Oversizing an array going into a MPPT

[hans29]

Hi,

I'm sure it's somewhere in a previous post but I couldn't find it.
Is it ok to oversize an array going into a MPPT? What happens to the extra current in this situation?
To be more specific, I have a MPPT rated 20A/100V/500W and two 335W panels. So 670W total, exceeds 500W.
Safe?

Thanks to anyone taking the time to answer.

[PNW_Steve]

Welcome Hans,

I am probably not the most qualified to answer your question but until the smart people arrive I will give it my best.

It is my understanding that many MPPT charge controllers have the ability to limit current to the battery bank and load.

Could you tell us what particular charge controller you have?

[hans29]

Hello Steve,

It is an integrated MPPT in a Mastervolt Mass Combi Ultra 24/3500-100. The two solar panels are Benq Sunforte 335W PM0096B.
Thanks for your input, I just find different answers to this one like "it's a great idea to oversize, this way your charger works longer at full capacity" all the way to "make sure the solar array doesn't exceed the specs of your charger" (not voltage, obviously you don't want to do that).
Confusing.

[inetdog]

Hans,
As a general rule, the MPPT circuit is capable of limiting the instantaneous input power it draws from the array, unlike a typical PWM controller. This means that any power above and beyond either what the CC currently needs or what it can safely handle will simply never leave the array. This will do no harm to either the array (which is perfectly happy being disconnected with no load at all) nor the CC.
That said, it is possible that some less than optimal circuit designs will be limited in how much excess power your can make available to them (possibly because of what goes on when the unit is trying to find the Maximum Power Point (MPP). If this is the case, the instructions for the CC should state clearly the maximum array power you can safely connect to it.

[hans29]

They do specify a 0-500W in the specs at the end of the user manual. But no specific warning about going over that number. So I'm inclined to think that 500W is the maximum wattage the CC can work with, and like you said, not a forbidden number to exceed when connecting an array... Sounds about right?

[inetdog]

They do specify a 0-500W in the specs at the end of the user manual. But no specific warning about going over that number. So I'm inclined to think that 500W is the maximum wattage the CC can work with, and like you said, not a forbidden number to exceed when connecting an array... Sounds about right?

Sounds fine, but I am not sure I would risk trashing my system on the basis of that reasoning alone. I would try to contact Tech Support for clarification. Or maybe someone on the forum has experience with that exact unit.
It is common to oversize an array in order to extend the number of hours in the day a CC or GTI can produce its maximum rated output.

[J.P.M.]

Welcome Hans,

I am probably not the most qualified to answer your question but until the smart people arrive I will give it my best.

You're as smart as anyone reading this. It's just that others may have more experience in the particular matter at hand.

[Mike90250]

Nearly all MPPT controllers can be slightly over paneled. How much over depends on how well their circuitry can manage throttling back the PV array, Each brand does it differently, and 20% of a 500w controller (hey, real MPPT controllers are rated in output amps not watts) or 20% of a 4kw controller will be quite different.

[hans29]

About the rating, that is something I didn't quite get : 20A and 100V are within specs on this CC and that is 1000W. Like you are saying Mike, I thought amperage/voltage was the number not to exceed, power in W just being a consequence of the first two. I'm guessing there's another limitation somewhere that makes them give the user this 500W limitation.

[inetdog]

About the rating, that is something I didn't quite get : 20A and 100V are within specs on this CC and that is 1000W. Like you are saying Mike, I thought amperage/voltage was the number not to exceed, power in W just being a consequence of the first two. I'm guessing there's another limitation somewhere that makes them give the user this 500W limitation.

I suspect that the total power limitation is related to the output current it can handle at the nominal 28V battery voltage. It states max 15A at 28.5V which is 425.7W. With a reasonable conversion efficiency that would correspond to about 500W solar PV input. It can get that 15A output from a lower current at higher voltage on the input side, and can also upconvert the DC to get 28.5V out from only 25V in. That situation corresponds to the 20A input.

[littleharbor]

Here's what one of the middle of the road controller mfg. allows into their controllers.

EP max panel power chart.jpg

[hans29]

Thanks a lot for your comments. I'm still waiting for Mastervolt to answer me about this...