Combiner boxes, am I doing the math right?

pabloesguapo

Let's say I want to add 18 new panels into my system. Let's assume they are all the same, for example I will use the Kyocera KU 265.  265watts, 31.0 Vmp, 8.55a Imp, 38.3 Voc, 9.26a Isc.  6 strings of 3 panels, into a 6 circuit combiner box.

Per string input will be:
795 watts, 92 Vmp, 8.55 amps Imp

Output will be:
4770 Watts, 92 Vmp, 51 amps Imp

And I put all that into a midnite classic 150 charge controller. 

Did I do the math right? 

Let's assume I double that, another 18 panels, another 6 circuit combiner.  etc, into a midnite classic lite in "follow me mode".  What would my numbers be going into the battery bank (48v)?

Raj174

The math is correct. Both arrays technically have the potential to provide the Classic 150s enough power to output over 80 charging amps to the battery, however it will more realistically be between 65 and 70 amps due to losses in the solar panels caused by heat.

MichaelK

Here is one detail to be concerned about.  Although the 150 is rated at 96amps at 12V, it is only rated for 83amps at 48V.  You can find this little detail on page 65 of the Classic manual (Classic and Classic SL Manual REV-L)  Now, assuming you have 4770W (on paper), and it's charging the battery at ~52V, that works out to be ~92amps (on paper).  This could potentially be a warranty issue.  In the real world, I'd predict your arrays putting out a total of about 79-80amps, which would be high, but safe.  If anything ever goes wrong, Midnight could potentially say you exceeded the documented limits stated in their manual.  I have no reason to think the Midnight people would claim this, but I'm a "Murphy's Law" kinda guy.  Just one more thing to think about.

I have the Midnight 200, and it's 48V limit is 65amps.  So, I set the current limit in my settings to 65amps.  I've seen my controller limit the current at noons on some high-output days, but nothing bad has ever happened.  For your peace of mind, I'd suggest also setting you limit to 83amps, mostly just to protect your warranty.

One other issue though is the size of your battery bank.  Most lead acid batteries prefer a charge rate no higher than C/8.  One of your systems is big enough to charge a 660AH battery bank.  With two it would be 1320AH.  How big is your bank?

softdown

You likely know this and I could be wrong. Thinking 96 amps at 12 volts is similar power to 24 amps at 48 volts. Could be related to the amount of power the copper windings can handle? 

Vic

Pablo,

Couple of other things;

MidNite has a very easy-to-use String Sizer:
http://www.midnitesolar.com/sizingTool/index.php

Try running that,  to see the advice on PV array config and size.

AND,  As we Over PV our Charge Controllers (CCs),  we tend to see the Vin to the CC rise,  due to current Limiting,  or from the transition to Absorb,  from Bulk.   If the oversize is huge,  then there will be Limiting in Bulk.  The higher input V from Limiting (results in lighter loading of the PVs) reduces the CC efficiency,  which results in increased CC heating,  particularly when the CC is delivering a lot of current.

Running Opportunity loads can reduced the Vin,  but,  still needs to be within the current capability of the CC.  With a very large PV array,  the Limiting will occur often.

You may have seen the following Classic Power Chart:
http://www.midnitesolar.com/pdfs/classicPowerChart.pdf

The Classic has very fast output current Limiting (unlike a number of other MPPT CCs),  so it seems to protect itself well.
Also,  MidNite Solar is a very customer-friendly company.   Have seen them go far out of their to way to accommodate customers,  and not nit-pick weather the customer may have screwed up something  --  great products,  great support and service  IMO.

FWIW,   Good Luck,  Vic

Photowhit

I've been using Midnite charge controllers somewhat over paneled on a 24 volt system for 8 years, It's a great company with good responsive people running it...

I think there's no problem 'over paneling the charge controller, it appears to have pretty robust safeties setup, It may throttle back the through put if it gets too warm....

As Michael said, what type of battery bank do you have? does it NEED the extra array?

Here's the power chart Vic linked to;

Every once in a while Robin or, more often @boB will stop in and help out here. I think this is pretty simple and they won't worry about it, but it's nice that they take time to stop in and say hi and straighten us out on things. They are the owners and designers at Midnight and formerly of Outback, even spent time in the dungeons of Trace/Xantrex (now Schneider).