Panel volts and amps output

rgk1rgk1 Solar Expert Posts: 107 ✭✭✭
I recently finished adding 2 more panels to my system and today was the first sunny day we have had. I wanted to check everthing to make sure the new panels were working properly after the final install. The amps from the panels started out around 7amps. The voltage however was about 61 volts. When I had all 4 panels temporarily connected last week before the clouds and rain set in it was reading around 80 volts (clear, but much colder then). Thinking something was wrong, I disconnected and checked each panel. They all showed 20.x volts. Since they were not connected to anything I assume that was the Voc.? Here is where I'm sure someone can explain the "Why" or assure me all is well I hope. In bulk the controller was at one point putting out about 33 amps to the batteries. As it transitioned to absorb and the amps to the batteries began to decrease, the voltage reading on the controller (confirmed with a dmm at the solar input terminals) rose to about 73 volts. If I turned on a box fan, the output amps from the controller went up and the reading for the panel voltage went down. I guess I am mistaken to think that the voltage from the panels would peak rather quickly in good sun and the amps would go up and down over the course of the day assuming full sun all day and the voltage not vary much. Correct, incorrect?
4-Kyocera 135 watt in series, 4-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/24 volt inverter, Tristar 45 MPPT controller, Iota 24 volt/25 amp charger.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts and amps output

    No worries.
    Panels are funny things; they're current, not Voltage, sources.
    With your four panels in series and nothing connected but a meter you get the Voltage open circuit (Voc). 20 Voc per panel * 4 = 80 Voc for the string.
    Hook them up through (I presume) an MPPT charge controller and suddenly input seems to bear no resemblance to output. The controller will take any "extra" Voltage available from the panels and turn it into charge current, if the batteries need it.
    In the course of a day's charging you get:
    1). Instant rise to Voc at the panels the moment any amount of light falls on them, but limited current for charging available. At this time the batteries would theoretically be at their lowest charge state and take all the current the controller can provide.
    2). Rise in panel power output as sun intensity increases and the angle comes more into line with the panels. This is when charge current rises drastically and the batteries charge at the fastest rate. As a result the battery Voltage goes up quickly and thus the need for current decreases.
    3). The change from Bulk charge to Absorb charge. Here the controller tries to maintain a fixed Voltage on the battery terminals. It will make adjustments as it sees fit according to the state of charge, power available from the panels, and power drawn off by loads. Comparing the input and output of the controller at this point is confusing and won't tell you anything useful.
    4). Switch to Float stage once the controller determines that Absorb has finished either due to time or current or a combination thereof (depending on the controller). This is similar to Absorb in that it tries to maintain a fixed Voltage against any draw on the batteries.
    5). Once there is not enough light on the panels to maintain Float, the controller will drop back into Bulk and try to add whatever power it can get from the panels to the system until the sun is too low and the charge system shuts down for the night.

    Remember that a solar panel is capable of putting out its full current into a short circuit: essentially zero Volts. With a matched panel/battery set-up (12 Volt panel & 12 Volt battery) and a PWM type controller you will see some correlation between panel output and battery input. With a high Voltage array and MPPT controller this analog is gone.
  • rgk1rgk1 Solar Expert Posts: 107 ✭✭✭
    Re: Panel volts and amps output

    I guess I was just expecting the voltage from the panels as measured at the controller solar input terminals to remain constant and the amperage from the panels to go up and down.
    4-Kyocera 135 watt in series, 4-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/24 volt inverter, Tristar 45 MPPT controller, Iota 24 volt/25 amp charger.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts and amps output
    rgk1 wrote: »
    I guess I was just expecting the voltage from the panels as measured at the controller solar input terminals to remain constant and the amperage from the panels to go up and down.

    Nope. :D

    Panels operate within the range of these extremes: Voc @ zero Amps to zero Volts @ Isc
    The exact output depends on how intense the light hitting them is, how directly it hits them, what the panel temperature is, and what the load on them is. All of these factors vary during the charge cycle, so you don't get anything close to a linear function.

    If you put both an Ammeter and a Volt meter on both the input and the output of a charge controller you will get:
    Panel Volts * Panel Amps = Panel Watts and Controller Volts * Controller Amps = Controller Watts
    In that case, Panel Watts and Controller Watts will be close, but the Controller Watts will always be lower due to the amount of power consumed by the charge controller itself.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Panel volts and amps output
    rgk1 wrote: »
    I guess I was just expecting the voltage from the panels as measured at the controller solar input terminals to remain constant and the amperage from the panels to go up and down.

    No, the MPPT controller will find and keep rechecking for the combined voltage and current available from the panels, to find at what point, at that instant, the most power in watts can be obtained from the panels. In other words, the controller loads down the panels, thus lowering their voltage, to the point where max watts output is obtained. And as this Max Power Point is Tracked, the panel voltage will change as required, to get the highest wattage available, or, when the batteries get charges, a lower wattage, so as to supply only what the batteries and loads require.
    EDIT: Sorry Cariboocoot, guess I was still working on my post while you had already posted yours :blush:
  • rgk1rgk1 Solar Expert Posts: 107 ✭✭✭
    Re: Panel volts and amps output

    Thanks guys, that makes sense.
    4-Kyocera 135 watt in series, 4-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/24 volt inverter, Tristar 45 MPPT controller, Iota 24 volt/25 amp charger.
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