Solar Performance in the Med

[StuM]

Originally Posted by lawrence_craig

So I did some further tests. At midday in unshaded sun, the solar array is giving out 19V (multimeter to the connections).

However when I connect it to the regulator, the voltage drops to 13-15v.

It’s a Victron SmartSolat MPPT 75/15. It’s only 9 months old.

I have removed and rewired the connectors but getting the same result.

Batteries when testing this are at 30-40% SOC. (I know they shouldn’t be that low but I am about to replace I think).

At that SOC, dropping to 13-15V may well be the Maximum Power Point.
How many Amps were you getting at that voltage.

[ReneJK]

Originally Posted by lawrence_craig

So I did some further tests. At midday in unshaded sun, the solar array is giving out 19V (multimeter to the connections).

However when I connect it to the regulator, the voltage drops to 13-15v.

It’s a Victron SmartSolat MPPT 75/15. It’s only 9 months old.

I have removed and rewired the connectors but getting the same result.

Batteries when testing this are at 30-40% SOC. (I know they shouldn’t be that low but I am about to replace I think).

the Victron needs 'battery voltage" + 5volt to start ?

You might try to put the panels in series , up the voltage so the MPPT can start
properly and quicker , it may help with your setup

[lawrence_craig]

Originally Posted by StuM

At that SOC, dropping to 13-15V may well be the Maximum Power Point.
How many Amps were you getting at that voltage.

I was seeing about 2 amps.

[noelex 77]

Originally Posted by lawrence_craig

So I did some further tests. At midday in unshaded sun, the solar array is giving out 19V (multimeter to the connections).

However when I connect it to the regulator, the voltage drops to 13-15v.

It’s a Victron SmartSolat MPPT 75/15. It’s only 9 months old.

I have removed and rewired the connectors but getting the same result.

Batteries when testing this are at 30-40% SOC. (I know they shouldn’t be that low but I am about to replace I think).

These results are unfortunately not conclusive. A Voc of 19v in sunny conditions is lower than expected, but not so low that it proves a problem with the solar panel.

A Vmp of 13-15v is also slightly lower than would be normal, but this could be a problem with the panel or the controller, but once again it is not so low that it proves there is a diffinitve problem.

The 2A current in full sun from a 150w panel into a low SOC battery definitely shows a issue. There are a couple of tests that can be done to diagnose why this is occuring. One of the easiest and safest is to connect the solar panel directly to the battery (the battery should be reasonably discharged for this test and the solar panel in full sun).

If the current is similar, or lower than the connection via the Victron controller, it indicates a defective solar panel (I think this is the most likely result). If the current is higher (it will be much higher) with direct connection it indicates a defective controller.

[CatNewBee]

Originally Posted by noelex 77

These results are unfortunately not conclusive. A Voc of 19v in sunny conditions is lower than expected, but not so low that it proves a problem with the solar panel.

A Vmp of 13-15v is also slightly lower than would be normal, but this could be a problem with the panel or the controller, but once again it is not so low that it proves there is a diffinitve problem.

The 2A current in full sun from a 150w panel into a low SOC battery definitely shows a issue. There are a couple of tests that can be done to diagnose why this is occuring. One of the easiest and safest is to connect the solar panel directly to the battery (the battery should be reasonably discharged for this test and the solar panel in full sun).

If the current is similar, or lower than the connection via the Victron controller, it indicates a defective solar panel (I think this is the most likely result). If the current is higher (it will be much higher) with direct connection it indicates a defective controller.

You can measure Isc directly connecting an ammeter to the panel and measuring the short cut current, see panel parameters for details, a 150Wp panel with 19V Voc will be Isc around 10A. Use a clamp multimeter or one with more than 10A

Panel can always be checked in direct sun light measuring Voc and Isc with a multimeter. You can short cut the wires and measure the current of a single panel.

[noelex 77]

Originally Posted by CatNewBee

You can measure Isc directly connecting an ammeter to the panel and measuring the short cut current, see panel parameters for details, a 150Wp panel with 19V Voc will be Isc around 10A. Use a clamp multimeter or one with more than 10A

Yes, this is a good test. I suggested this in post #21. It removes the effect of battery SOC confusing the results, but introduces the possibility that lower sunlight can affect the reading. Both tests are fine and will show a similar result if care is taken.

The only caution with these current tests, is many multimeters are limited to a maximum of 10A and then only for a short time (such as 30 seconds). Higher power panels will exceed this rating. One advantage of the test I suggested is that the result can be read from the battery monitor. Battery monitors can measure high currents so the 10A limitation is not a concern.

CatNewBees suggestion of using a clamp on multimeter is a good one. Not many boats have this test equipment, but it is invaluable for a multitude of electric checks so is a worthwhile purchase. With a clamp on multimeter the 10A restriction does not apply and either test is fine and safe.

[lawrence_craig]

Perfect, will give them a try.

[skenn_ie]

Solar panels generate according to the weakest cell. If even one cell is in the shade, or even partially in the shade, the whole panel will be restricted. If the panels are in series, then both panels will suffer. Rated power will only be achieved if the sun shines brightly at 90degrees to them.

[skenn_ie]

Does anyone know what the capacitance of PV is likely to be. Many supposedly MPPT controllers are actually PWM, so the panel voltage will fluctuate significantly without large capacitance.

[noelex 77]

Originally Posted by skenn_ie

Rated power will only be achieved if the sun shines brightly at 90degrees to them.

This is not correct.

STC or Standard Test Conditions are measured with solar irradiance of 1000W per sq meter. This is bright, but these levels can be exceeded. So panels can typically briefly deliver their full rated power if they are not orientated at 90°.

[CatNewBee]

But they are rated at 25 degree celsius, measured by a flash, in reality a panel can heat up to 60 degree C, losing a lot of power.

[CatNewBee]

Originally Posted by skenn_ie

Does anyone know what the capacitance of PV is likely to be. Many supposedly MPPT controllers are actually PWM, so the panel voltage will fluctuate significantly without large capacitance.

That is true if you choose the wrong panels. PWM is more efficient than MPPT when the voltage difference between Vmp and battety is less or equal 3V, then MPPT makes no sense and the controller passes through like a PWM controller. There are high voltage panels, like mine, with 60 or 96 cells, they really leverage the advantages of an MPPT.

Btw, what do you mean with capacitance?

[StuM]

Originally Posted by noelex 77

This is not correct.

STC or Standard Test Conditions are measured with solar irradiance of 1000W per sq meter. This is bright, but these levels can be exceeded. So panels can typically briefly deliver their full rated power if they are not orientated at 90°.

Especially if there are some cumulus clouds around but they are not directly blocking the sun. You can get diffused light hitting the panels in addition to the direct solar radiation.

[skenn_ie]

Capacitance, to minimise the voltage/current ripple that the controller "sees". The cells have some inherent capacitance, but I have no idea how much. A buck converter, regulated to keep the input voltage at MPPT, and with substantial capacitance on the panel side, is the way it should be done. The only power loss is in the switch, and that can be as little as maybe 2%. I am trying to motivate myself to build a flyback converter with a synchronous switch on the output side.

[noelex 77]

Originally Posted by CatNewBee

But they are rated at 25 degree celsius, measured by a flash, in reality a panel can heat up to 60 degree C, losing a lot of power.

True. The STC conditions are measured at unrealistic cell temperatures. Nevertheless, if the solar panels are performing correctly and the installation is satisfactory, you should occasionally, under very good conditions, be briefly measuring an output that matches, or at least is close to the panel’s rating.

This is true for panels that are mounted flat, even in locations when the sun is a reasonable angle from directly overhead.

The cloud edge effect that Stu has mentioned is one of the reasons this occurs.