Solar charger - tempature compensation really needed?

[john61ct]

Originally Posted by a64pilot

Another way if you don't mind the hassle is a simple remote thermometer, read the temp and change the voltage yourself.
Batteries are of course a huge thermal mass and therefore quite resistant to temperature changes, the temp difference in the hottest part of the day to the coolest part of the night is negligible.
They most variation I think I have ever seen my charging voltage is plus or minus .1 V.

However my bank is in the boat, specifically under the settee. Now if your bank is in the engine compt., then of course that is different, a long motor would likely raise the temp quite a lot.

True for people staying in areas where the weather is constant, but in say Maine of course you'll see bigger variations, sometimes in just a single day 8-).

I am realizing though that the temp sensor being internal to the controller is not such a big deal for mobile solar, lots more so for higher-amp sources.

[noelex 77]

Originally Posted by basssears

So you're not limited to 220w on this controller, you're limited to 220w at 12V... you can run up to 440w into this controller if you go 24v. I'm going to start with 200w @ 24v into this controller, knowing if I want I can add another 200w as long as I keep that at 24v as well.

For most controllers you can only do this if the output is 24v. The input voltage is irrelevant, as the limit is the output current and that will always be equal to or greater than the input current.

So you need a battery voltage of 24v. A solar panel voltage of 24v does not help. The Victron could be different in this regard, but I would be surprised.

[noelex 77]

Originally Posted by john61ct

Means if the charging voltage setpoint is 14.8V, the controller won't even try to start any charging until the input voltage from the panel(s) reaches 19.8

I dont think this is correct. My understanding is that it is just Vbatt +5v, which will generally be much lower than 14.8v at startup.

[john61ct]

Originally Posted by basssears

OK so start is based on charging voltage setpoint, not current voltage of batteries... good to know, thanks for clarification.

Actually I don't know, didn't think the controller would do a calculation each day based on battery V, but now you mention that would make sense, and those Dutch engineers are cleverer than most of us 8-)

[john61ct]

And yes, the 12V vs 24 is talking about the system / battery voltage, nothing to do with panel inputs, which make no difference as long as you're between the spec's min and max

[StuM]

Originally Posted by john61ct

Means if the charging voltage setpoint is 14.8V, the controller won't even try to start any charging until the input voltage from the panel(s) reaches 19.8

I think you'll find it will start when the output voltage of the panels is 5V greater than the current battery voltage,not the charging voltage.

So if you batteries are down to 12.5, charging will start when the batteries are at 17.5.

[john61ct]

Yes, makes sense, thanks.

Originally Posted by noelex 77

the limit is the output current and that will always be equal to or greater than the input current.

Could you explain what you mean?

Pretty sure that's impossible for PWM controllers, do you mean because of the step-down Voltage conversion done by MPPT?

And "always" is a stretch I think, there are some losses. . .

[noelex 77]

Originally Posted by StuM

I think you'll find it will start when the output voltage of the panels is 5V greater than the current battery voltage,not the charging voltage.

So if you batteries are down to 12.5, charging will start when the batteries are at 17.5.

That is my understanding as well.

It is not ideal for "12v" panels, but is not as bad as it sounds. The 17.5v is Voc not Vmp. Solar panels reach quite a high voltage well before they can produce any useful power.

Here is a photo of my controller. It is a blurry photo because it was still pretty dark, but the panel voltage has already risen to 18v. At this stage the controller is asleep, although it has woken up, found the panels will not produce enough power to compensate for the self consumption of the tracking circuitry, and has gone back to sleep.

I suspect the Victron is a little less sophisticated and picks a single wake up voltage. 5v over battery voltage is a bit too high in my view, but not as wasteful as it might sound.

[noelex 77]

Originally Posted by john61ct

Yes, makes sense, thanks.
Could you explain what you mean?

MPPT controllers (with very rare exceptions) can only reduce voltage and boost current.

Many people assume that if the controller is rated at say 20A providing the input is less than 20A everything is acceptable. However 20A is also the maximium output current and this will be the limiting factor for a MPPT controller.

[basssears]

Originally Posted by noelex 77

For most controllers you can only do this if the output is 24v. The input voltage is irrelevant, as the limit is the output current and that will always be equal to or greater than the input current.

So you need a battery voltage of 24v. A solar panel voltage of 24v does not help. The Victron could be different in this regard, but I would be surprised.

In one of my early economics classes I was taught "TNSTAAFL" (prof. would even yell it at us, "Tan-staff-el")... There Ain't Such Thing As A Free Lunch. Guess I'll have to start applying that to some of my solar thoughts.

Thank you for pointing this out, I thought those ratings were just talking about the input, did not realize I would have to have 24v output for that higher wattage to be valid (even though now that I think about it it makes sense, the output wiring inside the controller is obviously sized to a certain amperage just like the input wiring).

Thanks for the heads up.

-- Bass

[marty9876]

Few takeaways for me-

Using a Victron MPPT controller on a 12v string sounds like a poor match ( in fairness the manual does state 2x 12v in series or 1x 24v panel or more is recommended).

I'm starting to think with a lower voltage (12v) lower wattage ( < 200-300w) sting it might be better suited with a PWM controller. MPPT really shine in either higher voltage applications or cooler environments (e.g. Winter), neither of which apply to my specific scenario.

Seems completely wrong to be thinking PWM but perhaps on a small setup, with a fully programmable tempature compensation enabled PWM controller one could save some cash and be giving up less than 10% performance at most.

E.g. Victron BlueSolar PWM-Pro Charge Controller 12/24V-30A

[StuM]

Originally Posted by basssears

In one of my early economics classes I was taught "TNSTAAFL" (prof. would even yell it at us, "Tan-staff-el")... There Ain't Such Thing As A Free Lunch. Guess I'll have to start applying that to some of my solar thoughts.

That's more commonly TANSTAAFL (There ain't no such thing as a free lunch).

Popularised by Robert Heinlein in his SciFi novel "The Moon is a Harsh Mistress." but the acronym predates that book by decades.

[mitiempo]

Originally Posted by basssears

My take on this:

Stay you're batteries are half gone (12.2v)... your panels would need to be putting out 17.2v or higher before the controller would start feeding amps to your batteries, and once the controller starts feeding amps to your batteries the panels would have to keep putting out at least 13.2 volts to have the controller continue to feed amps to the batteries.

Is 17.7v the max your panels will put out? Sounds like at that max they would have to be operating pretty good conditions before they would even "start" that controller.

This is why I am going to standardize around 24v not 12v system... open circuit voltage on my panels is 22.7 so as a "24v" system (i.e. running in series not parallel) open voltage is 45.4, meaning I don't have to have a hugely efficient setup going to get this controller started.

You are reading the manual incorrectly. Victron's 12 volt or 24 volt relates to your battery system, not the panel array. If your battery bank is 12 volts you are limited to 200 watts of panels with the 75/15 controller - whether the panels are in parallel or series.

As far as the controller needing a 5 volt advantage to start this has never been a problem as far as I know, and I sell a large number of Victron MPPT controllers.

[john61ct]

Absolutely nothing wrong with PWM for panels whose output is in the standard lower ranges, some situations actually more efficient, and the money you save is better spent on another panel.

A sweet setup is Bogart's SC-2030 controlled by their Trimetric bank monitoring system. If you need more than 30A, two can be daisychained, get up to 60A output.

Input can be much higher than that, to boost average power in suboptimal conditions.

[mitiempo]

Originally Posted by marty9876

Few takeaways for me-

Using a Victron MPPT controller on a 12v string sounds like a poor match ( in fairness the manual does state 2x 12v in series or 1x 24v panel or more is recommended).

I'm starting to think with a lower voltage (12v) lower wattage ( < 200-300w) sting it might be better suited with a PWM controller. MPPT really shine in either higher voltage applications or cooler environments (e.g. Winter), neither of which apply to my specific scenario.

Seems completely wrong to be thinking PWM but perhaps on a small setup, with a fully programmable tempature compensation enabled PWM controller one could save some cash and be giving up less than 10% performance at most.

E.g. Victron BlueSolar PWM-Pro Charge Controller 12/24V-30A

That depends. If you are not exceeding 200 watts (by much at least) the Victron 75/15 would work very well. It is less expensive than the 30 amp Pwm controller you linked to. Victron Energy MPPT 75/15 Solar Charge Controller
As well the very popular wireless dongle from Victron only works with their MPPT controllers.

As I alluded to above the maximum panel wattage can be exceeded with Victron controllers - they will automatically cut it back to the stated limit. In many cases this makes sense as you will rarely get maximum wattage output from an array and it allows for a greater average current output for charging.

Do not however exceed the voltage maximum for the controller - there is no internal protection for over-voltage. As many panels can output a higher than stated voltage at times best to keep the maximum to say 60 volts for the 75/15.