Panel array Voltage VS battery voltage and charging efficiency.

[Checkthisout]

A Victron rep told me on a Youtube video that wiring your panels for the highest overall voltage nets you the best overall efficiency. The reason being that charging starts earlier in the day. I have a charge controller with a 150V input limit and my panels are 3 series at 44.4 volts for 135 volts input.

Guys on Solarpaneltalk said that keeping the panel voltage as close to battery voltage as possible nets you better efficiency.

Is the charge controller more efficient with less of a voltage difference because the fets won't have to switch as much? Is that the reasoning or is it something else? And why the conflict between what they are saying and what the victron rep is saying?

Anyone else experience this? If I rewired the system so that each panel fed the charge controller individually, would I see any gains in output due to increased CC efficiency?

 

[BentleyJ]

Most importantly, the MAX voltage limit specified by the manufacturer must never be exceeded. Voltage goes UP when temperatures go DOWN, its an inverse correlation and the solar panel datasheet lists the coefficient for that brand and model number. The 44.4V you mentioned is probably for Standard Test Conditions at 25C so the first step is to calculate the panel voltage at the lowest expected temp for your area make sure that 3 in series is OK.

Most charge controllers have MIN(startup) and MAX operating voltages, however, the voltage range where the MPPT functions is almost always a narrower range within the min/max range. Also, many manufacturers publish graphs of efficiency vs. output %. Without consulting the foregoing data, you would only be guessing at the optimum conditions for your particular set up.

 

[fratermus]

A Victron rep told me on a Youtube video that wiring your panels for the highest overall voltage nets you the best overall efficiency.

Most of the MPPT efficiency curves I've seen show greatest efficiency when Vpanel is ~twice Vbatt. Details will vary by the controller.

The reason being that charging starts earlier in the day

<shrug>

1. panels come up to normal voltage pretty early (something like 20% insolation)
2. there is very little power available during shoulder periods (sunrise, sundown) anyhow

In my A/B testing the additional harvest during shoulder period was microscopic. I like series arrangements but I would not use this as the basis for going series.

Guys on Solarpaneltalk said that keeping the panel voltage as close to battery voltage as possible nets you better efficiency.

For PWM, yes that would make for better harvest given the same panel wattage. MPPT needs a certain amount of headroom to do its tricks.

Is the charge controller more efficient with less of a voltage difference because the fets won't have to switch as much?

My understanding is buck conversion (as seen in MPPT) is more efficient when the voltages are closer together.

 

[Checkthisout]

Most importantly, the MAX voltage limit specified by the manufacturer must never be exceeded. Voltage goes UP when temperatures go DOWN, its an inverse correlation and the solar panel datasheet lists the coefficient for that brand and model number. The 44.4V you mentioned is probably for Standard Test Conditions at 25C so the first step is to calculate the panel voltage at the lowest expected temp for your area make sure that 3 in series is OK.

Most charge controllers have MIN(startup) and MAX operating voltages, however, the voltage range where the MPPT functions is almost always a narrower range within the min/max range. Also, many manufacturers publish graphs of efficiency vs. output %. Without consulting the foregoing data, you would only be guessing at the optimum conditions for your particular set up.

I interpret it more as a general electrical theory that would relate to all electronics.

Which is more efficient? A 120V dc power source being converted to 12 volts?

Or let's say a 60 v dc power source being converted to 12 volts?

Which voltage experiences more losses in the charge controller and why?

 

[Checkthisout]

Most of the MPPT efficiency curves I've seen show greatest efficiency when Vpanel is ~twice Vbatt. Details will vary by the controller.




<shrug>

1. panels come up to normal voltage pretty early (something like 20% insolation)
2. there is very little power available during shoulder periods (sunrise, sundown) anyhow

In my A/B testing the additional harvest during shoulder period was microscopic.


My understanding is buck conversion (as seen in MPPT) is more efficient when the voltages are closer together.

Agree. I don't see much harvest before occurring when the array voltage is slightly above battery voltage.

And ah. I didn't know mppt was buck conversion. In that case the closer the voltages, the less heating that occurs.

My cc doesn't seem to heat up much. When experimenting with a different 40 amp cc with 90 volts oc and 12v battery, I would have guessed about 50 watts were being lost to heat based on the amount of heat coming off the fins so about an 8% loss at full output.

Pure sotp measurement though.

 

[cehs]

I experimented with 2 (24V) solar panels with a 12V battery.
When I connected the panels in series (70V production) the wattage out was NOT as high as when I connected the 2 panels in parallel (33V production)

 

[Checkthisout]

I experimented with 2 (24V) solar panels with a 12V battery.
When I connected the panels in series (70V production) the wattage out was NOT as high as when I connected the 2 panels in parallel (33V production)

How much difference?

 

[cehs]

How much difference?

I do not remember the specifics but I was thinking about 10% difference.

 

[RCinFLA]

Efficiency of buck converter in SSC is a bit better (a few percent) the lower the ratio of PV input to battery output voltage, but the lower the PV voltage the more current for same PV power meaning more possible wiring line loss. I doubt you will see a 10% difference in efficiency between running at low PV voltage or highest PV voltage if wire gauge is sized appropriate for PV current. More series connected PV panels are more likely to have shading issues which can significantly degrade performance, like tree tops causing partial shading in morning or afternoon when sun is low to horizon.

As to what illumination level starts to produce useful power the only real limit is minimum overhead voltage required for MPPT controller. Different PV panels have more or less shunt resistance current leakage loss. Generally, higher quality panels have less shunt leakage current loss giving them better low light performance. Most panels start to degrade in output efficiency yield below 20% maximum sun illumination level of 1000 W/m^2 due to PV shunt leakage current loss.

MPP voltage does not change too much across output level so if there is 0.5 amps PV leakage resistance loss then at full sun PV produces 9 amp illumination current - 0.5 amps leakage loss = 8.5 A output but at 10% illumination level the PV produces 0.9 amp illumination current - 0.5 amp leakage loss = 0.4 A output. There are some other factors like oblique angle panel surface reflections and PV temperature, but this gives you the general idea of PV leakage current impact.

Once illumination level generated current exceeds panel leakage current the voltage will go to near Voc unloaded. It may not yield much power at low illumination but it will activate the MPPT controller.