One MPPT or Two?

[temptress]

So I was recently fitting a solar system to a cruising yacht in the Carribean and was faced wiht a challenge!

The entire system had been specified and delivered by a well know US based solar shop. They supplied 3 x 200w pannels, 3x1000w MPPT controlers, connectors and instructions on how they wanted them setup. So that is what I did.

However they insisted that each panned had its own dedicated Mppt controler connected directly to the domestic bank. If i had not had very clear instructions form the manufacturer, including a phone call, I would have wired the pannels in parallel to one Mppt controler (like I have done on my own boat). They were insistant that I needed a dedicated MPPT controler for each pannel.

For the avoidance of dbout an MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.

Now i can't get my head around the logic/engineering here. My training and instincts still tell me that one controler is what I need as more than one and they will step on each other.

Am i wrong here? Can anyone explain why the insisted on one controler per pannel?

 

[VicS]

So I was recently fitting a solar system to a cruising yacht in the Carribean and was faced wiht a challenge!

The entire system had been specified and delivered by a well know US based solar shop. They supplied 3 x 200w pannels, 3x1000w MPPT controlers, connectors and instructions on how they wanted them setup. So that is what I did.

However they insisted that each panned had its own dedicated Mppt controler connected directly to the domestic bank. If i had not had very clear instructions form the manufacturer, including a phone call, I would have wired the pannels in parallel to one Mppt controler (like I have done on my own boat). They were insistant that I needed a dedicated MPPT controler for each pannel.

For the avoidance of dbout an MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank. To put it simply, they convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.

Now i can't get my head around the logic/engineering here. My training and instincts still tell me that one controler is what I need as more than one and they will step on each other.

Am i wrong here? Can anyone explain why the insisted on one controler per pannel?

I'd have thought a separate MPPT controller for each panel to optimise the individual panel performances. Particularly relevant if the panels could receive different light levels.

One controller for all three if they are mounted side by side and always equally illuminated eg on the house roof.

 

[noelex]

The Vmp of each panel will be different especially on a boat with its multiple shaddows.
A separate controler for each panel allows the tracking to be better.

Against this the better controllers have a reasonably high self consumption so powering three controllers can erode, or eliminate the gains. The cost of three small controllers is also higher than one larger one and often there are less features.

 

[temptress]

I'd have thought a separate MPPT controller for each panel to optimise the individual panel performances. Particularly relevant if the panels could receive different light levels.

One controller for all three if they are mounted side by side and always equally illuminated eg on the house roof.

I don't think that is necessarily right. The Mppt is controlling the amps to the battery pack and surely they will step on each other?

 

[Ian_Edwards]

Because the MPPT controllers are nominally identical and sensing the same battery bank, they will switch from bulk, to absorption to float at the same nominal set points, so most of the time I think they will be OK and not step on each others toes. The battery bank will just absorb whatever current it can from the sum of the three input. However, when the battery bank is near a switching point and because the set points are only nominally the same, i.e. the set points will be slightly different because of manufacturing and component tolerances, there might be some odd behavior when the 1st MPPT switches to the next mode, it might drive the others to switch mode, or oscillate between two modes until the others catch up. It's hard to tell what these transition effects might be. But they probably won't last long and if the manufactures recommend the set-up, they have probably tested it and found no ill effects.

 

[VicS]

I don't think that is necessarily right. The Mppt is controlling the amps to the battery pack and surely they will step on each other?

Because the MPPT controllers are nominally identical and sensing the same battery bank, they will switch from bulk, to absorption to float at the same nominal set points, so most of the time I think they will be OK and not step on each others toes. The battery bank will just absorb whatever current it can from the sum of the three input. However, when the battery bank is near a switching point and because the set points are only nominally the same, i.e. the set points will be slightly different because of manufacturing and component tolerances, there might be some odd behavior when the 1st MPPT switches to the next mode, it might drive the others to switch mode, or oscillate between two modes until the others catch up. It's hard to tell what these transition effects might be. But they probably won't last long and if the manufactures recommend the set-up, they have probably tested it and found no ill effects.

An MPPT controller does two things.
It controls the output from the solar panel so that is running at its maximum power output. Normally with a terminal voltage in the region of 16-17 volts. The tech spec for the solar panel should state this voltage. It is this aspect of their operation which I think leads to the requirement for one controller per panel

It also controls its own output to the battery AFAIK in the same way that a PWM controller does, reducing the effective charge rate as the battery becomes fully charged by reducing the pulse width.
Maybe some operate like a multistage charger, with bulk, absorption and float stages. If so these could possibly affect each other and be unsuitable for a multipanel, multicontroller configuration.

 

[temptress]

An MPPT controller does two things.
It controls the output from the solar panel so that is running at its maximum power output. Normally with a terminal voltage in the region of 16-17 volts. The tech spec for the solar panel should state this voltage. It is this aspect of their operation which I think leads to the requirement for one controller per panel

It also controls its own output to the battery AFAIK in the same way that a PWM controller does, reducing the effective charge rate as the battery becomes fully charged by reducing the pulse width.
Maybe some operate like a multistage charger, with bulk, absorption and float stages. If so these could possibly affect each other and be unsuitable for a multipanel, multicontroller configuration.

I can't see how the MPPT 'controls' the output from the solar panels, it simple takes what it is given by the pannels and regulates it. Typicial Power point tracker is a high frequency DC to DC converter. They take the DC input from the solar panels, change it to high frequency AC, and convert it back down to a different DC voltage and current to exactly match the panels to the batteries. MPPT's operate at very high audio frequencies, usually in the 20-80 kHz range. The advantage of high frequency circuits is that they can be designed with very high efficiency transformers and small components. The design of high frequency circuits can be very tricky because the problems with portions of the circuit "broadcasting" just like a radio transmitter and causing radio and TV interference. Noise isolation and suppression becomes very important.

Unless it is slight variations in the voltage that cause issues. I always thought the MPPT altered the output (charging) voltage to maximise the AMPS the battery could take?

So I still don't understand why I NEED ONE MPPT per panel?

 

[Ian_Edwards]

If one panel is in the shade and another is in full sun a single MPPT control won't be able to optimism the output of both panels, it'll come to some compromise, depending on the combined voltage, current and impedance of the panels in parallel. Dedicated MPPT controllers will optimism each panel and increase the overall output. Whether this is worth the extra cost and complexity, is a different story.

 

[noelex]

I can't see how the MPPT 'controls' the output from the solar panels, it simple takes what it is given by the pannels and regulates it.

The controller does control the output voltage of the solar panel. It adjusts this voltage to find the point where the power produced is the highest. The voltage is adjusted up and down, to find and track this maximium point.
Any panels exposed to different conditions will have a different optimimum voltage. Thus one controller per panel can potential extract more power.

A single controller for two panels must select a compromise voltage so the output is lower.

 

[temptress]

The controller does control the output voltage of the solar panel. It adjusts this voltage to find the point where the power produced is the highest. The voltage is adjusted up and down, to find and track this maximium point.
Any panels exposed to different conditions will have a different optimimum voltage. Thus one controller per panel can potential extract more power.

A single controller for two panels must select a compromise voltage so the output is lower.

Sorry but how does the controler control th eSolar panel? Surely as it is downstream of the panel it gets as input whatever the panel sends it?

 

[VicS]

I can't see how the MPPT 'controls' the output from the solar panels, it simple takes what it is given by the pannels and regulates it. Typicial Power point tracker is a high frequency DC to DC converter. They take the DC input from the solar panels, change it to high frequency AC, and convert it back down to a different DC voltage and current to exactly match the panels to the batteries. MPPT's operate at very high audio frequencies, usually in the 20-80 kHz range. The advantage of high frequency circuits is that they can be designed with very high efficiency transformers and small components. The design of high frequency circuits can be very tricky because the problems with portions of the circuit "broadcasting" just like a radio transmitter and causing radio and TV interference. Noise isolation and suppression becomes very important.

Unless it is slight variations in the voltage that cause issues. I always thought the MPPT altered the output (charging) voltage to maximise the AMPS the battery could take?

So I still don't understand why I NEED ONE MPPT per panel?

It controls the current drawn from the solar panel so that it is delivering the maximum power...... thats why it is called maximum power point tracking. The specs should quote the panel terminal volts and the current at this max power point..

Draw less current then the power output will fall also if you attempt to draw more current the internal resistance of the panel will cause the terminal volts to drop and the power output to fall.

Control of the output to the battery is a separate issue. As I said earlier I believe this is by pulse width modulation.

 

[VicS]

Sorry but how does the controler control th eSolar panel? Surely as it is downstream of the panel it gets as input whatever the panel sends it?

By controlling the current drawn so that the panel operates at its maximum power point

 

[noelex]

The voltage of a solar panel is quite easy to control. You don't even need sophisticated electronics if you hook your solar panel up to your battery without any controller the solar panel voltage will be the same as your battery voltage (ignoring losses in the wiring).

The difference with a MPPT controller is that the solar panel voltage can be different to the battery voltage. This voltage is controlled at the point where the maximum number of watts is produced. This ideal voltage is constantly changing with shadows, temperature, light level etc so the solar panel voltage is constantly adjusted by the MPPT controller. This is what MPPT stands for. Maximum Power Point Tracking is adjusting the input voltage from the panel so the maximum power is produced.

Draw more current from the solar panel and the voltage goes down, draw less current and the voltage goes up. There is a point where the volts x amps (the number of watts) is the highest. It is at this point the controller is aiming for, but the correct answer is constantly changing.

The Vmp or maximum power point voltage is lower that the the maximum voltage the panel can produce Voc

 

[VicS]

If you connect the solar panel to a variable load together with a voltmeter and an ammeter

Record volts and amps as the load is varied , multiply volts by amps to get the power in watts and plot a graph of power against volts you will get a curve which rises to a maximum and then falls again.

The peak is the maximum power point at which an MPPT controller tries to control the solar panel. the corresponding volts is V_MP

 

[William_H]

Solar panels were and are still designed to charge a 12v battery with a direct connection. The design here means the number of cells cosen to make the panel. Cells giving about .4 volt each and are wired in series (then banks in parallel. The number of cells was chosen to give a useful charge voltage say about 13v on a relatively dim day. This means that in real full sun the voltage is higher than needed. hence a 12v panel will usually produce 18 to 20 volts with no load. As said the cells have a significant internal resistance. This means that as current is drawn the voltage falls. Now a regulator is usually fitted to stop the panel overcharging or boiling the battery. hence not a concern for a small panel large battery.
An MPPT controller both stops the battery being overcharged but more importantly takes current from the panel at the rate for amx power (transfer) for that particular level of sun shine.
As for the OP question I would mostly guess that while as said 3 controllers might extract a little more power from the whole system it would mostly improve the sellers sales figures. olewill

 

[changeman]

You are right about them stepping on each other.
If one panel is in full sunshine producing nearly full power from its controller, the other two controllers, possibly in partial shade, will detect what they assume to be a charged battery bank and switch off.
Much better to connect all panels through one controller and it will sort out the input from each panel.
What is also disturbing is the size of the controllers. 3 of 1000w units are expensive and way too overspecced for their configuration. 3 of 200w (or 20amp) controllers would suffice for their configuration or a single 1000w (or 80amp) unit would easily suffice for the panels in parallel.
Makes me wonder if they really know their stuff.

Regards
Chris

 

[VicS]

You are right about them stepping on each other.
If one panel is in full sunshine producing nearly full power from its controller, the other two controllers, possibly in partial shade, will detect what they assume to be a charged battery bank and switch off.
Much better to connect all panels through one controller and it will sort out the input from each panel.

Regards
Chris

Not so much that the controllers with their panels in partial shade will "detect what they assume to be a charged battery bank and switch off". More a case of the lower output voltage from the shaded panels not feeding any current into a circuit already at a higher voltage.
You cannot get flow from a low pressure source to flow into a higher pressure system. To use the water analogy.

The same thing will happen if all the panels are connected in parallel to the same controller. There will be no output from the shaded panels until their illumination is high enough for their output voltage to reach the same level as the illuminated panel.

In fact if the panels each have their own MPPT controllers, by virtue of the way they work, this will occur sooner . What your are using as an argument against separate controllers is really an argument in their favour.

 

[noelex]

There are really no problems with multiple regulators. The only slight danger is that you can end up with a slightly longer absorption time in some situations. As the correct absorption time is very debatable and variable anyway this is of little practical consequence, but with an adjustable controller setting a slightly shorter absorption time on each controller is a good idea especially if the default is on the long side.

If anyone can think of hypothetical situation with two controllers where the effective charging algorithm would be poor, or cause problems, then I am all ears.

 

[temptress]

So it seems we can't agree in here.

I have my panels connected is parallel to a single Mppt and they work fine. THIS manufacturer insisted that there was one Mppt per panel.

Seems to me that as long as the input voltage and amp rating allows then one MPPT is all that is required.

NOW, how about wiring the panels is series to increase the input volts to say 60? Assuming the controler can handle the input would this be better?

 

[VicS]

I have my panels connected is parallel to a single Mppt and they work fine.

Of course they do. they would just work better if they been connected to separate controllers.

It would need some detailed measurements to determine how much difference it makes. In fact it would probably be impossible to control the variables to make a valid comparison without doing it under laboratory conditions

NOW, how about wiring the panels is series to increase the input volts to say 60? Assuming the controller can handle the input would this be better?

carry out some tests, under laboratory conditions so that you have control of all the variables, and let us know the results.