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gone-sailing asked

PV voltage 0.1v below battery voltage

Hi All, I currently have a single 78w solar panel installed on the deck of my boat connected to a SmartSolar Charger MPPT 75/10 and my Victron SuperCycle AGM battery bank. I have noticed an unusual issue that happens occasionally. Despite having lots of sunshine, the MPPT charge controlled shows the PV Voltage to be roughly 0.1v lower than the battery voltage and so the MPPT switches off. The 0.1v difference remains constant for a period of time, sometimes a whole day. I have a Victron Multiplus and so the system is currently also hooked up to 240v AC and charging.

I asked the same qiestion to the Solar Panel manufacturer and their answer was this is a "feature" of the Victron MPPT. If that's the case I am trying to figure what conditions causes this to happen and how to clear it. Any suggestions welcome.


By way of an example, here is a screen shot from my VRM.
1655220763502.png


Yesterday, 13th June, the sun came up and PV Voltage raised as expected, peaked above the battery V+5 volts and switched to bulk, then flat lined to 0.1V below battery voltage and the MPPT switched off. Later before end of day the issue cleared, PV voltage went back up, MPPT switched back on then gradually it went dark. Yesterday was sunny all day.

Today was a similar pattern, a couple of hours of PV voltage above battery voltage then it flat lines to 0.1V below.

Thanks.

MPPT Controllers
1655220763502.png (104.5 KiB)
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7 Answers
snoobler avatar image
snoobler answered ·

I suspect a temperature issue.


Panel specs assume cell temperatures of 25°C. In reality, sun on the panels raises the cell temps to somewhere in the 40-50°C range. This lowers the panel voltage.


You can test for this by cooling the panel with a water spray. Additionally, you could simply cover it to allow it to cool and then expose it to the sun. If either option restores panel performance, it's likely a temperature issue.


Depending on the panel construction, partial shading of the panel effectively shuts down portions of the panel. Shading a third of a 36 cell panel will cut the output voltage by 1/3 and the panel will not function at all.


It could also be related to temperature compensation. Is that enabled? If so, what is providing the MPPT temperature data?

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seb71 avatar image
seb71 answered ·

I currently have a single 78w solar panel

That looks to be a small panel. Maybe too small.
What are the Voc (open Circuit Voltage) and Vmp (Maximum Power Voltage)?

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sharpener avatar image sharpener commented ·

Hi Seb,

According to the spec sheet, Voc = 23.12V and Vmp = 19.72V

It is soon to be joined by 2 more 175w panels, each with separate controllers.

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kevgermany avatar image
kevgermany answered ·

Do you have load on the system? The battery voltage hardly varies, as if the multi is in float mode and the battery is fully charged.

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snoobler avatar image snoobler commented ·

Wouldn't the panel show Voc if there's no current flowing? No load and no PV current usually means Voc:


1655270745554.png


As you can see, as my current decreased, my PV increases to near Voc.

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1655270745554.png (69.4 KiB)
gone-sailing avatar image gone-sailing commented ·
The multiplus is in float mode. There are small loads on the system like the fridge and 4G router.
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seb71 avatar image seb71 gone-sailing commented ·
A fridge is not a small load when you only have a 78W PV panel.
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sharpener avatar image sharpener seb71 commented ·
True, but the 78w panel won't be my only charging source. I don't see how the size of the panel can cause this flat PV voltage issue?
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seb71 avatar image seb71 sharpener commented ·
When you have a load, the voltage drops.
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kevgermany avatar image
kevgermany answered ·

Sorry, wasn't able to comment further yesterday.

Ignoring the PV power line for now.

There are two voltage axes, both using different scales(orange on left for panels, red on right for battery).

PV voltage is good, but it looks to me as if you're reading it from the battery voltage scale and this is creating the confusion.

Battery voltage is flat throughout implying that the MP is keeping it fully charged, probably in float mode.

The Victron MPPTs do start in bulk mode, then switch to absorption/float according to settings. In bulk mode current is dependent on load and battery acceptance of charge. The green line show that there is no load/battery not accepting charge much of the time, but at times the load kicks in and current passes from the charger. This is peaking at about half panel max.

Looks to me as if all working well, apart from the solar panel being undersized.


On the VRM side it looks as if you've found a couple of areas for improvement. The Grey line having no data or Y axis, and the voltages being displayed with different scaling. I'll raise this as bugs. FYI, the custom widgets are a new and useful feature, but there are some wrinkles to iron out.


Please come back here if there's anything that's not clear in my reply. And thanks for raising it, helps a lot.

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seb71 avatar image seb71 commented ·

PV voltage is good

I would not say that. Starting from 11 June and especially in 13 June, there are periods during the day when the PV voltage is about 13V.

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kevgermany avatar image kevgermany ♦♦ seb71 commented ·
I agree. Misreading on the dual scale. Sorry



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kevgermany avatar image kevgermany ♦♦ commented ·
Grey power line update, missing because you switched it off. I should have seen that.
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gone-sailing avatar image gone-sailing commented ·

>There are two voltage axes, both using different scales(orange on left for panels, red on right for battery).

You are very right. I wish VRM custom widget would use the same voltage scale for voltages. The PV Voltage range is much greater so at that scale any smaller fluctuations are hidden. So what described earlier as "flat lining" is not a totally flat line at all. Enhancement request please!

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gone-sailing avatar image
gone-sailing answered ·

Some more information. Yesterday, I switched off the Multiplus at 9am so the only charge source was the MPPT.

1655359319127.png

After about 10 mins the PV Voltage resumed and the MPPT switched back on. All was OK for a couple of hours, then the PV Voltage flatlined again. I switched the Multiplus back on at about 4pm but the PV Voltage stayed flat until dusk. Whilst the Multiplus was off, SOC of charge dropped to 98.7% due to the loads on the system.

It's not clear from the VRM chart due to the PV voltage scale, but the PV Voltage tracked the battery voltage at about 1.1/1.2 below battery voltage.

I still don't understand what is going on.


1655359319127.png (167.2 KiB)
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klim8skeptic avatar image klim8skeptic ♦ commented ·
Do you have any loads on the load output of the mppt?
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seb71 avatar image seb71 commented ·

I still don't understand what is going on.

The 78W PV panel is too small. Replace it with a bigger one.

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kevgermany avatar image
kevgermany answered ·

Please post screen shots of the Multiplus and MPPT settings.

Have you switched on DVCC?

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gone-sailing avatar image
gone-sailing answered ·

We just finished wiring in the two big 175w panels yesterday. So far I haven't seen the problem happen again. I'll let the system settle in for a few days then report back.

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