Need a bit of advice.

[Usmanj]

Hi! I have a 2.Kw off rid system self installed with a classic 150. 4 strings of 3 220w panels in 72v config and 48v battery bank. the wire is 12mm in cross-sectional diameter. Now i have noticed that if I calculate watts generated by the equation "Power=Volts X Amperes" then i am losing around 200 watts during peak production hours i.e. product of V X I for PV being shown on classic is 200watts more than the product of V X I for battery bank without any connected load. Is this normal? Will connecting 2 panels in series with 6 strings make a difference? or am I losing power in the wire? Any suggestion or advice is greately appreciated. Thanks.

[Halfcrazy]

So I assume you are saying the array is rated for 2000 watts and you are seeing 1800 watts? If so this is normal as PV panels usually only produce 80-85% of the rated power they are labeled for.

Ryan

[Usmanj]

Thanks Ryan. Let me clarify. On the MNGP of my classic the status screen shows on the pv side(left) X volts and y amp. i multiplied the values to get watts being produced at that moment. At the same time on the right side the battery is showing A volts and B amps. when i multiplied these the value i got was 200 watts less than the product of X and Y basically XY=AB-200 watts. so where are these 200 watts going?

[Vic]

Hi Usmanj,

EDIT:  * Removed the first sentence -edit end *

There several things in the original post that I do not understand regarding the PV array;

Four strings of three PV is equal to 12 PV modules.  IF each of them is 220 watts STC,  then 12 X 220 = about 2640 W STC.  Perhaps you are de-rating from STC to nominal ?

AND,  regarding the wire size,  which wiring is 12 mm, and is that Diameter of the wire,  or cross-sectional area?   It DOES make a difference.  What is the size of the wiring connecting the PVs to the CC,  and what is the size of the wire connecting the CC to the battery bank?  Also, the wire size connecting individual batteries to each other?

Thanks,  Vic

[Usmanj]

Yup it is 2640 watts STC. With De-rating it comes down to somewhere around 1800 watts. 12 mm is 0 AWG with 8.25 mm diameter. i have used the same wire for everything. the distance b/w the array and cc is 40ft. My problem is even when there is no load the multiplier of voltage and amps for pv is more than the multiplier voltage and amps for battery bank. during peek hours there is a difference of around 200 watts. I want to know will increasing the wire size solve the problem? or reconfiguring the array in 48 volts configuration i.e 2 panels in series will give me some benefit in case of energy being stored in battery bank? i am about to double the pv array to 5280 watts STC with 24 panels. will the same wire work?

[Vic]

Hi Usmanj,

Thanks for the reply.

First,  I do not know just where those 200 W might be going.  Recall that at one time boB mentioned that the output current reading on the Classics is more accurate than that of the input.  I do not know.  Although,  a 10-ish percent different between In and Out is quite a lot.

There is not enough information on the specs of the PV modules that you are using,  but have guessed that the Vmp spec is about 24 V  (the 72 V config/3 that you mentioned earlier).  If that is the case,  then that is too low voltage to use with a 48 V battery bank,  even for SLAs IMHO.

Thanks for clarification of the wire size,  but,  still,  are you using this 12 mm wire for the run from the combiner box to the CC?  Assume that the CC is close to the PVs but 12 mm wire is huge.  If I read it correctly,  the distance between the CC and the PVs and possibly the Combiner is only 40 feet. ( I do not know the meaning of b/w reference).

I believe that if the wire run from the PVs to the CC is 12 mm,  that increasing this wire size will essentially make no difference in the power produced by your system.

And since you mentioned that you used 12 mm wire for everything,  guess that it is used to connect individual batteries,  for the cables to the Inverter from the batts,  and from the CC battery disconnect/breaker to the battery terminals (?).

Could you please give specs on the PV modules that you are using,  your batteries,  Inverter and approximate location (while you are at it).  If you placed this info in a Signature,  you would need do it only once,  and could help others respond to posts in a more meaningful manner.

As Halfcrazy mentioned,  have you checked the battery terminal voltage at the battery bank?  There is a voltage Calibration adjustment for the Classic,  if it needs it,  but one should have an accurate DMM/DVM to know weather the CC already has good calibration.

Normally,  one would advise to check ALL connections to make certain that all are torqued to spec,   if there is any question about system losses.

OK more later and thanks for any additional data on your system components.  Vic

[Usmanj]

Thanks Vic
Specs for the PV modules are:

Pmax        220W
Vmax        29V
Impp        7.60A
Isc            8.15A
Voc           36.8A

I am attaching 2 pics showing the status screen showing In and Volts, amps, and watts being generated at that specific moment. On the in side V=70.9V, A=26.6A and the power should be 1885.94Watts. but it is showing 1634 watts. here lies my confusion. If i look at the out values the power comes out to be 1629 watts(54.1v x 30.1A).

[Vic]

OK Usmanj,

Thanks for the info on the PVs,  and for the signature line.

Just now,   am at a location which has no Classic installed.  Seems that the two status screens are on a different page,  as Kwh is on one and Voc on the other ...   dunno about this.

BUT back to several of the previous questions:

1.  Have you measured the battery voltage at the battery terminals.  Hos closely does this agree with the Classic's Batt voltage reading at that time? ??

2.  What Brand,  Model number and Technology (ie,  FLA, AGM,  Gel etc) are your batteries? ??

3.  You mentioned using the 12 mm cable for ALL connections.   Does this include the cable connection twix individual batteries,  and to/from the inverter  and CC?  ??

4.  Roughly,  where are you located? ??

Furthermore,   you probably should look at the voltage readings at the CC terminal block for Vin and Vbatt,  using the negative terminals on the CC as the return for the voltmeter.  How well does this agree with the CC's readings at that time ?

What is the cable  length between the CC  and the batteries,  and between the batteries and the inverter?

You had asked about using strings of two of those PVs you have,  but,  at 29 Vmp volts,  and a 48 V battery bank,  it seems too low a voltage for the CC to operate well,  and with Hot PVs it is probably a no go.  The Classic (and most other MPPT CCs)  are principally Buck Converters,  which reduce the input voltage to the required battery voltage,  and CCs also need a bit of voltage head room for proper operation.

It may not seem obvious,  but a number of the questions above are being asked because the battery technology affects the required voltage that the CC needs to supply,  your location affects the temperatures that the PV modules and to some extent,  the batteries are likely to experience,  and so on.  Information is a large help in envisioning your system.
Thanks!   Vic

[zoneblue]

The classic  is really at its heart a big buck converter,right?. So it will never be 100% efficient.

The way i monitor my classic is like so:

Device:   Classic 150, PCB Rev 4, (Firmware: 2012-8-13)
Date:       2013-07-01  04:04pm
State:   Active - Float
Lifetime:   191.3 kWh
Today:   2.5 kWh - Float: 3.7 hrs

Battery Temp: 13.5 DegC
Controller Temp: 39.6 DegC
Power:  89.0W
In:    67.8V 1.1A
Out:    25.5V 3.5A
Efficiency: 119.7%

See that last figure thats the difference in watts in and watts out. In bulk it sits around 96%, but at various times it will drop as low as 50% as the controller resweeps and trys to reduce power. I confess it doesnt go above 100% much, thats magic, or silly, one of the two.

Confused about your cable.

[Usmanj]

Thanks Vic and Zoneblue for the input.
Sorry couldn't reply earlier, was busy with work. First i need to make a correction. I rechecked the wire's cross-sectional diameter. Its 5mm i.e AWG#4 copper. Sorry for the confusion about the wire size.
1. Checked the batteries, Same voltage on the meter and classic.
2. Batteries are AGM, Rocket ES-12 Korea 200AH, only ones i could get my hands on.
3. Same cable used for everything, from array to cc, from cc to batteries, b/w individual batteries and from batteries to inverter.
4. i am located outside USA, in south east Asia.
P.S.
    Can anyone guide me about sizing the wire gauge. I am a bit confused. The Vmax for my array is 87 amps at the moment. I have started making the Frame to hold the next 12 panels and am planning to shift the whole array to 4 panels in series with 6 strings total, Vmax of 116V. But almost all the wire size calculators I have found are not taking the voltage of my array in account, all they ask is nominal battery voltage and max amps. Is that correct or should I calculate using the Vmax or Vmpp of the array? Hoping to decrease the losses by increasing the array voltage. Thanks.

[Vic]

Hi Usmanj,

Thanks for the reply and added info.

First,  I could not find the Classic model that you have  Is it  a Classic 150 ?  Please provide that.

If you are using a Classic 200 or 250,  the proposed string voltage for four PVs in series will probably be OK.  If you are using a Classic 150,  this voltage is TOO HIGH in my opinion.  The real test of this is to use the MidNite String sizing tool.  The coldest low temperature for your location will provide the necessary variable,  plus the string configuration,  etc.

In looking at your present string of PVs,  I would expect that your maximum charge current would be abound 40 Amps or so.     Strings of three PVs would mean that the maximum input current to the Classic would be about 18 - 20 Amps.

You can use this approximate value in the Voltage Drop calculator.  You are normally looking for  1 -  2% voltage drop.  Since you plan to double your PV array,  you would want to double the approximate input current to the CC for the required value.  Believe that your PV string Combiner is located near the PVs.

Each PV string will require a Fuse or Circuit breaker that is rated for DC,  in the combiner.  And you will probably want a circuit breaker near the Classic,  as well as a breaker on the output of the Classic.  The main work that these two breakers perform is to protect the wire from getting too hot,  and possibly causing a fire.

In your country,  there will be different requirements for wire and the maximum current that this wire can carry than in the US.  The current rating varies depending on the temperature rating of these wires.

And one question about the battery AH Capacity.  For your battery to have 1600 AH Capacity,  using 200 AH individual 12 V batteries,  then you would have 8 strings of four batteries in series.   This seems unlikely.    So do you have two strings of four batteries?   When batteries are added in series for a higher voltage,   the AH of the string does not increase along with the voltage,  just the voltage increases.  The total amount of power the string can supply to a load DOES increase by the amount of the voltage.

More later,   back to work.   Thanks,    Vic

[Usmanj]

Thanks Vic for clearing so many things.
You are right about the batteries. I have 2 strings of 4 batteries i.e 400amps in 48V config.  I have a classic 150 and according to the sizing tool Max Voc would be Marginal at 19'C, the temp at which HyperVoc will be entered. max temp the panels would face is 48-50'C and minimum temp is 0 to -1'C. So should I go for 4 panels in series or not? You are right about the in and out current, the max in is under 20 amps mostly and max charging current is seldom above 30 amps, haven't seen 40 amps yet. because its summer, the usual voltage for PV array varies b/w 69-80 volts depending on the environmental conditions. So what voltage value should i put in the voltage drop calculator? the Vmax (87V), the average voltage (75V) or the battery voltage(48V)? For 100 feet distance the voltage drop is 3.02 volts for 4awg copper.     

[Vic]

Hi Usmanj,

Thanks for the added info.

First,  you do NOT  strings of four or more of those PV modules.  For a Classic 150,  strings of three is just right.  This gives your system a string Vmp of about 87 volts.  Four in series would  make the string voltage about 116 V,  but the Voc of around 150 Volts in normal weather,  and when it is cold,  this string Voc for four PVs would easily exceed the maximum input voltage rating of the Classic 150,  and could damage it.

But your array/s of 12 PVs will work well as four strings of three PVs --  This is what one system here uses,  and works well.

On which voltage to voltage to use for the PV wiring voltage drop percentage  calculation,  I would just use the PV  Vmp voltage multiplied by the number of PVs per string -- 3 --  which is that 87 volts nominal.

With the PV modules that your are adding,  your system will be at or perhaps exceed the maximum current rating of a single Classic.  This could cause some loss of power at times,  and could cause some additional heating of the Classic.   At some point,  you might want to consider adding a second Classic to share the current.   And,  thinking of this possibility,  you may want divide the 24 PVs into two separate arrays,   use a Combiner that allows two individual PV arrays,  and run two independent cable runs from the combiner to the CC/s.   As you probably know,  if you do choose to use two Classics,  each one will need its own independent PV array.

These are just my opinions.  I did go through a similar process when configuring a similar system here,  and chose to use two Classics.   Thanks,  and good luck with the new system.    Thanks,   Vic