One controller or two?

[jbinbi]

I have 2 285 watt panels, currently with 2 inexpensive $99 mppt controllers. Running in parallel. 12v system.

I am buying some victron equipment, and was also going to replace the controllers. I can put the 2 panels in parallel to a 50 Amp controller, or get 2 30 Amp controllers.

Both my old controllers work. Any reason to get 2 controllers from an electric respective?

[Dsanduril]

If there is any chance that the two panels will have different solar exposures (i.e. one shaded partially while the other is in full sun), and there usually is on a boat installation, then independent MPPT trackers for each panel will theoretically provide more total output.

Ideally this would be a dual-MPPT input controller so that there is only one "charger" keeping track of the battery charge state while operating independent MPPT channels for each panel. These do exist in large commercial array installations, but haven't seen them in small, boat size controllers.

The one potential issue with two controllers is if one controller restarts or otherwise misses something during charging. Imagine this - Both controllers are operating, but Controller A restarts for some reason. Controller B is able to provide enough current to keep the battery voltage at 14.1V and is in bulk mode. After restart Controller A sees the voltage at 14.1V and assumes it should go to float and so does not provide any output (because the battery voltage it sees is above its float setpoint). Whether this happens depends entirely on the software in the controllers and most vendors don't provide those details. A good controller should 'test' whether it should be in float or bulk after a restart by providing output and monitoring voltage response, but....

In the end, my preference for most boat installations is one MPPT controller per panel, but it is not a simple yes/no question.

[john61ct]

Besides the greater output from each panel getting its own MPPT, you also get redundancy if one fails.

And in your case the wiring is already in place.

[jbinbi]

the panels are located right next to each on the back of a catamaran, so if the sun is behind, they get identical exposure. If in front and the sail is up, then one sometimes gets more shade than the other. So the answer is, they do get somewhat different outputs, but with the V tied together adding, I don't really know how much efficiency you lose. If it is < 10%, no problem with that, if 50%, then yes, I would go with 2 controllers.

With the 2 controllers, everything is a bit more complicated with victron for monitoring, etc. Was just hoping a higher quality single controller (with spares as backup) would make life simpler?

I am not really worried too much about redundancy as I have 2 more working controllers.

[john61ct]

The issue is just the greater incremental efficiency of the MPP tracking when done independently. Partial shading can cause the algorithm to jump around, almost like what happens with mismatched panels.

Sounds to me like a single SC is better for you, but there shouldn't be any addition of voltages, go parallel not serial.

[rwells36]

Originally Posted by john61ct

Besides the greater output from each panel getting its own MPPT, you also get redundancy if one fails.

And in your case the wiring is already in place.

How much is the greater output?

[Bleemus]

Two Victron SmartSolar MPPT units would be my choice. Bluetooth built in and monitoring is simple via the Victron app on my tablet and phone.

[john61ct]

Originally Posted by rwells36

How much is the greater output?

The difference would vary depending on conditions, even minute to minute.

[rwells36]

Originally Posted by john61ct

The difference would vary depending on conditions, even minute to minute.

I have three 315w all mounted on helm roof close together all three have a flat orientation without any obstructions.

[Dsanduril]

I assume from the 285W size that these panels are not nominal 12-14V panels with a Voc of 17-19V. If they are then two controllers makes no difference, if one panel is shaded enough to cause a bypass diode to conduct then that panel's Vmp will drop below 13 volts, making it useless for charging and so it will drop out of the equation.

But, with 285W panels I'd guess that Voc is in either the 36V or 48V range. In this case, as an example, lets say that in full sun the panels could produce 200W at 40V Vmp with the current, real world conditions being encountered on the boat. If one panel gets shaded enough to cause one bypass diode to conduct then its Vmp will drop by 1/3 (depends on the number of bypass diodes and the amount of shading but that in general the diodes divide panels of this size into thirds). That means that the panel's output capabilities, all other things remaining the same, is now 2/3 x 200 = 133W and the Vmp is around 27V . The maximum power available from the system is then 200W (the full sun panel) + 133W (partially shaded panel).

Case 1 - Single controller
The controller will start an MPPT sweep. As it rises from say 20V (pretty typical for this setup) it will see power output rise as current stays steady from both panels and voltage is below the lowest Vmp. At 27V current from the shaded panel will start to drop off very quickly (it has moved beyond the 'knee' in the I-V curve) while current will drop off very slowly in the full sun panel. At this point both panels will be at about 27V, with the full sun panel in the broad, relatively flat section of the I-V curve producing just a little more than 2/3 available power, while the shaded panel will be at the knee, also producing 2/3 power. Because there is a slight slope to the curve for the full sun panel it will be somewhere between 0.7 and 0.75 Pmax, while the partial shade panel will drop off quickly from 0.67. Pmax at this point is about 1.4 (.67 + .73) of the maximum available power at 27V or 1.4 x 200 = 280W.

This is the MPPT for a single controller. If the voltage continues to rise the shaded panel will drop production rapidly, by 30V there will be none from that panel and you then get production only from the full sun panel at 200W. So, as long as the MPPT controller works properly it will select a point with production from both panels, but the full sun panel will be operating at a far suboptimal voltage.

Case 2 - Dual controllers
This is far easier to describe; the full sun controller will select 40V and 200W output, the partial shade controller will select 27V and 133W output. Total output is therefore 200+133 = 333W.

So, in this example you'd get an extra 53W (19%) or about 4A of charging current. The question then becomes "how often does this scenario occur?" How often will shading on one panel be such that Vmp drops this far? While leaving the other panel in full sun? That all depends on the installation, the conditions, the sun angles.... For the OP, with two panels side-by-side in a flat rack at the stern of a catamaran probably not too often. Then it becomes a cost and complexity question - how much are 2 20A controllers vs. 1 40A controller? Offset by the redundancy aspect.....

Not a simple question. In the real world, under real world operating conditions you may see a few percent overall increase from having two controllers, is that worth it to you? You'll probably never be able to control conditions enough to measure the difference, but for all the reasons stated I prefer multiple controllers where possible - YMMV.

Now, if only the market would provide a multi-MPPT input, single battery charge controller....

[john61ct]

The fact that two of Victron's smaller SCs are often actually cheaper than one big one is another factor to consider.

Perhaps not relevant to the OP, just thought I'd mention it, being an unusual pricing scheme.

[jbinbi]

They make a 20, 30 and 50. 2 20s are well priced vs. One 50. Not so with 2 30s, they are 2x the cost of the 20s. 285 panels in theory can possibly max out at 20amps, and if I ever upgraded the panels, with the 20s I have no room.

I guess 2 20s would work well vs. 1 50.

Appreciate all the input. Btw, with the sails down , once the sun is forward of the mast, there is a bit of uneveness.

[john61ct]

Yes your panels would get maximum value from 2 x 20's

Note you only need one dongle for custom programming both, but would only get the logging from one at a time.

The new Smart line incorporates the dongle functionality, may add $50 to the cost.

[Bleemus]

The SmartSolar ones I got were only about 28 bucks more than the BlueSolar which is a nice savings over the dongle plus the benefit of one less connection to corrode and malfunction. Get SmartSolar.

[Group9]

I have two panels wired into one controller, but if I was starting from scratch, I'd go the two controller route. I just don't feel like running another wire through a really difficult route. But, it eould have been easy to do both at the same time and the math supports it.