MPPT and VOC limits

solardad · November 2018

Are MPPT controllers / chargers designed to run efficiently at their upper limits? Is it a good idea or ok to design a system where the array voltage is runs near the max of a controller? Anyone have issues with a similar design?

Designing a system where by the overall array should ideally be broken up into two sub arrays (shading issues), 6 panels and 6 panels that feed into dedicated MPPT chargers. The 3s2p setup would be 144v20a for each. The control that I am looking at has a max of 145 and absolute cold max of 150 (Victron 150/60).

I could jump to the next model, 250/100, which would provide over 100v of headroom but is that necessary? Expectation that a panel would go beyond the stated VOC limits?

I was hoping by choosing the smaller model I could cost effectively handle both sub arrays separately - main reason is to deal with shading issues (1~2hr at either end of the day). The150 series is roughly 1/2 the cost of the 250.

Thanks

mcgivor · November 2018

The question is, what nominal voltage is the system designed for, if charging batteries the voltage should be 50-100% above battery nominal, using higher will be slightly less efficient, generally speaking.

littleharbor2 · November 2018

Way too close for comfort. The first really chilly, clear, winter morning you may find you've ruined your controller and will have ended up spending double the money to get back to square one. And still have lower Voc. limits.
What is your system voltage? Can you wire 2s3p?

solardad · November 2018

Thanks for feedback

@mcgivor 48v s the plan

@littleharbor2 I have distance to deal with. Array will be in the backyard, roughly 150’ from the house. So trying to avoid massive cables by boosting the volts. I was planning on combining at the array which would require a 6awg to keep my vdrop under 2%. Going down in volts I guess would work also.. just move the combinderbox to the house and have a short run to the mppt... just more wires to run. I think I Iike this idea... smaller wires (but more of them) and safer headroom.

littleharbor2 · November 2018

Not sure of your location/availability, but there are other brands that do high voltage. Midnite Classic 200, Magnum PT 100, Morningstar TS-MPPT-60-600V-48 . While a bit pricy the Morningstar will take up to 600 volts, eliminating the need for combiner boxes and fusing in many cases.

solardad · November 2018

@littleharbor2 I have thought about those also. Related to my scenario I just came across an old post related to the idea that you want to limit the upper end of the voltage the MPPT has to handle for stepping it down to the system voltage, in my case planning 48v.

Not sure if this is still the case, I was unable to find any cautions from Victron, but if I run with a 96v I"ll be at 100% or 144v I'll be at 150%. Is there still a need to be wary of that % over the system voltage OR have controllers improved in performance that this is a non-issue stepping down from these high voltages? Related to @mcgivor warning earlier.

jonr · November 2018

Controller design varies, so it's not good to generalize. Specs suggest that the Victron needs only 5V above Vbat-max to work - not 50%. Generally (:-)), DC-DC converter efficiency drops with increasing voltage differential.

littleharbor2 · November 2018

You will lose a negligible amount of efficiency and so be generating a bit more heat. These controllers have been designed to operate within the voltage ranges listed. In your case with long wire runs higher voltage is warranted so that's why you would use a high Voc. controller.

You're not specifying what voltage you're talking about. When you say 96 or 144, is this the Voc. listed on the panels? You still need to factor in the temperature coefficient for anything other than 25 degrees Celsius, or 77 degrees Fahrenheit. This is why you need to be careful on those cold, clear mornings. It's not raw power (current) that kills your controller. it's high voltage, which will likely come with very little current on those early mornings when your controller first wakes up.

Estragon · November 2018

There is still a bit of a trade-off going with higher voltage controllers. Midnite has output curves on their website that you can use to get a sense of this.

With most controllers, you want to look up the lowest temperature ever recorded near your location, and make sure the array is configured so max temp adjusted output voltage is at least 15v or more under to allow some leeway for temp variance from local terrain etc. If, for example, you have a 150v controller, and input voltage hits 151v, most contollers will flip an overvoltage bit (voiding warranty), and may (or may not) be damaged. Midnite contollers have what they call "hypervoc", which as I understand it, will instead take the array offline at 150v, and not "flip the bit" until voltage reaches 150v plus bank voltage (eg 150v + 48v =198v).

IMHO, you could relax the 2% voltage drop criteria a bit in sizing wire. Panels are cheap enough vs wire these days that (within reason) it may make more sense to add a bit more pv if need be to offset a bit of wire loss.

solardad · November 2018

@littleharbor2 96 & 144 is based on the panels Voc, 48.21. 2s3p and 3s2p respectfully

@Estragon thanks for the info

jonr · November 2018

Definitely calculate and use the worst case, low temperature Voc. For example (only!), this can be 20% higher than the 25C Voc.

mike95490 · November 2018

The PT 100 is 187V operating, 200VDC death, and 100A output
http://www.magnum-dimensions.com/pt-100-mppt-charge-controller

WaterWheel · November 2018

When choosing a controller brand it's nice if it is the same brand as the inverter so they can communicate. This makes programming the controller easier. I've never used Midnight Solar but I understand it may be very programmable without communicating with the inverter control panel.

Estragon · November 2018

Yes, Midnite classic controllers can be monitored and controlled through their "LocalApp" software. With appropriate network setup, it could also be used remotely.

I've found no need to access inverter data or settings remotely, but it would be simpler to do if same brand.

MPPT and VOC limits

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