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ahug avatar image
ahug asked

Solar output voltage drop under load (Smartsolar 75/15)

Hi all,

I have a small solar setup in my converted van. 2x 100W panels wired (10AWG) to a Smartsolar 75/15 with newest firmware. The controller feeds a 105Ah Trojan lead acid deep cycle battery. For the load side I use the load output of the Smartsolar 75/15 as I'm only running small loads (mostly <5A).

What I realized is that my Solar output voltage drops from the should be 18-19V as soon as there is a bit of load (>1A) to 14-16V. This leads to the situation that especially during absorption the battery output is below the desired voltage. Also the overall watt I'm getting from the 2x 100W panels seems really low (~70-80W with direct midday sun exposure).

Any ideas what the issue could be? Wire sizing shouldn't be the issue as I'm running 10AWG wires between the panels/controller/battery, which should be more than enough). Could suboptimal connections/wire connectors create this issue (e.g. I have a breaker switch between the panel and the controller)? And is my assumption correct that the issue must be somewhere between the Solar Panel and controller, or could an issue on the battery/load size also create this effect?


Thanks for your help!

Alex


MPPT Controllersvoltagewiring
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3 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Hi ahug,

This is the wire calculator I use. https://www.calculator.net/voltage-drop-calculator.html

Wire your panels in series. Your 75/15 will be ok with input voltages up to 75 volts. Do not go over 75 volts!!

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ahug avatar image ahug commented ·

Thanks klim8skeptic, good idea! I originally wired them in parallel to get better performance in partially shaded areas (where we often end up parking our van), but wiring them in series would probably solve my current issue.

Nevertheless, I don't quite understand why I'm getting the voltage drop in the first place. With the 10AWG wires I'm using (length ~3m) I should only be getting minimal voltage drop (<1%). Any ideas what could be causing this? My only idea is faulty wire connectors (I used normal crimped connectors to connect to the breaker and the wire then connects directly to the MPPT terminal).

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solardude avatar image
solardude answered ·

What is the ambient temperature outside? 10%-20% voltage drop under full load is not uncommon if you have high outside temps.

What is the Vmp of your panels? If it is 18V-19V, that is under test conditions, not real world. Most summer voltages I see on 12V panels are 14.5V-16V under full load.

The other answer is correct, wire in series and you will be much better off. Better efficiency all around.

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ahug avatar image ahug commented ·

It was between 25-30°C, the solar panels themselves must have been quite a bit warmer though. Vmp of the panels is 17.6V.

I'll change the wiring to series and will see what happens. But sounds like that will be the fix.

Thanks for your help!

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wkirby avatar image
wkirby answered ·

The Voltage drop is unlikely to be over your wiring.

You are witnessing the Voltage drop of the PV array. This is characteristic completely normal. It would be great if PV panels had zero ESR and held their rated Voltage indefinitely regardless of the load, then we would not need MPPT controllers.

You can tell just from the label that the Voltage varies a lot between VOC and VMP. Mix in some clouds and then the VMP will be even lower.

You were right to wire them in parallel to mitigate the shading problem, but the smaller "12V" panels will bring about the low Voltage problem that you have experienced, so series wiring will bring you better gains overall.

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ahug avatar image ahug commented ·

Thanks! Appreciate the additional explanations!

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Stefan Östman avatar image Stefan Östman commented ·

I am having the same issue. Very similar setup. But I have my panels wired in series. My voltage directly on the cells disconnected from Victron 75/15 is around 39 volts but as soon as I connect them to Victron it drops to slightly over 14 volts and top power I have ever seen is 84 watts on a sunny cloud free day with 18 degrees Celsius outside temp. Not even the temp performance drop can explain the difference between 200W panels and 84 W production.


I am feeding 220 Ah of battery. And it is impossible to bet get them into float state

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solardude avatar image solardude Stefan Östman commented ·

Is the 14V reading at the input or the output of the controller? That is a drastic drop if it is the input. That sounds like too small of wire or a bad panel. What size wire, and how long is the run from the panels to the controller?

How old is the battery? Lead acid?

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Stefan Östman avatar image Stefan Östman solardude commented ·

The voltage drops kind of stepwise from VOC which is 21.5+21.5 for my panels connected in series, as soon as i connect them to the PV input terminals on the 75/15 to around 14 volts in the PV INPUT on victron. so feels like it is the MPPT algoritm in victron that causes it. max current from panels are 6.5 A, so power then gets 6,5x14=91 W(but i have never seen more than 83 W)

Cables are 12 square millimeters (about 6AWG) which is more than enough and it is only about 8 meters between panels and the victron 75/15.. The batteries are made for solar installations and are only about 15 months old.

I tried today to connect the panels in parallell instead and that gives better result during daylight, but in low light the voltage then drops too low so the 75/15 does not start the charging (battera voltage +5V needed)

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