Question: victron 100v 50a. 24v solar panel. how many panels can I use?

[ejfluhr]

As an example to the OP, I have used the Victron SS MPPT 100/50 with 2 JA Solar 400W panels Voc=50v, Vmp=40v, Imp=10A, wired in parallel. Charging a 12v LiFePO4 battery with active loads, I saw a max of 660W (=50A x 13.2V).

 

[RoadTurtle]

I received a response back from SolarRich

"The panels VOC will be close to 100V at 7 degrees Fahrenheit.", which is close to the 5F I had previously calculated.

This a huge discrepency to the number you just posted.

 

[sunshine_eggo]

I received a response back from SolarRich

"The panels VOC will be close to 100V at 7 degrees Fahrenheit.", which is close to the 5F I had previously calculated.

This a huge discrepency to the number you just posted.

I used data and math. I don't know what you or they are using. I showed you the specification and my calculations. Put up your calculations.

What might be a little telling is 42°F is 5.6°C. It's actually pretty atypical that specifications are using °F. The vast majority of the time, it's °C, and I get someone is making a units error.

We can also look at it this way... for every 10°F down from 77°F, it's 2.9% up (0.29%/°F). 30°F below 77°F would be 3 * 2.9% = 8.7% pushing voltage to 98.0V from 90.2V. Pretty clear to see it's not going to take much more to get to 100°F from 47°F.

 

[RoadTurtle]

I'm leaning towards it's supposed to be C, especially considering the number is pretty typical for C. I already sent a note off to them asking if there's an error. I had previously calculated it to 5F, but tossed the paper I used to write it out.

 

[RoadTurtle]

I used data and math. I don't know what you or they are using. I showed you the specification and my calculations. Put up your calculations.

I re ran my calculations, and did it differently than you. I came up with a similar number to yours. I know where I made the mistake in my prior calculation, I used the wrong coefficient. Also, looked at my panels voltage over the last few days, and got to 48 volts at somewhere around 50F. Guess I'm not going to 2s. For the occassional time I might like to have moar power, it's not worth replacing the SCCs. It was one thing to just add a couple more panels... replacing SCC cost benefit ratio doesn't make sense..

 

[sunshine_eggo]

I re ran my calculations, and did it differently than you. I came up with a similar number to yours. I know where I made the mistake in my prior calculation, I used the wrong coefficient. Also, looked at my panels voltage over the last few days, and got to 48 volts at somewhere around 50F. Guess I'm not going to 2s. For the occassional time I might like to have moar power, it's not worth replacing the SCCs. It was one thing to just add a couple more panels... replacing SCC cost benefit ratio doesn't make sense..

Is there a reason you couldn't go 4P? You may need to upgrade the wiring between the PV and MPPT once all four panels are combined. If you're under the 60A PV input limit, you're good. Higher Voc/Vmp doesn't make a lot of difference in efficiency. In fact the conversion from higher voltage to battery voltage loses efficiency. It's usually offset by reduced losses in PV to MPPT wiring (higher voltage/lower current). There's often slight gains in harvest in very low light conditions.

 

[HRTKD]

What might be a little telling is 42°F is 5.6°C. It's actually pretty atypical that specifications are using °F. The vast majority of the time, it's °C, and I get someone is making a units error.

I started to run a calculation with their coefficient and when I saw it in °F I stopped. Major WTF there. FWIW, their coefficient was .01 different than the coefficient for my 320 watt panels that are listed in °C. With the assumption that their coefficient was °C I did come to the same conclusion you did, which surprised me. My 320 watt panels in 2s work great with my Victron 100/50.

 

[sunshine_eggo]

IF that -.29%/°F should be for °C, then:

9.2%/-.29%/°F = -32°C

That's -7°C or 19.4°F

 

[RoadTurtle]

I started to run a calculation with their coefficient and when I saw it in °F I stopped. Major WTF there. FWIW, their coefficient was .01 different than the coefficient for my 320 watt panels that are listed in °C. With the assumption that their coefficient was °C I did come to the same conclusion you did, which surprised me. My 320 watt panels in 2s work great with my Victron 100/50.

I received confirmation from Rich Solar that it is in fact supposed to be C, and they are going to fix it.

 

[RoadTurtle]

Is there a reason you couldn't go 4P? You may need to upgrade the wiring between the PV and MPPT once all four panels are combined. If you're under the 60A PV input limit, you're good. Higher Voc/Vmp doesn't make a lot of difference in efficiency. In fact the conversion from higher voltage to battery voltage loses efficiency. It's usually offset by reduced losses in PV to MPPT wiring (higher voltage/lower current). There's often slight gains in harvest in very low light conditions.

I ran 10g to the SCCs, so I'm good with 4P. It's just my life would be so much easier to 2s2p I may tear out the project box on the roof and redo it. It will be easier to add extra wires at that point. I think for now, I'm going to stay with the 6 panels on the roof and 2 ground deployed panels and see how it goes. I never planned to run aircon, but the fact I can has me very interested. I'm not adding any more batteries (6), so that really is the limitation.

We have two longer off grid travel plans before we land at our winter spot (FHU). I'll get a better assessment of my system capabilities after those trips. I know right now, I've started the fridge on electric, and solar is taking care of that and charging the batteries with a thin overcast cloud bank.