MPPT for 12 volt solar panel

Solar2020
Solar2020 Registered Users Posts: 2 ✭
in Off Grid Solar & Battery Systems #1
I am using 7AH battery. If I connect battery  to direct 40 watt PV panel it draws 2.2 amps at 14.04 Volts (30.88 watts) from panel. If I connect battery through MPPT it draws 1.7 amps at 16.5 volts (28.05watts) from panel.
It shows MPPT is not working. Whether MPPT is not suitable for 12V (17.4 Vmp) solar panel system ?
Please guide. Thanks.
SatD.
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,118 ✭✭✭✭✭
    #2
    MPPT controllers are designed to work on higher input voltage. your input voltage likely isn't high enough. Another thing to consider is that your batteries are charged enough that you are in absorb or float mode. At that point MPPT isn't needed so a real MPPT controller will revert to PWM charging. 
     What brand and model controller do you have? Many cheap, so called MPPT, controllers are fake.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #3
    As Littleharbor2 says, plus solar panels are rated using "marketing numbers"--Running the solar cells at ~75F/25C (a few seconds under a "sun sumulator lamp"). We all know that things out in full sun get hot, including solar panels and solar cells (at least 20C or 36F hotter than ambient temperatures).

    Normally, we use around 75 to 77% as a derating number:
    • 40 Watt panel * 0.77 panel+controller derating = 30.8 Watts typical, best case, solar noon, cool/clear day, on occasion, panel pointing at sun, etc.
    I use the same derating factor for both PWM and MPPT controllers. Even though, the two controller types have very different reasons for the derating number--The "typical" warm weather/full sun derating works out to be about the same.

    The only time MPPT can gain "significantly" better harvest is, typically, in subfreezing weather (very cold panels, Vmp-array rises as cell temperature falls).

    But, in general, a well designed MPPT system will harvest similar amounts of energy as a well designed PWM system.

    The place that MPPT systems usually work better is with higher voltage solar panels (typically used for grid tied systems--I.e., Vmp~30-36+ volts are 1/2 the cost per Watt, vs "12 volt" panels with Vmp~18 volts), and on systems where the array is upwards of 10's to 100's of feet away from the battery shed. You can run a higher voltage (lower current) array with much smaller/cheaper copper wiring back to the shed/charge controller.

    Your system seems to be working correctly--But, it is possible your battery needs to be recharged--As the battery aproaches full charge, it limits the charging current naturally (hold 14.4 volts to 14.8 volts, depending on battery specifications, for 2-6 hours to full charge--longer "absorb" charging time the deeper discharged the battery was).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Solar2020
    Solar2020 Registered Users Posts: 2 ✭
    #4
    @ Littleharbor2 and BB thanks for your guidance.
    I appreciate your knowledge and helping nature. Your inputs are very much useful to me.
    I have not brought the MPPT charger but I built it as per Tim Nolan and I was testing it. During testing I found that direct battery charging with solar panel is better than through MPPT which shows me MPPT is not working. Hence I raised above query.
    Afterwards I found inductor is getting hot  hence today I used two toroidal cores instead of one and then replaced it and I found MPPT is working.
    But as per your remarks, it is found that MPPT is draws more power when battery was at  more discharged state. Afterwards when battery getting some charge then MPPT lags behind marginally to the direct charging of battery with solar panel.
    At present I used only 7AH battery. I think if I use battery of 40AH and above then MPPT will work better than direct charging with solar panel.
    Once again thanks for your valuable guidance.
    SatD.



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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #5
    You are very welcome SatD.

    I highly suggest that you measure/document your loads/power needs, then use that to spec. the Battery Bank and Solar Array to meet those loads.

    Otherwise, you do run the risk of a system that either does not support your loads/needs, or a system that costs more than it should/poorly matched components (too big of battery bank, to small of array, too large of AC inverter for system, etc.).

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