Maybe dumb question on MPPT controller and panels

fastline

I am just getting myself to speed with the current technology.  Yet again, I cannot really find specifics regarding the electronics.  Seems there are MPPT and PWM, as they are advertised.  I guess I would like to confirm my vague understanding as of now.  MPPT uses a DC-DC buck/boost converter to boost voltage from PV panels to get above battery voltage so the batteries will take charge?  It appears the MPPT can still modulate or "clamp" current so they are likely using PWM after the boost converter?  

If that is the case, that is actually very exciting because I had this thought 10yrs ago that PV panels always roll back on voltage, but that value is critical to charging batteries.  Technically you can modulate the current just by modulating the voltage set point above battery voltage, but I am sure testing has shown what needs done.  

What I am noticing is the open circuit and loaded voltage of panels would otherwise be very difficult to use.  I notice some 48V panels with about 47V open circuit voltage, and 38V loaded.  So the MPPT will boost 38V to my target set point?  I am guessing that is the case since running two panels in series will yield voltages that don't seem typical in inverters or chargers.  

Estragon

Almost all mppt are buck converters, designed for use with panels in series strings.  47 Voc is on the high side. 

You need enough voltage in the string so the controller sees ~130% of charging voltage at max power, so ~75v for a 48v nominal charging at ~58v.  150v is a common limit for max Voc.  47Voc panels might be a bit too close in strings of 3 in a cold climate, so may look at something like a Midnite classic200v

My 48v array uses strings of 3 in series for ~100Voc typically, maybe 120v really cold.

There used to be a very few (maybe genasun?) boost controllers, but they're rare.

Once the bank gets to absorb voltage, an mppt works a lot like a pwm to hold voltage while current drops off as the bank fills.

fastline

Gotcha!  So it sounds like panels are series wired to ensure voltage would always be well above charge voltage, and buck converters work to drop voltage into the target charge voltage range?  

Is the max voltage threshold set by the max input voltage off the charge controller?  Seeing there is a large difference between open and loaded cell voltage, I am not sure which is being considered?  

I did find a good presentation by Texas Instruments which helps me better understand MPPT as a closed loop system that would constantly monitor relations between current and voltage and constantly update parameters to maintain maximum power.  I would assume it may not matter how panels are connected as it would simply look at the V/I relationship and go with a a pulsed output that would mimic series resistance that keeps the panels pushing max power.  

Am I close on this at all? 

I do like the idea of running a higher array voltage to reduce wire size and make moving the power to an indoor charge controller more financially feasible.  

I do see where one of the Outback controllers has a max input voltage of 290VDC.  I do like that.  One small concern I have, and probably will want to confirm is that protection provisions are made to ensure internal failures would not result in full input voltage could never be applied to the batteries?  I would assume they are using MOSFET technology for the output, in which those are notorious for latching upon failure.  I know smart guys designed them, but want to learn about failure modes.  

Estragon

The controller max voltage is Voc at record low temp for your location (lower temp = higher voltage).  Some will tolerate a somewhat higher voltage in a non-operating mode (eg Classic 150v will go to "hypervoc" from 150-198v with a 48v nominal bank, there are also 200 and 250v versions).  Higher voltage controllers generally cost more so unless you really need to go a long distance, it's generally better to spend on wire.  There are also small efficiency losses bucking from higher voltages.

Vmp is used to ensure there's enough voltage for the controller to work properly when panels are hot.

Midnite and others have string calculators which can be used to look at different wiring schemes for specific panels.

I haven't heard of a lot of failures in which a controller lets string voltage boil batteries.  They do generally have high battery shutdowns.

There are a variety of mppt algorithms (sweep timing, etc.), but yeah, the general idea is to keep max I/V power point under changing light/heat conditions.

littleharbor2

There have been some Midnite Classic 250 controllers for sale lately for a crazy low price, 240.00 - 300.00. They were a special order that don't have all the functions of a regular classic. Look them up on eBay. 
 The Classic 250 has a hyper Voc. limit of 298 volts on a 48 volt system.

Photowhit

littleharbor2 said:

There have been some Midnite Classic 250 controllers for sale lately for a crazy low price, 240.00 - 300.00. They were a special order that don't have all the functions of a regular classic. Look them up on eBay. 
 The Classic 250 has a hyper Voc. limit of 298 volts on a 48 volt system.

Through eBay  they  come  without   warranty,  you  might  look   at buying them through Midnite. If they are less than  5 years old they may still have some  warranty.  They may have sold them all off as a group and someone is deep discounting  them, but no update to the forum listing at Midnite's solar forum;

http://midniteftp.com/forum/index.php?topic=4606.0

petertearai

There is a warning on midnight's  forum .

petertearai

Sorry on midnights home page .

BB.

Looks like some confusion/issues over who is responsible for warranty (or probably no warranty):

http://www.midnitesolar.com/

DC SOLAR AUCTION!!
It has come to our attention that there will be an auction featuring a large number of MidNite Classic 250 Charge Controllers. These, As the auction notes, are not standard Classic 250s. They were custom built for an OEM with specific design requirements. In order to reduce cost of these units, many parts and features that were non-essential to their application were removed from these controllers. Because these controllers were purchased as an OEM product and have now been transferred to a third party not associated with the OEM or MidNite Solar, they do not come with any warranty from Midnite Solar. Please email [email protected] with questions about these units.
Thank You
MidNite Solar

-Bill

Photowhit

Yes confusion, and the reason I provided the  link to Midnite's forum. That says;

Kyle at Midnite Forum said:

The Classic 250-CP is a Standard Classic that was a custom order.

They DO NOT have multiple MNGP slots so they can not be used in Follow me.
They DO NOT have AUX 1 or 2 terminals so they can't be used with a WBJR or any AUX functions.
THey do not have ARC Fault protection.
MNGP can not be programed for Voice.
None of these features can be added back to this board.


They DO HAVE an Ethernet Port so they can be used with MM2 and the Local App.
They can be updated normally using our normal Firmware via USB
They have Ground Fault Protection
BTS input jack is functioning

Most of them still are under warranty and will carry that warranty until they reach the 5 year mark.

Price on this item is $250.00 each. First come first served.

Please email [email protected]  to order.

Kyle

Well I tried to make the;

 "Most of them still are under warranty and will carry that warranty until they reach the 5 year mark. " 

...stand out by highlighting and using the head line function, but it applied to everything. Guess  I'm never going to  enjoy the software here, though this is an improvement.

I understand that Midnite has unhooked it's self from the forum, but bet they would have asked them to take this down if they didn't plan on honoring it.

I certainly don't need one, but had been thinking it'd be nice to have a 2nd MNGP, and would have a backup for cheap. Though not being able to use 'follow me' is sort of  a killer.