Solar Math Question and BZ 500 report

StevenBStevenB Solar Expert Posts: 71 ✭✭✭✭
First my question:
I have 2 Kaneka 60's in parallel...and a Pentametrix to monitor in real time.
Nominal Power 60W
Open Circuit Voltage 92V
Short Circuit Current 1.19A
Voltage at Pmax 67V
Current at Pmax 0.90A

Will the Amps observed at any given time, always produce the same corresponding Volts? ...or will varying conditions of sunlight and temperature, vary the volts produced for the same amps in different conditions?
I know Ohms law will produce a constant relation between amps/volts/watts, but is the amps to volts ratio always a constant in all conditions?
BZ500 CONTROLLER = avoid it at all costs. Forum members already told me this, but it was already installed....and the results were so erratic, and unrealistic I tested the rest of my system to make sure that's all that was going on.
Then replaced it with an Xantrex C40....terrific results immediately. Manages everything great....even in overcast conditions, it squeezes out amps. And the same unit can be used for charge, load, or diversion control.


  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: Solar Math Question and BZ 500 report

    You have the C40 which is a PWM controller (basically, an "On/OFF" type controller").

    So, as long as the PV panel voltage is >battery+~2 volts -- then temperature and other variables will not affect the total charging of the system because any "excess voltage" does not affect current output of the solar array.

    While panel voltage drops with increasing temperatures (P=I*V gives lower power on MPPT type controllers--but does not affect PWM power output)--Panel Current output, SLIGHTLY, increases with higher temperatures (probably need a lab setup to see any increase in current).

    In the end, once there is a minimum amount of sunlight on the panel (and Voc > Vbatt)--the output current is basically proportional to the amount of sunlight hitting the panel.

    On my Grid Tied system, I can see anywhere from 2,000 to 3,000 watts (3.5 kWatts of solar panels) on a "sunny" day--depending on temperature, air quality, sky conditions, season time of day, etc... It is difficult to actually look outside and guesstimate what my system's output would be.

    Add the fact that you are charging a battery bank--once the battery is >80-90% state of charge--the charge controller will start cutting back on current into the battery anyway (assuming full sun), and so the panel's maximum output may no apply at that point.

    You can read about solar charge controllers from a couple of FAQ's here for more information:

    All About Charge Controllers
    Read this page about power tracking controllers

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,359 ✭✭✭✭✭
    Re: Solar Math Question and BZ 500 report

    Volts are determined by tempeture, and a bit by loading

    Amps are determined by the brightness of the light, as it gets brighter, the panels get hotter, and loose voltage.

    With Pmax 67V, I trust you are running a 48V system ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • StevenBStevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: Solar Math Question and BZ 500 report

    Thanks Bill, Thanks Mike,
    I am running a 48 volt.
    Bill can you explain "on - off" a bit more? The C40 does have to potentiometers to set Bulk and Float separately. I consulted with Xantrex matching up my battery specs, and adjusting a bit to their suggestions.

    One of the reasons I asked about the difference in amp to volt ratios is that with the Pentametrix instrument connected from the battery barn, to my home office...I was able to observe real time the amps and watts...and observe the sun brightness. And dividing the watts by the amps, get volts....and they did raise slightly as the sun got brighter. But I was sure if it was just a margin of variance in the Pentametrix.
  • RCinFLARCinFLA Solar Expert Posts: 1,393 ✭✭✭✭
    Re: Solar Math Question and BZ 500 report

    The PWM controllers are 'ON-OFF' function in that the connection to the panel is switched on and off only.

    Remember that the panels are like illumination based current sources assuming the stack of cell totals a voltage greater then the battery voltage. The cell stack is around 0.5 vdc per cell but goes down at bit when hot and up a bit when cold. A 36 cell panel would have an operating voltage of about 18 vdc at maximum power point. This does not mean you cannot put a 12v battery across this panel. The voltage on the panel will drop to battery voltage at the current the illumination creates. If it was a 6 volt battery the voltage would drop to that level but the current would remain nearly the same because it is primarily based on illumination.

    PWM controller monitors the battery voltage during the on-off cycling of charging. The controller will stay 'ON' supplying the current the panel illumination produces until the bulk voltage on the battery is reached. Then it will start a duty cycle ON-OFF cycling to drop to the average float level setting.

    An MPPT controller has a power conversion switching power supply that adjust the load on the panels to optimize the voltage times current product (maximum power).

    A PWM controller just lets the panels get loaded down to battery voltage.

    Make sure your controller can handle the maximum Voc voltage. This usually occurs on a very cold morning just as the sun comes up. Enough illumination for the panels to start producing power but not enough time for the panels to get warmed up from the cold.

    I just looked up the BZ500 controller and it is an MPPT controller. But the maximum input voltage is spec'd at 100 vdc. You are running a bit close to this maximum for Voc.

    The Xantrex C40 has a maximum input voltage of 125 vdc so it should be safe with your panel setup.

    You're pretty weak on current generation. 1 amp may not take a battery bigger then 100 A-H to bulk voltage.
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: Solar Math Question and BZ 500 report


    RC has the discription down for PWM controllers... Nothing more than a very fast on/off light switch (of course--electronic switches).

    MPPT type controllers are typically "buck mode" power supplies... Which more work like an AC transformer--taking high voltage/low current (from the solar panels) and converting it to low voltage/high current (to the battery bank).

    MPPT can have some advantages (a "good" MPPT can give you 10-15% more power over a good PWM). Or, if your panels are a long way (10's to 100's of feet away)--you can wire your panels in series to up their voltage (and drop their current) to send power to the local battery bank (100+ volts at the solar panels, 12,24,48 volts--really 15,30,60 volts at the battery).

    For larger arrays (over 200-400 watts minimum) MPPT controllers can be more efficient (for smaller arrays, MPPT tend to use more power to run the conversion electronics--so small panels means a higher percentage of wasted power).

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