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Best string config Axpert MKS-II 450V MPPT

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Need some advice here. I want to use 400Watt PV panels with the Axpert MKS-II 450V MPPT inverter.

The Inverter manual states max open circuit PV voltage is 450V with max input current 18A.

The PV Panel Spec is max OC volts is 49.1V and max current is 9.62A

How many panels and how to string them for the best possible power production keeping safety and reliability in mind.

 

Thanks

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8s2p should be fine. That's 16 panels for 6400Wp. You'll probably see a little lower output due to the MPPT limiting the current input to 9A per string.

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Would suggest 2 strings of 6panels to get 240vdc +- way withing the inverters MPPT range of 120v ~ 450v 

 Total of 4800w with bit left for "play" 

Would suggest 6 panels towards sun up

And 6 sundown position for maximum on solar load time.

But the Mark11 also run well with 4 panels together for 160w with first hand exsperiance runnings pool pumps ect.

All is your load profile and Battery size dependent.

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Posted (edited)
1 hour ago, gooseberry said:

16 panels for 6400Wp

Ensure that that's not over 18 A total Imp. There as been an SCC burn up and warranty refused because the connected panels were capable of 20 A.

Edit: 6400 Wp is way more than I would recommend. 4500 W is officially allowed; 4800 W will probably work without too many problems. More than that is begging for trouble.

Edited by Coulomb

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1 hour ago, Coulomb said:

Ensure that that's not over 18 A total Imp. There as been an SCC burn up and warranty refused because the connected panels were capable of 20 A.

Edit: 6400 Wp is way more than I would recommend. 4500 W is officially allowed; 4800 W will probably work without too many problems. More than that is begging for trouble.

My thoughts exactly. So I think given the design limits and PV panel specs I would opt for 9 PV panels  in series. This will have the Input amperage at 9.6 and a max of 442 Volts OC. Still within the limits as well as solar edge effect.

10 panels in series would also be risky in solar edge effect.

I was thinking 12 panels connected as 2P 6S, but that would exceed the max Input 18A.

Rather safe than sorry.

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31 minutes ago, JustinSchoeman said:

9S is over the open circuit voltage limit...

Oops,

I thought 49,1VDC X 9 = 441,1VDC and that it is less than the OC value of 450VDC of the inverter? Is the margin too close?

Panels would not get close to the 4500 Watt PV power the inverter is capable of. I'm sure I am missing something here.

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The panel open ciruit voltage is at 25C.  It goes up by a few percent as temperature goes down, so you will end up over voltage on a cold winter morning.  Not likely to cause an issue, but always best to operate equipment only withing rated limits.

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58 minutes ago, JustinSchoeman said:

The panel open ciruit voltage is at 25C.  It goes up by a few percent as temperature goes down, so you will end up over voltage on a cold winter morning.  Not likely to cause an issue, but always best to operate equipment only withing rated limits.

OK, it makes sense.

 

Then what would be the best panel to use to get most the most power/ Seems the 400 watt option will have some sacrifices.

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There is about a 7% increase in voltage from 25°C to 0°C. So it's best to think of the practical voltage limit as 450 / 1.07 = 420V.

To get decent use from these SCCs, you'll need panels with an Imp of less than 9A, so you can use two strings and stay under 18A. Note that Imp increases with temperature, and panels easily get past 50°C, so the Isc at 25°C should be well under 9A.

The 18A design limit was presumably made when 8A was a big panel. I think you'll also find that 400W panels are awkward to work with. I saw somewhere that they were intended only for solar farms at ground height, not rooftops. 

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2 minutes ago, Coulomb said:

There is about a 7% increase in voltage from 25°C to 0°C. So it's best to think of the practical voltage limit as 450 / 1.07 = 420V.

To get decent use from these SCCs, you'll need panels with an Imp of less than 9A, so you can use two strings and stay under 18A. Note that Imp increases with temperature, and panels easily get past 50°C, so the Isc at 25°C should be well under 9A.

The 18A design limit was presumably made when 8A was a big panel. I think you'll also find that 400W panels are awkward to work with. I saw somewhere that they were intended only for solar farms at ground height, not rooftops. 

I surely will keep this in mind. Thanks, it all makes perfect sense now.

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Most decent quality MPPT Controllers can hande 150% over amprage however when it comes to Axpert type inverters I remain dubious..so with that said going a couple of amps over the limit should not damage the MPPT however the ideal is to stay within safe margins.

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10 hours ago, SteveFury said:

going a couple of amps over the limit should not damage the MPPT however...

You'd think that. And yet 2 A over the limit allegedly resulted in this:

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How does that work when the strings should be fused and the fuse rating is always recommended to be about 50% above the string max current... on my panels it says to use with a 15A fuse. What use is the fuse if the inverter does this when it goes 2A over the input limit?

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1 hour ago, gooseberry said:

What use is the fuse if the inverter does this when it goes 2A over the input limit?

The fuse is to protect panels from fire if there is a short in a panel, not protect the inverter's charge controller.

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