Voltage at panels in darkness?

[pda1]

In darkness I disconnected the positive and negative wires coming from the Combiner box and to my surprise there was 26.6 volts at the wires. Obviously they are getting that voltage from the batteries (24v system).

Is that the way it's supposed to be?

Are the panels being backfed at night?

Thanks,

Peter

 

[Bud Martin]

Voltage is measured at the PV input terminals of the SCC, correct?
There are capacitors at the PV input of the SCC, you should see Voltage start dropping after a while.

 

[time2roll]

How many amps?

 

[pda1]

How many amps?

Didn't measure the amps. Will do in a while.

 

[Bvillebob]

I suspect it's just from stars, the moon and / or streetlights. It's voltage, but there's no amperage to go with it, there's no real power being generated.

 

[Bud Martin]

How many amps?

He is measuring the Voltage at the PV input that is disconnected from the panels so I hope he IS NOT going to put meter in Amp mode and put the probes on the PV + and - Input terminals, OP may get a big surprise.

 

[time2roll]

I would be surprised to see a couple milliamps. Virtually nothing should be coming from the controller. Same as virtually nothing from the panels.

Is there no controller?

 

[Bud Martin]

If the capacitor bank at the PV input is discharged then there will not be much of the current to discharge. My MPP Solar takes about 5 minutes for the PV input caps discharge, I verify that by turn off my PV breaker.

 

[pda1]

Here are the results taken at night;

At the Pv input to the SCC- 26.6v Amperage- 0.15 A (+/-) ammeter is a clamp-on so the readings fluctuate.
At the SCC output to the batteries- 27.11

Without using the clamp on ammeter and taking a amperage reading on the positive PV side to the positive of the SCC input- .3 amps.

SCC is a Victron 150/60 mppt.

 

[sunshine]

Without using the clamp on ammeter and taking a amperage reading on the positive PV side to the positive of the SCC input- .3 amps

That is minus 0.3amps?
You shouldn't be getting more than the standby current whichever way you look at it....unless at 27.1v the CC is still on in float mode and the current is somehow finding it's way back though the resistance of the panels. The resistance of the panels can be worked out from the specs.
Try at a lower battery V and see if the CC is now in idle mode and float is off.

Edit= It wouldn't be the capacitors as they would have bled off back through the panels from that position since there are no blocking diodes.

 

[pda1]

That is minus 0.3amps?
You shouldn't be getting more than the standby current whichever way you look at it....unless at 27.1v the CC is still on in float mode and the current is somehow finding it's way back though the resistance of the panels. The resistance of the panels can be worked out from the specs.
Try at a lower battery V and see if the CC is now in idle mode and float is off.

Edit= It wouldn't be the capacitors as they would have bled off back through the panels from that position since there are no blocking diodes.

No, it was 0.3 a

I'm thinking it might be the capacitors on the inverter feeding back. Will give it another try tonight.

 

[svetz]

I'm not 100% sure what you're measuring or if you've disconnected the PV panels from the system to isolate the measurements. It's not uncommon to see voltage on panels at night, but usually, there shouldn't be significant current. if you've isolated your string and short-circuit the panels for a second or two the voltage and current should go to zero as their capacitance fades. If there is current, it could be a grounding or dissimilar metals issue, atmospheric electricity (e.g., St. Elmo's fire), or even phantom current (possibly induced current by wires running parallel under or over the PV run, although this would be an AC voltage). If the voltage and current are on the SCC side then possibly the blocking diodes in the SCC have shorted out or become excessively leaky. As you have Victron you'd want to call their tech support number for troubleshooting, if they agree there is a problem ask them to start the warranty process for a replacement.

Typically an SCC has a built-in blocking diode to prevent back-feeding the panels at the night. Bypass diodes are typically built into the Jbox.

 

[pda1]

I'm not 100% sure what you're measuring or if you've disconnected the PV panels from the system to isolate the measurements. It's not uncommon to see voltage on panels at night, but usually, there shouldn't be significant current. if you've isolated your string and short-circuit the panels for a second or two the voltage and current should go to zero as their capacitance fades. If there is current, it could be a grounding or dissimilar metals issue, atmospheric electricity (e.g., St. Elmo's fire), or even phantom current (possibly induced current by wires running parallel under or over the PV run, although this would be an AC voltage). If the voltage and current are on the SCC side then possibly the blocking diodes in the SCC have shorted out or become excessively leaky. As you have Victron you'd want to call their tech support number for troubleshooting, if they agree there is a problem ask them to start the warranty process for a replacement.

Typically an SCC has a built-in blocking diode to prevent back-feeding the panels at the night. Bypass diodes are typically built into the Jbox.

Tonight when it's dark this is what I'm going to do (tell me if this is right or wrong)-

At the SCC;

- Remove the wires coming from the panels
- Short the panel wires
- Check for voltage at the panel wires (as mentioned there probably will be some small voltage)
- Check for voltage on the PV panel input terminals of the SCC

I will not be removing any wiring from the SCC output "to the battery" terminals. The only wires that are going to be removed are the wires from the panels to the SCC "From Panels" terminals.

How do that sound for a process?

 

[svetz]

- Short the panel wires

If you have a PV cutoff between the SCC and panels, you can just use it rather than disconnecting anything.
Measure the voltage first a minute or so after disconnecting, if it's high (> 24V) you might want to short them through a hefty resistor rather than directly.
Please be careful... dark & electricity don't go well together.

When you separate the shorted PV wires and again check for voltage (if the moon is out you might see some but it should take a bit to build up). Check both DC and AC (AC might be induced voltage). If there is no moon and you see DC voltage the potential difference might be from corrosion of dissimilar metals (e.g., aluminum frame and stainless brackets). If you're in Spain with all the Sahara dust it might just be charge from the atmosphere.

I will not be removing any wiring from the SCC output "to the battery" terminals.

You'll still want to measure the voltage on where the SCC side (e.g., where the PV wires plug into). That will tell you if the SCC has a bad diode back-feeding the panels. I wouldn't short that to find the current though... if the blocking diodes are bad it could easily be the full battery output and blow a fuse or far worse. You already know the power from your prior measurements (where the panels act as resistors to the current).

Hope that makes sense, be careful and good luck!

 

[pda1]

Testing was done at night with overcast sky.

Here are the results with the panel wires disconnected (circuit breaker in Combiner) from the SCC-

Panel string 1= 26mv
Panel string 2= 26mv
Panel string 3= 26mv

Voltage at wires going to SCC "Panel input"= 26.91v

Voltage at SCC terminals "Battery"= 27.44v

SCC was in "Bulk" charge mode

With the entire system connected to the panels a current of .17 (point one seven) was measured.

Hope that makes some sense.

Thanks,

Peter

 

[svetz]

The panels look fine (e.g., nothing to worry about regarding corrosion, grounding, phantom/induced, atmospheric effects).

Call Victron tech support and tell them your current and voltage from the SCC at night to the panels and let us know what they say.
5W per string isn't much, but it all adds up. I could be wrong, but it sounds excessive to me.

 

[12VoltInstalls]

sounds excessive to me.

I’m no expert or EE but that sounds wonky to me too

 

[sunshine]

Call Victron tech support and tell them your current and voltage from the SCC at night to the panels and let us know what they say.

Why would the CC be in Bulk Mode at night?

'Voltage at SCC terminals "Battery"= 27.44v SCC was in "Bulk" charge mode'

This amperage may disappear when the B drops below a certain V setting and the CC goes into standby?

Also 'Voltage at wires going to SCC "Panel input"= 26.91v" What is the reverse R of solar panels ? Must be high to get a connected V of 26.97?

 

[pda1]

Why would the CC be in Bulk Mode at night?

'Voltage at SCC terminals "Battery"= 27.44v SCC was in "Bulk" charge mode'

This amperage may disappear when the B drops below a certain V setting and the CC goes into standby?

Also 'Voltage at wires going to SCC "Panel input"= 26.91v" What is the reverse R of solar panels ? Must be high to get a connected V of 26.97?

Don't know why it would be in bulk. I assume the SCC is "dumb"- doesn't know it's dark outside. Realistically it could be because the battery voltage drops below 27v which causes the SCC to go into Bulk charge mode.

What does this mean? - What is the reverse R of solar panels ? Must be high to get a connected V of 26.97?

I've asked this overall question at the Victron forum here- Mppt backfeeding panels at night?

At the SCC "Battery" terminals I was reading the battery voltage when the PV wires are disconnected.

 

[pda1]

It's dark outside.

Below is a screenshot of the MPPT readings using VictronConnect. Yes, there is 27.01 volts at the panel terminals of the SCC (that figure is from my voltmeter and not the figure you see below- it's close enough. Even with the pv panel wires are removed there is voltage.

Notice that Battery---State of Off

(A side note- I'm curious as to what the Virtual load output---State---On is?

About my comments on Bulk charging. I'm going to check the readings early tomorrow morning to see if the Battery----Voltage will be drop below 27v which should cause the controller to go into Bulk charging (so I think).