Is This Wrong?

BC_WarrenBC_Warren Registered Users Posts: 8 ✭✭
I had my solar panel stolen last week and bought and installed a new one this week.  (That part is immaterial, but it still bugs me).

The panel is rated at 100 watts.  It claims it can generate 21.5 V on an open circuit and deliver 17.1 v to the battery.  It is apparently fused at 5.8 A.

Here is what I fear may be a problem.

My battery has only about 12.9 V charge, which the voltage regulator optimistically calls 99% full.  
When I put a voltmeter to the output of the solar panel, I get 19.5 V
When I put the voltmeter on the input side of the controller, I see it is getting between 12.9 and 13.5 V.

The controller shows it is providing no further charge to the battery.  I would like to see the battery up above 14 V before it is considered approaching full.  

Advice and insights are very welcome.


  • SolraySolray Registered Users Posts: 246 ✭✭✭
    A 12 volt battery charged to 14.4 will not remain at 14.4. Are you using the data from the controller to know what voltage it reached when the charge was considered full?
  • Raj174Raj174 Solar Expert Posts: 751 ✭✭✭✭
    If there is no load on the battery, then it is considered full at 12.7 volts.


    3600W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 195AH LiFePO4, Kohler Pro 5.2E.
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    I'm assuming you unplugged the panel to take the voltage reading, and it was in full sun at the time? Also that you plugged it back in and when reading the voltage at the CC input, it was still in full sun?

    If these assumptions are correct, there are a few possibilities. A defective panel or connector seems most likely. When the old panel was stolen, could the wiring have been damaged during the theft?

    If you have or can get a load (like an automotive light bulb), you could check whether the panel voltage collapses under load at the panel output. If it looks okay at the panel output, repeat by disconnecting the CC input test and with load there. If the voltage collapses under load there, wire or connection damage is likely.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • littleharbor2littleharbor2 Solar Expert Posts: 1,518 ✭✭✭✭
    edited August 2017 #5
    A battery will clamp the voltage at slightly higher than it's own voltage. If you have a pwm charge controller it will only pass whatever amperage your panel is producing. You lose the potential power between the voltage of the battery and the Vmp. of the panel. An MPPT controller, on the other hand will take all potential power, at whatever voltage is coming in from the panels and step down the voltage while stepping up the amperage, giving nearly all the wattage available from the panel.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    More or less, solar panels are current mode devices. Full sun with panel square to sun, it will output around 5.8 amps with 0-17.1 volts on the output (a little lie, really about 15 volts with hot panel on hot day...). Above 15 volts, the output current will fall to zero (over 19-20 volts or so).

    So, the Vmp of the panel needs to be higher than the Vbatt+Vcontroller-drop+Vwiring-drop.

    With 13.x on the Vpanel input, it would appear that your controller is "turned on" and sending current to the battery bank.

    Note that the chart above is resting voltage. If the controller is charging, the battery voltage should rise to 14.75 volts (or whatever absorb voltage it is set for). And if enough sun, hold absorb voltage for 2-6 hours.

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Also, your wiring and fuse should be 1/0.85 times larger than the Imp rating of the panel:

    5.84 amps / 0.85 NEC derating = 6.9 amps (next standard fuse/breaker larger)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BC_WarrenBC_Warren Registered Users Posts: 8 ✭✭
    Thanks, folks -- that has given me a lot of leads to check out.

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