Question about the Classic's input voltage.

[verdigo]

Is it normal for the Classic to pull the array (Classic's displayed input) voltage down to a couple of volts above the battery voltage? The picture below caused some concern over at the NAWS forum with one of the moderators there by causing me some concern.

http://i1000.photobucket.com/albums/af125/verdigo59/IMAG0469.jpg

[WizBandit]

Depends on what your VMPP on the array feeding the Classic is...
Looks to me like you might have two 60 cell PV modules feeding it, the VMP for those are around 30VDC with a VOC of 40 plus VDC each...

[Westbranch]

The Classic will find the MPP where it generates the most Watts. 

Don't see what the issue is, it's charging  with a lot of watts coming in...  unless something does not fit with the PV specs etc...

[Mtn Don]

I believe the concern about the array voltage being just above battery voltage is that if the array drops any further there will not be sufficient voltage to push the charge into the battery. When the weather is hot with sun shining brightly the panels get hot. Hot panels have reduced voltage output. If the total rating of the strings Vmp is not much higher than the battery bank voltage the system could stop charging even though the sun is shining brightly.

[Westbranch]

Don, I agree but without the PV specs it may just be the MPPT algorithm doing its thing as long as he has enough head space ( voltage)  between bat V and MPP V to run the CC and cover line loss.... 

I saw 25.4 PV Volts at less than 1 amp  on mine last week and the CC outputting 25.5 V into the battery at .1amp,  it happens a lot right around Batt V and afternoon sun. the Classics will sure squeeze it out if its there.  BTW inverter was on but no loads, other than tare...

[Mtn Don]

Yes; we have a Kid and it too is amazing at what it can squeeze out of a few early morning or late evening sun rays. Panel and environmental specs would help settle the theory.

[Vic]

Elsewhere, Dennis has said,  "The panels are in strings of two 37 volt vmp".   (74 Vmp strings.   This appears to be on a fairly modest Hybrid GT system.

We have always run relatively high String Vmps on systems here -- one at 106 V and another at 93 Vmp nominal,   all on 48 V battery banks.

BUT,  I was quite surprised to see that the 2320 Watt STC array (from memory)  could deliver that amount of power at such a low Vin,  and at that close to Vbat.

Have always tried to give the MPPT CC a bit more room to operate,  at a bit of a hit on efficiency.

All FWIW,   Vic

[Resthome]

Sure looks like these panels are getting a lot of shading on them.

[dgd]

I saw 25.4 PV Volts at less than 1 amp  on mine last week and the CC outputting 25.5 V into the battery at .1amp,   

Eric,
Althought its only 0.1volt the Classic does not have voltage boost so 25.4v making 25.5v is likely a little inaccuracy in the voltage readings. The tweaks options may need applied to get the voltages correct using a decent voltmeter (fluke).

Then again, boB said some time ago he was looking at boost so he may have quietly sneaked it in the code 

dgd

[verdigo]

Elsewhere, Dennis has said,  "The panels are in strings of two 37 volt vmp".   (74 Vmp strings.   This appears to be on a fairly modest Hybrid GT system.

We have always run relatively high String Vmps on systems here -- one at 106 V and another at 93 Vmp nominal,   all on 48 V battery banks.

BUT,  I was quite surprised to see that the 2320 Watt STC array (from memory)  could deliver that amount of power at such a low Vin,  and at that close to Vbat.

Have always tried to give the MPPT CC a bit more room to operate,  at a bit of a hit on efficiency.

All FWIW,   Vic

Viv at one point I have seen 2500 watts and you are correct "out of a 2320 watt array". I didn't and still don't have the experience to have been concerned about the low input voltage.

I thought the classic was just doing it's thing. My plan is to buy four more panels and the advice I was seeking was whether I need four more or perhaps just two more of the same panels in strings of two.

The Classic sizing tool indicated I was getting close having a hyper VOC issue in colder weather with strings of three.

I guess I will go ahead with my original plan for 4 more panels, and try them out in strings of three and see what is best.
What I gather is if I can make the same or more power in strings of two it would be more efficient than having the CC work harder down converting higher voltages from strings of three?

Thanks for the advice

[Vic]

Hi Dennis,

The system with 106 Vmp strings  was on the hairy edge using the venerable OB MX-60 CC.  Highest recorded Voc on this system is now at 141 V.   This in a very moderate CA climate.  The PVs in this system are 35.4 Vmp,  in strings of three.  This really IS a bit too high,   but wanted to play with harvesting PV DC for an electric Water Heater,  so higher V is a bit better.  Probably would not want to run any higher string Vs than these (here at least).

In theory,  strings of two of your 37 Vmp PVs should be fine,  but on hot days,  doing an EQ might not work out so well (if you use FLAs).

But you seem to be able to make good power at about Vbat + 2V,  so on most days you may not have an issue.

David,  recall that boB did say that he saw a slight Boost function on a Classic at one point,  with low output currents,  but with the Classic charging a battery,  which was showing a voltage a bit above the Vin to the Classic ...  just from memory.

OK,  Good Luck,    Vic

[tecnodave]

Dgd,

The equipment label on my Classic says:

               Midnite Solar Classic 150
               MPPT Solar Wind Hydro Buck Boost Charge Controller


I take that literally that it can buck or boost.........that is what a buck boost controller does


David

<edit>.   Ok that was a memory fart......that was not on my Classic but in the not too up to date online manual which I downloaded.   td

[dgd]

The equipment label on my Classic says:
               Midnite Solar Classic 150
               MPPT Solar Wind Hydro Buck Boost Charge Controller
I take that literally that it can buck or boost.........that is what a buck boost controller does

Thats interesting. Have you tried charging, say a 24v battery bank, with  pv input of maybe 18v mpv (single 12v nominal solar panel)  on a bright sunny day?
I'm pretty sure its a no-go but I would be very pleasantly surprised if in fact it did charge the battery bank - then the label would be spot on 

dgd

[dgd]

David,  recall that boB did say that he saw a slight Boost function on a Classic at one point,  with low output currents,  but with the Classic charging a battery,  which was showing a voltage a bit above the Vin to the Classic ...  just from memory.

Vic,
I must have missed that. A slight boost, I guess, caused by some chance arrangement of oscillating charging caps adding an ac component to the PV voltage which maybe a diode bridge later boosting rectified DC output voltage OR maybe just the caps and diodes being a voltage pump 

dgd

[boB]

Tecnodave, Where and what label was this that said the Classic was a buck-boost ?  I don't remember that but maybe
a real old one had something like that on it ?
The Classic does not boost.  At least, not in normal operation.  What you probably saw was where the input voltage
appeared to be less than battery voltage.  Not exactly sure why but I have a couple of ideas why.