A few more questions about solar panels...

[Qwazert]

As you may recall...I had some issues with my solar panels/batteries a few weeks ago.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30348700.cfm

I've since bought a new controller (because the old one would show the batteries as Charged even though they obviously weren't) and I've come across another puzzle....
The controller is a RENOGY Voyager, rated at 20 amps which should be sufficient for my 100W panels.

Here are the details of my solar panels: The first is an offshore-made flexible type...


and the second is a rigid style, like the type you get mounted to a roof...


As you can see, the specs are fairly close and do NOT exceed the ratings on the controller.

Here's the interesting part; when I connect only ONE panel I get a charging current of around 2-3A.
If I connect the second one in parallel, the charging current either stays the same or decreases slightly.

The controller has built in reverse polarity protection and it gives no errors, so I'm 99.9% certain that the panels are connected properly.
So why am I getting LESS current with the second panel connected?
Does the controller sense that less is required because the batteries are fully charged (still connected to shore power)?

[CA_Traveler]

Qwazert wrote:

As I understand things, connecting them in series will not be much of a benefit, since I need more charging CURRENT, as opposed to higher voltage.

In fact the amps would be the lower Imp panel.

Serial panels with a MPPT controller can be a better option depending upon a number of parameters. MPPT converts the higher voltage power to the battery voltage and increased amps. Basically the OP would get about the same amount of battery V and A for charging as the PWM controller with parallel panels.

[CA_Traveler]

As stated remove any other charging source and add 10A+ load (lights etc) and see what you get with 2 panels.

But be aware that the contoller amp output FIRST is used by any house loads and any excess then charges the batteries. ie With a 8A output 2A may be house loads and 6A battery charging.

[CA_Traveler]

I have two 12V panels...not a 20.8 and a 22.6...they are both considered as 12V panels.

That's Voc (open circuit) which is useful for testing panels. Vmp (maximum power) is used for power along with Imp. Voc and Isc (short circuit) are useful for testing a panel.

12V panels are generally 36 cells and Vmp is 18-20V. They are used as one or in parallel to charge 12V batteries with a PWM controller. The wiring loss is much less of a factor as long as the maximum battery charging voltage is maintained which is usually 14.4-14.8V. Panel temperature is another consideration.

72 cell panels can charge a 24V battery. 60 cell panels provide very limited voltage drop for 24V batteries.

[red31]

the little lights on the controller indicate when the controller is in BULK mode.
Bulk mode is the only mode where the panels full power goes to the battery. The panels should operate at approx battery voltage during BULK mode. As the battery voltage rises, panel operating voltage will rise to match that of the battery.

Other modes, power is limited to maintain a constant voltage. The panel is turned on/off to keep the voltage at a certain set point.

So the controller needs to be in BULK mode for your test!

[Qwazert]

CA Traveler wrote:
This may help. Solar panel cells are not batteries and are electrically diodes and hence the higher voltage panel cannot backfeed the lower panel. Hence no heating occurs in the lower voltage panel and blocking diodes are not needed. Most modern controller prevent the function of isolating the panels from controller and hence the battery is not discharged at night. Ie Blocking diodes are not needed.

Scroll down the the 4 parallel panel diagram with different amps and voltage for an example. https://solarpanelsvenue.com/mixing-solar-panels/

Interesting article, and it does clear up some of the confusion.
For instance, I have two 12V panels...not a 20.8 and a 22.6...they are both considered as 12V panels.
Also, this new charger has "multiple intelligent protections against reverse polarity, over charge, short-circuit, reverse current, overload, and over discharge, keeping your battery fully protected."
So indeed...diodes are likely not needed.

[time2roll]

Please post the results 😉

[Qwazert]

Wow...I am getting a lot of conflicting answers on this one.
At the time of the original post, I had BOTH panels connected to the SOLAR input of the controller. Now it makes sense that since the batteries were fully charged AND connected to shore power, that there wouldn't be much current required from the panels.
I did test the panels individually with the same controller and the panel output was roughly the same in both cases; about 2 - 3 amps. When connected together, the output was much the same.

Tomorrow (or as soon as this current heat wave ends) I will try disconnecting shore power and will reconnect each panel separately and again in tandem.
As I understand things, connecting them in series will not be much of a benefit, since I need more charging CURRENT, as opposed to higher voltage.

[CA_Traveler]

This may help. Solar panel cells are not batteries and are electrically diodes and hence the higher voltage panel cannot backfeed the lower panel. Hence no heating occurs in the lower voltage panel and blocking diodes are not needed. Most modern controller prevent the function of isolating the panels from controller and hence the battery is not discharged at night. Ie Blocking diodes are not needed.

Scroll down the the 4 parallel panel diagram with different amps and voltage for an example. https://solarpanelsvenue.com/mixing-solar-panels/

[2oldman]

Qwazert wrote:
Does the controller sense that less is required because the batteries are fully charged (still connected to shore power)?

You have these panels hooked up AND connected to shore? I guess the simplest answer would be your 2-3 amps is all the batteries are asking for, and adding another panel doesn't make any difference.

Turn shore off, put some load on the batteries, drain them a bit, then try again.

[time2roll]

Assuming controller is PWM the panels will operate just above battery voltage and provide Isc in the bulk phase of charging.

[pianotuna]

Panels in parallel need to be the same voltage (within 1 volt or less).

[Gdetrailer]

CA Traveler wrote:

time2roll wrote:
Put a 20 amp load on the battery and test again.

X2 Your 2 parallel panels can deliver 5.4A+5.17A = 10.57A at 17.6V (the lower voltage panel). This power loss from the higher voltage panel will not be a factor for a PWM controller as long as your wiring losses do not limit the maximum charge rate the battery will accept, ie the absorb voltage for flooded batteries.

:h

Hmm, not so sure about that.

In parallel the combined voltage will result in a lower voltage than the highest panel voltage rating which does affect PWM output. Some of the voltage (hence some of the wattage) developed is wasted (IE is lost) as heat in the lower voltage panel.. The lower voltage panel due to additional heat may result in a further reduction of harvest.

This is the same thing as paralleling old wornout battery(ies) with new batteries.. The old batteries will draw down the voltage of newer batteries until they reach equilibrium in voltage..

Mismatched voltage panels for maximum harvest are better suited to be combined in series and use a MPPT controller.

There is a considerable 5.2V difference between the two panels open voltage rating, that is about 11 - 12 cells difference (each individual solar cell in a panel develops about .5V).. My guess is the max harvest might be in the range of 7A-8A at best, not 10.57 as it isn't going to be a linear addition of current.

This is why adding isolation diodes might prove beneficial since Schottky diodes only drop .3V and would help maximize the harvest in parallel with mismatched panel voltages with PWM controller.

I would agree that if the OPs batteries are already charged and topped off, they will not see more than a couple of amps.. In this case for testing purposes they need to turn on at least a 10A "load" then check the current being harvested from the panels.

[SJ-Chris]

You can also test each individual panel separately two different ways....

One way is with a volt meter / amp meter. In sunlight, connecting the meter to the MC connectors of the solar panel you should be able to read the Voltage and then the amps. This way you know your panels are both good.

Another way is to hook them up to your controller by themselves (one at a time). Do each of them work (according to your solar charge controller)?

If your batteries are fully charged, you will not see a difference if you hook up 100w, 200w, 1000w....your battery can only accept a certain amount of amps and when fully charged that amount is quite low (usually .3 to 2 amps). Are your batteries also hooked up in your rig? If so, you can turn on a bunch of lights/etc to create a load (draw) on your batteries. This will allow more current to flow from your solar panels.

Keep us posted and good luck!
Chris

[CA_Traveler]

time2roll wrote:
Put a 20 amp load on the battery and test again.

X2 Your 2 parallel panels can deliver 5.4A+5.17A = 10.57A at 17.6V (the lower voltage panel). This power loss from the higher voltage panel will not be a factor for a PWM controller as long as your wiring losses do not limit the maximum charge rate the battery will accept, ie the absorb voltage for flooded batteries.

[time2roll]

Put a 20 amp load on the battery and test again.