MX60 eff with PV V closer to bank V?

System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
Seems the MX60 is More eff when the PV voltage is closer to the bank voltage that further is that correct?

Say a 24 volt bank with 36 V of nominal panels would be more in to the bank than 72?

Lets disregard wire size in this discussion as to say the lower voltage will get a comparatively larger wire so as to not factor in the loss!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: MX60 eff with PV V closer to bank V?

    From what solar guppy has said (designs solar inverters/chargers), the higher the DC voltage, the higher the switching losses (capacitive losses in the FETs)... And he does recommend not running any higher voltage than you need...

    But, don't ignore the P=I*V and P=(I^2)*R equations... Doubling the voltage will have the current--but with the same resistance, will 1/4 the loss due to I^R*R heating... I don't know the exact trade off in efficiencies between low FET voltage vs low current (at high voltage). I probably would not try to over think it... If you have a long wire run, running at higher voltages (to reduce I^2*R losses) is probably the side I would error on.

    And with a MX 60 being current limited to 60 (or 70) amps... Raising the battery bank voltage from 12, to 24, to 48 will 2x or 4x the wattage that one controller can manage when charging your batteries.

    Lastly check Min and Max voltages, you do have to make sure that your Vmp at maximum temperature is still ~2 volts more than your battery bank's maximum charging voltage (max temperature and equalization) (worst case condition, max temp and minimum panel voltage).

    And, you do have the other issue, maximum open circuit voltage (when the panels are cold, and the battery bank is full)--cannot exceed the MX 60's maximum voltages (operating, never exceed).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
    Re: MX60 eff with PV V closer to bank V?
    BB. wrote: »
    Lastly check Min and Max voltages, you do have to make sure that your Vmp at maximum temperature is still ~2 volts more than your battery bank's maximum charging voltage (max temperature and equalization) (worst case condition, max temp and minimum panel voltage).
    -Bill

    Will the MX60 not boost PV voltage to complete a charge ? I would not expect it to deal with 18V panels, charging a 24V battery, but if it was a hot day, and you had "25V" panels, and 24V battery, it can't boost a couple of volts ?
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #4
    Re: MX60 eff with PV V closer to bank V?

    Nope, All Mppt contollers on the market are buck converters with about a minimum of 1 to 2 volt difference on Vin to Vout ... Typically have 97-99% maximum duty cycle ... stuff no one specs in the product data sheets.

    boB's "bun in the oven" has a goal of buck or boost ... which presents some interesting design challenges :roll:
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #5
    Re: MX60 eff with PV V closer to bank V?
    Seems the MX60 is More eff when the PV voltage is closer to the bank voltage that further is that correct?

    Say a 24 volt bank with 36 V of nominal panels would be more in to the bank than 72?
    Yes, that's correct. Take a look at the efficiency charts on page 66 of the current MX60 manual.

    HTH,
    Jim/crewzer
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #6
    Re: MX60 eff with PV V closer to bank V?
    Solar Guppy wrote: »
    boB's "bun in the oven" has a goal of buck or boost ... which presents some interesting design challenges :roll:

    Not in regular old PV mode. Just buck is correct in regular old PV mode.
    I think their are buck-boost MPPT controllers out there, but no good reason to do that really. Just wire the array so it is normally higher than the battery by a few volts at least.

    It is true that dis-regarding wire loss, the "converter" is more efficient with lower input voltage. However, there are more advantages than just higher "system" efficiency by raising the input voltage.... For example, if you have partial shading of an array, say one module... THEN the MPP voltage can be below the battery voltage, so with more modules in series, you may still be above the battery voltage. Another advantage is when the modules are hot. Anytime the sun shines on them, they're going to be higher temperature than ambient.

    boB
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