Low PV Voltage Readings

Re: Low PV Voltage Readings

From a quick cruse of the net... The C-40 is a PWM type charge controller. When passing maximum current, the PV input will probably be within a volt or so of the battery terminals (think PWM--pulse width modulation--imagine a switch simply turning on and off quickly between the input and output--100% charging current, switch closed. 50% charging current, switch is only closed 50% of the time. 10% charging current, switch is only closed 1/10 of the time)...

For an array this large, it would make sense if you use a MPPT type controller (Outback MX-60, etc.)... Those controllers find the Maximum Power Point (Tracking) and the input voltage will most likely be much higher than the battery bus when all is working well (think like an AC transformer--but is really a DC to DC converter circuit which behaves like a transformer).

I assume that a string of 20 panels is really 4 strings series x 5 panels in parallel. Assuming your panels are:

Module: Kyocera KC120-1
Max Power Voltage - Vmp 16.9 Vdc
Open Circuit Voltage - Voc 21.5 Vdc
Voltage Temp Coeff - Vtoc -0.0824 V/°C
STC Rating - Pmp 120 Wstcdc
Max Power Current - Imp 7.1 Adc
PTC Rating 105.7 Wptc

4x Voc (21.5vdc) = 86 volts at 77F cell temperature (voltage will drop on warm days)

5x Imp (7.1 amps) = 35.5 amps of current per controller...

So... If your battery voltage (state of charge) is low, the controllers should be dumping maximum current into your battery bank (PV voltage = Battery bank voltage). Once the batteries have done most of their charging, the controller(s) should begin to throttle back on their current--and you will see the RMS voltage ("average" voltage) of the array start to rise.

If the array voltage never rises (and you see current of 20-35 amps through the controllers), then either your batteries are never getting charged (too much load) or your controllers are set to the wrong charging voltage (set too high?) or not working correctly.

The last possibility is that (if you are in a hot climate and get temperatures at, for example, 95F) you do not have enough panel in series to charge your battery bank.

Using the Xantrex array calculator (for easy numbers), at 95F ambient, one would expect Vmp to be roughly 52.8 volts (assuming 4 panels in series--guessing from your Voc of 80-95 volts).

To fully charge your battery bank you should probably want the array to support 62 VDC (for equalization) or 58 VDC (normal charge), plus 1-2 volts for controller drop (60-64 VDC total from the array)...

So, on hot days (and cool batteries), it is possible that your array current will drop as it is not able to supply full current to the batteries while charging (and leaving your batteries--possibly--chronically undercharged even though, from a "wattage" perspective, you more than enough power to keep your batteries charged--again, assuming reasonable loads).

You can fine-tune the numbers (actual battery temperature range and your actual ambient and/or cell temperatures) as this may be a "close" call as to whether or not this is a "reliable/capable" system for your location.

But, from the size of this array--using an MPPT type controller would give you more power overall and always have enough voltage to fully charge your battery bank. (for example, instead of 4 panels in series x 5 parallel strings, you could use 5 panels in series with 4 parallel strings (give or take numbers of parallel strings--depending on the size of the MPPT controllers).

-Bill

Re: Low PV Voltage Readings

Well PMCall,

I took a stab at it... And according to your drawings, I was wrong--you have a 5 series x 4 parallel strings. And, if this is so, this should work perfectly fine with your batteries/charge controllers (high enough voltage to charge the battery bank in all conditions).

But, the Voc voltage does not make sense (assuming that I have everything correct)... Assuming the Kyocera KC120-1 panels and you are around San Diego--so 70-75°F days are probably pretty normal, assume 100°F cell temperature:

100°F-77°F stc / 1.8 °F/°C = 12.8 °C cell temperature rise over standard test conditions

21.5 Voc + -0.0824 V/°C * 12.8 °C = 22.55 volts (at 100°F cell temperature)

22.55 volts * 4 series panels = 90.2 volts @ 100°F
22.55 volts * 5 series panels = 112.75 volts @ 100°F
Your Measurement of Voc = 80 and 95 VDC @ ~70-80°F???

Your plans say 5 panels in series, your measurements seem to imply only 4 panels in series... So now what. Is one or more panels mis-wired (in all arrays)? Or is your volt meter out of calibration? Was the system not wired to the drawing? Is it not the KC120-1 panels that I guessed? All of these are very possible situations here.

One thing that would help--do you have a typical maximum current reading you see from each of your C-40 controllers? You will either see around 28.4 or 35.5 amps maximum from each properly working controller on a nice clear day with clean panels (temperature should not matter--assuming your batteries need charging and/or you are selling power).

Also, what is your typical battery voltage range and temperatures (charging, resting, charged). Do you measure specific gravity on occasion too?

And, with the SW4048's selling power--you are correct. They are acting like your battery charger by load dumping (aka "dump controller") your excess power to the utility mains (that not needed by the battery for charging). The C-40's are really only needed to prevent the batteries from overcharging (such as when inverters are off-line or you are generating more power than they are programed to sell).

You have the system that all of us Grid Tie only guys want... Grid Tie plus full off-grid battery backed operation. Cool. :cool:

Lastly, you said that you wanted to diagnose your system. What is your question/problem/issue?

If it is the C-40 PV Bus ~= Battery Bus? If that is your question--I would believe that would be standard operation for your system. You would have to replace the C-40's with a MPPT charge controller (like the Outback MX-60 or possibly the new Xantrex MX-60) to see the PV bus voltage substantually different than the battery bus voltage (and MPPT might give you 10-20+% more power too). Or, you would have to program the C-40's to start cutting back on battery bus voltage before the SW4048 started cutting back (or simply turn of the SW4048's and see how the C-40's regulate to float, etc.).

Or is it the SW4048 misbehaving? Since you have too of them, and one behaves differently from the other with the same settings--then you should probably call Xantrex and see if they can help (it may be a faulty unit and it would be best to get it repaired under warranty--if possible).

Or is it some other issue that caused you to start looking at PV bus voltages.

Hope I am helping... I should probably take more time and shorten this post...:roll:

-Bill

PS: I am pretty sure that SW4048 have their own AC to DC battery chargers internally too... So how those are set may cause interesting things too (like charging from your AC mains at times instead of from your solar panels).