pmax vs. open circuit volts etc?

StevenB
StevenB Solar Expert Posts: 71 ✭✭✭✭
Here is the basic specs for a 60 watt Kaneka:
>>> Nominal Power 60W
>>> Open Circuit Voltage 92V
>>> Short Circuit Current 1.19A
>>> Voltage at Pmax 67V
>>> Current at Pmax 0.90A
Can someone help me understand the difference in theese volt and amp descriptors....and how each one is used in system calculations?

Thanks,
Steven

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    >>> Nominal Power 60W
    >>> Open Circuit Voltage 92V
    >>> Short Circuit Current 1.19A
    >>> Voltage at Pmax 67V
    >>> Current at Pmax 0.90A

    Power = Volts * Current

    The Maximum Power (or nominal power) is based on the optimum current/voltage drawn from the solar panel. So:

    Vmp*Imp=67 volts * 0.9 amps = 60.3 watts

    Voc (Voltage open circuit) is the maximum voltage the panel will output with no current flow at ~77F (typically standard test condition) or Voc=92 volts

    Voc needs to be known for the maximum voltage a charge controller/electronics will see (and not blow up).

    Isc (Current short circuit) is the maximum current a shorted solar panel will output; Isc=1.19 amps in this case.

    Isc needs to be defined so that wire gauge and fusing can be defined for larger systems and protection against shorts/fires.

    Note, the above numbers are temperature sensitive (Voc, Vmp rise as the panels get colder; Isc, Imp rise a tiny bit as temperature increases).

    The changes are enough that you need to know the minimum / maximum temperatures for your particular installation when designing/picking your system components.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?

    Thanks so much! This is awesome.
    The BZ Controller recommends doing a line test by taking the PV positive wire connected to the controller and touching it to the Battery positive, and view the controller read out. With the 2 panels in parallel the assumption is....if there is no power "leaking" from some other issue...............the readout should be 1.8amps?

    Can you comment on this procedure, in terms of why and what's happening?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    They are just testing your solar panels and wiring to validate that everything is OK (with the controller out of the circuit).

    You are loading the solar panels (voltage and current) to near their actual operation conditions. If done correctly, this is a very good way to just about 100% guarantee that the panels and the wiring is OK.

    To get a better idea about how solar panels work -- Take a look at their IV (current voltage) curves. Here is a nice data PDF sheet that shows the I, V, Power, and temperature effects on a solar panel.

    Solar panels, once that have a minimum level of light, are, more or less current sources (constant current output proportional to amount of sunlight hitting the panel). Very different than the typical chemical battery which (more or less) is a constant voltage source and it's current varies with load.

    In and of itself, this type of testing is not a bad thing to do...

    HOWEVER, the devil is in the details.

    If your your batteries have been freshly charged--there is probably Hydrogen gas around the tops of the batteries and inside the cells. Connecting a "hot wire" near the batteries like that will very likely cause a spark.

    You do not want to be anywhere within a 100' of a battery bank that has been "lit off" with a hydrogen explosion (not good for anyone / anything nearby).

    Also, if you have a short out at the array--bypassing all of your fuses/breakers can spot weld your cable end to the battery bank terminal--and cause a fire.

    Batteries are dangerous animals--and with many in series/parallel bank--they are actually more dangerous than working on a 120/240 VAC live circuit.

    If you do the test... Make sure that there are no sparks that can get to the battery area--and make sure that you have some sort of current limiter (fuse, breaker) inline so that a short will pop your circuit protection.

    (Standard Internet Warning--I don't know your system or skill levels. And you should always double check that you understand and agree with anything you are told to do by some nameless guy on the Internet :roll: ). I don't want to see anyone hurt or killed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?
    StevenB wrote: »
    Thanks so much! This is awesome.
    The BZ Controller recommends doing a line test by taking the PV positive wire connected to the controller and touching it to the Battery positive, and view the controller read out. With the 2 panels in parallel the assumption is....if there is no power "leaking" from some other issue...............the readout should be 1.8amps?

    Can you comment on this procedure, in terms of why and what's happening?

    WARNING,,,WARNING,,, WARNING
    Please read the threads about BZ Products on this forum!! The consensus amongst those that REALLY know this stuff,, is that they are,,, to their core, POS.

    I'm having trouble with my ability to search tonight,, but there are some eye opening comments.

    T
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    Here is one long thread:

    BZ Controller

    Actually started in November of 2007 by you Tony/Icarus. :D

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?

    Sorry Bill,, as I said,, I am having browser trouble,

    T
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    Not a problem Tony--I was just surprised when I saw that you where the one that started that whole BZ thread with a simple question (does anyone know about the BZ controllers?). ;)

    BZ is actually one of the hardest searches to make work here on the forum... They always comeback empty (at least for me--I guess the "BZ" string is too short and adding other terms just returns a ton of non-interesting results).

    I use this Google Search Term and it works very well here (and other websites that may not have good search tools):

    bz controller site:wind-sun.com

    The first part is a normal Google Search (bz controller). The second sets the Google Search to look on the wind-sun.com site only (site:wind-sun.com).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?

    I always knew I had memory problems,,,,What were we talking about?

    T

    I guess that is why I remember that there was such a thread!
  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?

    BB - thanks (and thanks to everyone I'm reading the BZ stuff next).
    I was told to expect a spark.

    I have AGM batteries ...so my understanding is that gas explosion is not possible?
    I'll double check with my Master Electrician bud who wired this, about where all the breaker/fuses are located....I know they are there.....as long as there is one somewhere in between, that's what you say is needed?

    Trying to get him over to hold my hand while I do it. He's attempting to raise my education too
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    AGM/Sealed batteries certainly have less of an explosion hazard--But a small risk is still there (especially if the batteries are overcharged).

    Better safe than sorry.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: pmax vs. open circuit volts etc?

    The Controller and Batteries are in a 6' x 8' "battery barn" ....but the charge on the batteries is never "overcharged" (50.5 - 52) .

    Keep the door open for a long time before I arc the spark? Or direct the neighbor with the great looking wife to do it?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: pmax vs. open circuit volts etc?

    Again--I don't know your installation/skill level/or how gorgeous the wife next door is (;)).

    Yes the risk is low with a properly vented battery installation (I blow air past a car battery just before I connect the leads myself).

    Myself, I would look before I leap (very sad story):
    CATSKILL, N.Y. June 29th, 2009 (AP) - Police say a New York man running to the aid of another motorist fell to his death from a bridge, apparently because he didn't know there was a large gap between lanes.

    ... around 10:45 p.m. Sunday when he noticed a vehicle overturned in the opposite lane.

    Investigators say Ryan stepped over a concrete barrier unaware there was a 17-foot wide gap between the eastbound and westbound lanes of the bridge...

    ...driver of the other vehicle suffered minor injuries.
    Be careful...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset