not double Isc. So if your measuring Isc, which is of course very easy to do, subtract about 10% to estimate Imp. Looking at a few panels it seems like Imp is about 10% less than Imp. with a 230W panel and say 13.5V at the batteries, I'd expect max current to the batteries to be about 230/13.5=17.0 *.95 (converter efficiency)=16.1A under ideal conditions. typical conditions somewhat less. And output current is going to be dependent on MPPT controller output voltage since it is a power converter, power out=power in * efficiency. Remember a MPPT controller is NOT a current converter, it is POWER based.

rather than looking at output current, look at output power. Vbat*I and compare that to input power. Use the trimetric to measure the controller input voltage and estimate input current as Isc*0.9 to get an estimate of Imp unless you can measure the current with the panel connected to the controller. I'd expect output power to be about 5%less than input power, since MPPT controllers are typically claimed to be in the range of 94-98% conversion efficiency. Which of course well all know is under some set of conditions that just happen to give the best numbers!! Graphs for my morningstar show efficiencies in that range over a reasonable portion of the input power range.

Unlike a PWM controller that is a current regulator, current out= current in.

that is until battery voltage goes up enough for either controller to switch to float mode for either PWM or MPPT controllers.