Parallel two solar controllers??

[HeywoodJ]

QCells are a bit of a wiring odd-duck, most of them (I think) have three junction boxes; one for positive cable, one for negative cable, and one for bypass diodes.

If you can access the bypass diodes (QCell also pots those on many panels) you could remove the central diode, hook a cable up to each terminal, voila, you now have two 160W modules in a single frame. Not an ideal solution, but one that might work with the parts at hand and avoid any question of paralleling.

[hpeer]

Originally Posted by skipmac

Qcell 320. Got them from a friend that does home solar installs. Leftover last year's model at his cost.

This year a different brand rated 340W at about the same size and weight but due to potential supply chain problems boss didn't want to let go any of the newer ones.

Below is the spec sheet for these panels.

Not a great spec sheet but good enough.

The short circuit current for these panels is 10.09A.

That is the absolute max you can get out of them.

The Impp is probably about 8 amps.

How did you derive you were going to get 27amps? Put 3 or 4 panels in series?

[HeywoodJ]

The Imp is 9.60A @ Vmp of 33.32V (which means splitting probably wouldn't work because you would end up with two 16V panels at STC).

The whole point of the DC-DC conversion in MPPT controllers is to take low current, high voltage electricity and turn it into high current, low voltage electricity with a DC-DC converter. 320W@12V = 26.7A, 320W@14.4V = 22.2A. In reality, at NOCT, the panel will generally probably be able to provide 18-20A into a 12V (nominal) battery system. But we design around MPP to get as much as we can when the getting is really good.

[RaymondR]

They weren't MPPT but I had three 20 amp controllers working in parallel and it did not seem to bother either them or the batteries. About the worst I can envision happening is that one will be a bit lazy and allow the other to do the work when the battery voltage gets towards the fully charged state.

[BjarneK]

I finished my experiments and using two MPPT controllers for one solar panel to charge the same battery or even two different batteries with a shared ground turns out to an even worse idea than I expected.

The short version: DO NOT DO THIS. It can result in overloading a controller which may result in it breaking or maybe even catching fire. The Victron MPPT controllers throttle the output when the internal temperature goes too high, so it may not catch fire, but I would not bet my boat on it.

The long version is a bit complicated, so first some background on the Victron MPPT controllers. They measure all sorts of data including output (battery) voltage, output current, input (solar panel) voltage, input power, internal temperature etc.

The output current is limited to 15 amps and this is done by the firmware of the controller (i.e. by software). The value can be set lower if desired. Such current may be measured as a voltage drop over a shunt resistor or by measuring a magnetic field in the output. My observations indicate that the current is measured on the negative output, which is the normal approach. The specific mechanism is not important, so let us just assume that a shunt resistor is used on the battery ground.

Another thing to note is that this is not an isolated controller: the negative of the battery and the negative of the solar panel are connected internally. This leads us to a schematic of the internals of the controller like the attached image. In that image CONT. is the actual controller, SH. is the shunt resistor and the dashed line is a connection for measuring the voltage over the shunt.

Imagine that you have two controllers, A and B, attached to the same battery and the same solar panel. Now the negative of the output current in controller A has two paths the the battery negative: Through its shunt as normal and over to the negative of the solar panel, into controller B, through the shunt in controller B and back to the battery negative. This means that actual output current of the controller and the current measured by its shunt resistor may not be the same. This is not good since the measured current is used to limit the output of the controller.

In some cases, one controller may be active and the other not. This could be if one controller decides that the charging is done while the other one is still charging (e.g. due to small differences between measured battery voltage). Other situations where one controller may be charging and the other not would be if one is broken or if the battery connection to one controller was disconnected for some reason.

Anyway, in this situation, the active controller may severely underestimate its output since some of its output goes through the shunt in the inactive controller. In this case, the firmware of the controller will just increase the output current until its measurements reach the 15 A that it is rated for. In the worst case, this may in reality be 30 A, overloading the charger which could break it or maybe even make it catch fire.

During my testing, I had one controller outputting 20 A due to this issue and then I turned it off to avoid breaking anything. At that point in time it was reporting an output current of around 12 A, so it would have gone higher if the sun had been full on.

[thomm225]

Originally Posted by skipmac

Upgrading my solar panels and the new panels exceed the rated capacity of my current controllers. The two controllers are rated 15 amps each and the new panels are rated 27 amps.

I have a new controller on hand for one panel but have two panels.

Wondered if I could use the two 15 amp controllers in parallel to take the output of one 27 amp panel. Would the controllers share the output equally or would a difference in internal resistance of the controllers send all the panel output to one or the other?

For about $$30.00 you can get two 30 amp PWM controllers that also have the absorption phase.

Pretty simple fix.

I bought a Victron a few years ago for $90.00 and it has since failed whereas these 2 PWM's I have now are still going strong

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

[Dankoozy]

Originally Posted by skipmac

Upgrading my solar panels and the new panels exceed the rated capacity of my current controllers. The two controllers are rated 15 amps each and the new panels are rated 27 amps.

I have a new controller on hand for one panel but have two panels.

Wondered if I could use the two 15 amp controllers in parallel to take the output of one 27 amp panel. Would the controllers share the output equally or would a difference in internal resistance of the controllers send all the panel output to one or the other?

It sounds like a bit of a hair-brained scheme alright. You have two controllers chasing the same Maximum power point that is constantly changing due to how much current the other controller is drawing.

[Andrew Lippman]

It seems the general wisdom is that parallel controllers works fine. Indeed, it provides redundancy and better operation when some panels are in the shade.

https://www.morningstarcorp.com/para...ate-pv-arrays/

https://cleversolarpower.com/multipl...e-controllers/

It also appears that they don't have to communicate, just as having the solar operational when the alternator or shore power is also charging the batteries.

I would check with Victron. I have had great luck with Justin Larrabee in the US: salesUSACanada@victronenergy.com. I have also had great luck with Victron equipment in general.

[skipmac]

Originally Posted by HeywoodJ

QCells are a bit of a wiring odd-duck, most of them (I think) have three junction boxes; one for positive cable, one for negative cable, and one for bypass diodes.

If you can access the bypass diodes (QCell also pots those on many panels) you could remove the central diode, hook a cable up to each terminal, voila, you now have two 160W modules in a single frame. Not an ideal solution, but one that might work with the parts at hand and avoid any question of paralleling.

These panels only have two junction boxes and two connections. Look pretty standard on initial inspection.

[skipmac]

Originally Posted by hpeer

Below is the spec sheet for these panels.

Not a great spec sheet but good enough.

The short circuit current for these panels is 10.09A.

That is the absolute max you can get out of them.

The Impp is probably about 8 amps.

How did you derive you were going to get 27amps? Put 3 or 4 panels in series?

Not sure what the short circuit amps spec means in this context. Assume that's 10A at Voc of 40.13V. I estimated 26-27 amps max at 12V based on a rating of 320 W. 320W/12V = 26.67A which is in the range of the max one would theoretically see from the controller into a 12V battery bank.

And before anyone jumps it, I know that isn't an exact figure and to charge it must be 13-14V etc. Just putting out round numbers for sake of simplicity.

[skipmac]

Originally Posted by thomm225

For about $$30.00 you can get two 30 amp PWM controllers that also have the absorption phase.

Pretty simple fix.

I bought a Victron a few years ago for $90.00 and it has since failed whereas these 2 PWM's I have now are still going strong

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I have two Victrons. A 70/15 that's 6-7 years old and a 75/15 2-3 years old and both working perfectly. Haven't done a side by side test but in general an MPPT controller should be more efficient and give you more charging from a panel than a PWM controller. Unless the Victron's start dying I think I'll stick with what I have.

[thomm225]

Originally Posted by skipmac

I have two Victrons. A 70/15 that's 6-7 years old and a 75/15 2-3 years old and both working perfectly. Haven't done a side by side test but in general an MPPT controller should be more efficient and give you more charging from a panel than a PWM controller. Unless the Victron's start dying I think I'll stick with what I have.

Yeah should be.

I was just saying you could get the larger PWM's if you were worried about too much current on the 15 amp controllers.

Also, I didn't notice much difference between the efficiency of the Victron 75/15 MPPT I had as compared to the PWM controllers I had in before but that could be due to only having 115 watts worth of solar going into my controllers.

The Victron has since failed but that could be due to all the experimenting I was doing at the time with different setups and I was continuously connecting and disconnecting panels, controllers, and batteries

Btw with the last PWM Controller I bought I noticed it has an absorption phase where it lets the batteries charge for a while at 14.4 volts or so with a float setting of 13.5. That's the first time I have seen that out of one of the lower priced PWM controllers

[skipmac]

Originally Posted by thomm225

Yeah should be.

I was just saying you could get the larger PWM's if you were worried about too much current on the 15 amp controllers.

Also, I didn't notice much difference between the efficiency of the Victron 75/15 I had as compared to the PWM controller I had in before but that could be due to only having 115 watts worth of solar going into my controllers.

The Victron has since failed but that could be due to all the experimenting I was doing at the time with different setups and I was continuously connecting and disconnecting panels, controllers, and batteries

Thanks for the idea. Since Santa Claus brought me a 50 amp rated MPPT controller I can manage one of the new panels with that and will keep one of the small Victrons for the second panel. May lose a bit of the max output occasionally but at this point will have excess capacity.

[hpeer]

Originally Posted by skipmac

Not sure what the short circuit amps spec means in this context. Assume that's 10A at Voc of 40.13V. I estimated 26-27 amps max at 12V based on a rating of 320 W. 320W/12V = 26.67A which is in the range of the max one would theoretically see from the controller into a 12V battery bank.

And before anyone jumps it, I know that isn't an exact figure and to charge it must be 13-14V etc. Just putting out round numbers for sake of simplicity.

10A means you will never ever get more than that.

Your assumption is very wrong.

That is why I included the links and diagram, to provide a better explanation.

I have made the mistake you are making. SOME manufacturers and dealers will lead you astray.

Just trying to help.

How many panels do you want to mount?

If it is 2 (seems logical for your size and type boat) then what I sketched out a ways back shows the math.

If you think I made a mistake please point out where.

I always enjoy a bit if humble pie!

[thomm225]

Originally Posted by skipmac

Not sure what the short circuit amps spec means in this context. Assume that's 10A at Voc of 40.13V. I estimated 26-27 amps max at 12V based on a rating of 320 W. 320W/12V = 26.67A which is in the range of the max one would theoretically see from the controller into a 12V battery bank.

And before anyone jumps it, I know that isn't an exact figure and to charge it must be 13-14V etc. Just putting out round numbers for sake of simplicity.

I believe the chart shows 10 amps at short circuit. (max current flow) Maybe that helps.

VOC (Voltage Open Circuit) is no load/open circuit so no current flow which is why the voltage is so high

In electronics, we used to call this float voltage but that would really cause confusion with controller float setting if they called it that