Classic 250 Charge Current

[elbrechtel]

Hello,

I am dimensioning a 2 kW system with the following specs:
8 panels in 2 strings of 4.
Stringvoltage is 173 Volt @ 5,4A, I calculate with safety factor 1,25 = 216,5 VDC
I need to cover 250 ft before reaching the Charge controller.
My batterybank is 24V
I have a 3500VA Outback FX Inverter

My question is the following: the classic 250 has a Max Current Out limit of 62A@24V. Does this mean that I will never be able to utilize the full Wattage of my array? (2kW) Since 62A x 24V = 1488 Watt ?
Why does the classic 150 not have this limit but a much higher 94A@24V?
Am I making a mistake here? If somebody could help me, that would be greatly appreciated.

Brecht

[boB]

Brecht,

You are correct.  The reason the higher voltage CC has a lower output rating is that the controller is
less efficient.  The components have higher resistance because of their ratings, unfortunately.

However, the system efficiency that one might gain by running the input at a higher voltage
can overcome the cost you would need to expend for the same efficiency running at a lower
input voltage by using larger wire....   OR, the system efficiency will be higher because the losses
of the long PV wire will be less because of the lower input current.

[dgd]

Brecht,

With a string voltage of 173v and string of 4 panels that means each panel has a voltage of 43.25v
Is this the Open circuit voltage? and if so what is the Maximum power voltage? about 35v  to 36v?

dgd

[tecnodave]

Brecht,

This has to do with the voltage conversion ratio. You are proposing about conversion ratio of about
6 to 1. The Classic 150 is operating at about 2.5 to 1 to 3.5 to 1input volts to output volts. The MPPT converter is more efficient at the lower conversion ratios so the classic 250 are rated at a lower current because of the extra heat generated by the lower efficiency.

On the power handling of the classics you are using the wrong voltage,  a 24 volt battery on charge is 28.8 to 29.6 on charge. This is the correct number to use for that calculation.    28.8 X 62 = 1785.6. Right where Classic charts are 1786. No worries your C250 will handle that array (1488 watt) with ease.


td

[vtmaps]

My question is the following: the classic 250 has a Max Current Out limit of 62A@24V. Does this mean that I will never be able to utilize the full Wattage of my array? (2kW) Since 62A x 24V = 1488 Watt ?
Why does the classic 150 not have this limit but a much higher 94A@24V?

Welcome to the forum. 

The reason is efficiency and heat.  The more down conversion of voltage the controller has to perform, the less efficient it is.  When power is lost (due to inefficiency) it creates heat, and heat is what sets the limits. 

We are not talking about major inefficiencies... if 1500 watts is passing through the controller, even a 1% loss of efficiency is 15 watts.  15 watts is a lot... put a 15 watt bulb in a metal box the size of the classic and feel the heat.

The classic 150 cannot always handle 94 amps...  that assumes an input voltage of  90 volts.  With a 120 volt input the classic 150 can only handle 82 amps. 

The classic 250 limit drops down from 62 to 60 amps when the input voltage rises from 180 volts to 200 volts.

What I will now tell you about is the wonders of higher voltage...

If you have a 48 volt system your controller will have less down conversion of voltage to perform.  Your controller can handle 55 amps with an input voltage of 180 volts.  55 amps X 48 volts = 2640 watts.

The classic 150 can handle over 5000 watts if its input is 70 volts on a 48 volt battery.

I presume that your panels are 250 watt with Imp = 5.4.  That means Vmp= 46.3.

If you get another panel and put your panels in three strings of three panels, a Classic 200 could handle over 2000 watts.  Ideally you would want #6 copper cable to go the 250 ft, but even #8 would be OK.

Finally, to wrap up...  We have been discussing the LIMITS.  In my opinion it is not a good idea to be pushing the limits continuously.  Good conservative design does not do that.

--vtMaps

edit: wow... three other posts came in while I wrote this 

[elbrechtel]

thanks for your quick reply`s. 
The guy i am helping just gave me the exact specs for the panels, they are slightly different from the above.

Pmax<W> 280.0
Vmp<V> 35.6
Imp<A> 7.76
Voc<V> 44.1
Isc<A> 8.31
Max  Series Fuse Rating<A> 15.0

If I make 4 strings of 2 panels and do the 250 ft run like that (88.2 V), connect the panels to a classic 150, what size cable do you recommend?
Is it more economical to buy two Classic 250? And connect 1 string of 4 panels to each? Or to do the run on lower (88 V) voltage?
Making a 48V battery bank is not an option, I am stuck with the 24V.

thanks,

Brecht

[vtmaps]

Pmax<W> 280.0
Vmp<V> 35.6
Imp<A> 7.76
Voc<V> 44.1
Isc<A> 8.31
Max  Series Fuse Rating<A> 15.0

If I make 4 strings of 2 panels and do the 250 ft run like that (88.2 V), connect the panels to a classic 150, what size cable do you recommend?

First of all, when you are trying to figure out the maximum voltage of your string you use Voc and temp coefficient.  When you are trying to figure cable size you use Vmp.  Thus I see a string voltage of 71.2, not 88.2.

Are you sitting down?   You will need to use #0 awg copper cable to have a 2% voltage drop.

If you use #4 awg you will have 5.5% voltage drop, representing an actual voltage drop of 3.9 volts.  That voltage drop is only at full nameplate power of 2210 watts... rarely achieved, and then only briefly.   The power lost in the cable would be 122 watts at full nameplate power.  NOTE: the power lost in the cable goes as the square of the power produced.  Thus, at half power the loss would be only 30.5 watts.

In my opinion, a little loss of efficiency in the cable is not such a bad thing... If you're so worried about it, it might be more cost effective to just add another panel than to upgrade the cable from #4 to #0.  Many folks get into the trap of raising their string voltage to reduce cable loss.  What happens is that they shift the power loss from their cable to their controller.  Better to heat up the cable than the controller.

--vtMaps

[elbrechtel]

Good point of keeping the losses more limited to your cable and reduce the stress on the controller.
I will go for a #4AWG cable and think about a #2AWG perhaps. Is read the classic goes up to 4AWG for the connection. How would you reduce the size to 4AWG? Is there a clean looking method?

Brecht

[elbrechtel]

And thank you for pointing out that cable size is calculated with Vmp. it makes much sense, since this is when current flows.

Brecht

[vtmaps]

Good point of keeping the losses more limited to your cable and reduce the stress on the controller.
I will go for a #4AWG cable and think about a #2AWG perhaps. Is read the classic goes up to 4AWG for the connection. How would you reduce the size to 4AWG? Is there a clean looking method?

Your combiner to controller cable does not connect directly to the Classic.  It has to go through a circuit breaker.  You can use #4 from the breaker to the controller.  A few inches of #4 won't add any significant resistance. 

On the PV negative side, a split bolt connector can get you from #2 to #4.  You could also land the PV cable on the negative battery bus and run #4 from that bus to the classic.  (batt neg and PV neg are interconnected in the classic).

--vtMaps

[elbrechtel]

Thank you so much! Really appreciate your help. Couple more questions:

If i would use 2 x 250 Classic with 4 panels in series on each (1120 Watt). Can I run just one cable (8 / 6 AWG) and split it at the end? How do I connect 2 classics in parallel? Is it possible for them to communicate to each other?
Still making the trade off..

Brecht

[vtmaps]

If i would use 2 x 250 Classic with 4 panels in series on each (1120 Watt). Can I run just one cable (8 / 6 AWG) and split it at the end? How do I connect 2 classics in parallel?

No, each classic must have direct access to its own array.  No trouble sharing one battery, though.

Is it possible for them to communicate to each other?

Classics have industry leading communications.  Read about "follow me". 

--vtMaps

[tecnodave]

I read in another thread that full parallel operation is coming to the Classic's soon.

td

[TomW]

I read in another thread that full parallel operation is coming to the Classic's soon.

td

I seem to remember that myself but  for The Kid. It would be a cool feature for either.

Tom

[mahendra]

that will be cool but would it also work for classics of different voltages and well as different pv voltages?