Panel voltages and 48V battery

stephendv
stephendv Solar Expert Posts: 1,571 ✭✭
in Off Grid Solar & Battery Systems #1
I'm looking at these panels (http://www.posharp.com/photovoltaic/solarpanel.aspx?pid=c5d108c6-7f06-40a8-91d5-d5f8990b1970) for use with a 48V battery setup and an MPPT controller, but their voltages are difficult to fit into the system.
Voc = 43.2
Vmp = 34.9

Ambient temperature around here varies between -5C and 40C.

Option 1: If I connect 2 in series, I'd have Vmp = 70V, say 1 V loss in cabling and 1 V in MPPT = 68V at the battery. At 40 degrees, the V would drop about 6%, so that's roughly 64V which would be on the boundary of the equalization voltage.

Option 2: connect 3 in series, Vmp will be fine, but Voc will be 130V. At 0 degrees C, Voc will be about 10% higher = Voc 143V which is dangerously close to the limit of the MPPT controllers.

Which is the better option?

(I'm assuming that the temperature coefficient of -0.4%/degree C is ambient temperature and not the panel temp - which would be much higher).
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Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Panel voltages and 48V battery

    sounds too much like a catch 22 and that means option 3, go with a different pv.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #3
    Re: Panel voltages and 48V battery

    I would go with option 1.

    Not a big deal on equilizaton at cold. Just wait til it warms up a bit.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #4
    Re: Panel voltages and 48V battery

    These panels are available at a very good price, so would really like to use them. The equalisation problem on the 2 in series configuration will be an issue in summer when the panels get hot.
    Is my assumption of 1V loss in the MPPT controller and about 1V loss in the wiring between PV and controller realistic? Array will be about 2.4kW at 70V and 15m from the charge controller.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Panel voltages and 48V battery

    Actually, the Voltage Change is the actual cell temperature and Ambient Temperature is only part of the equation... The other part (besides solar radiation) is wind and radiative cooling (mounting panels against roof vs mounted out in free air on a pole/rack). Ideally, the recommendation is to not mount flat and to have at least 5" behind the panels to allow for better cooling.

    For the panel you asked about, it should be around -157 mV/C and worst case assume 35C (63F) cell temperature rise over ambient (STC = 77F). (they do not list the temperature coefficient--just guessing on my part).

    So, the Vmp at 100F and 63F rise; and Voc cold at 14F:
    • -0.157 V/C * 1/1.8 C per F = -0.0872 V/F
    • 34.9 Vmp + (-0.0.0872 V/F * (100F+63F-77F)) = 27.4 volts Vmp Hot
    • 26.8 Volts * 2 panels in series = 54.8 Vmp Hot for 2 series panels
    • 43.2 Voc + (-0.0.0872 V/F * (14F -77F)) = 48.7 volts Voc Cold
    Even without any voltage/controller drop--that is barely float voltage for a 48 volt lead acid battery bank on a hot day.

    Three panels in series will work for Vmp-array Hot:
    • 3*27.4 Vmp hot = 82.2 volts Vmp Hot
    • 3*48.7 Voc cold = 146 volts Voc Cold
    Notice that Voc Cold with three panels is very near the maximum rated voltage for most high end MPPT charge controllers (typically 150 Volts never exceed).

    This is the problem with panels with Voc in the ~40-70 volt range on 48 volt battery banks... They cannot be used in many climates on 48 volt battery banks with MPPT (or PWM) solar charge controllers. They would work fine with 12 volt or 24 volt battery banks.

    I used 100F and 14F for the Hot and Cold panels plus 63F rise (assuming poor air flow). You can change the equations to your particular needs (put in a spread sheet--much easier ;)).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #6
    Re: Panel voltages and 48V battery

    Thanks all, looks like I'll have to wait for the Classic before buying these panels :cry:
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #7
    Re: Panel voltages and 48V battery
    stephendv wrote: »
    Thanks all, looks like I'll have to wait for the Classic before buying these panels :cry:

    I could not wait any longer and had to order an Apollo last week. 200 VOC 80A
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #8
    Re: Panel voltages and 48V battery

    The Apollo reseller in Spain said that they'd stopped selling the units because they had to return 50% of them with faults. And I read on another thread that Wind&sun did the same, for the same reasons. Does not inspire confidence.

    Found another outfit that do HV controllers out of Taiwan, up to 120A and 200V:
    http://www.rich-electric.com/index.php?option=com_content&view=article&id=36:sunstar-solar-charger-mppt&catid=5:solar-power&Itemid=9

    But alas, only the smaller and lower voltage units are currently available.
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #9
    Re: Panel voltages and 48V battery

    And you think that they just keep selling bad products? These guy's go to space and s--t happens. I know about what was posted here and while I respect it, I needed a solution. We"ll let ya know!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #10
    Re: Panel voltages and 48V battery

    Please do! Don't hear much about these units. They look great from the spec, especially like the built in battery monitor. How long have you had it - any issues, comments?
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #11
    Re: Panel voltages and 48V battery
    stephendv wrote: »
    Please do! Don't hear much about these units. They look great from the spec, especially like the built in battery monitor. How long have you had it - any issues, comments?

    OK Stephen, it has been running for 2 weeks now and array of Sanyo's at 52Vmppt 3 X 3. The rep said the low V. CC was the source of early firmware problems. I do not know how that squares with our boss here at W & S. This unit has a 200V rating and the rep said the only failures with the unit have been related to exceeding 200V.

    It was easy to install but I did not use the battery monitor. The owner really wanted to use the Sanyo doubles but with a Vmp of 69V x 3 it was a bad dream.
    All the Sanyo's are exceptional at high temperature performance coming close to nameplate in summer air temps in the mid 90F range.

    As with a lot of gear in this recession backlogs are long so keep that in mind. Nice piece of gear the Apollo. If you need beyond 150v+ it is available! Another tool in the box as they say!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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