24V array to charge a 48V battery bank. Possible or not?

BajaSunBajaSun Solar Expert Posts: 72 ✭✭✭✭
I always thought a flexmax 60 would take whatever you put into it and convert it to what you needed out of it? Not the case? This used sytem which supposedly worked cwme with five (custom rack) conergy 175W panels, flexmax60 and a FX3048 inverter. First installer shook his head and only used three panels in series and was worried about that because of the high voltage and left the other two panels out of the array. Needless to say having a hard time keeping the bank charged so rewired the three panels in parallel (hoping to add the other two panels) get about 40V but controller stays sleeping...
What to do???

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 24V array to charge a 48V battery bank. Possible or not?

    Welcome to the forum.

    An FM60 can take any input within its input parameters and use it to charge any standard system Voltage providing that too is within range.

    For a 48 Volt system you need at least 70 Vmp array to charge it. The Conergy 175 panels are probably 35 Vmp so you would need two in series minimum. Then you run in to the other problem: Voc. The Voc on them is probably 44 or so, thus three in series is like 132. Add a spot of cold weather and the Voc will exceed the FM60's input maximum.

    There is no way to make five of these panels work under the circumstances (they'd have to be "12 Volt" panels all in series to work that way).

    An MPPT controller can reduce Voltage, but it can not increase it. If the array Vmp isn't at least high enough for charging the system it won't do anything. Probably the best you could do with what you've got it two parallel strings of two of the panels in series. This would leave out only one and give you 700 Watts to work with. It would only manage about 11 Amps @ 48 Volts, which isn't very much charging power at all.

    If you can get the exact specifications for those panels we can work up some options for adding more PV to increase your array size as needed.
  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: 24V array to charge a 48V battery bank. Possible or not?

    MPPT solar charge controllers are a specialized form of "switching" power supplies. And there are three major classes--Buck (voltage dropping), Boost (voltage raising), and Buck-Boost (two in series, can do both dropping and increasing of voltage).

    Basically, a Buck switch mode converter is the cheapest/most stable/most efficient form of converter. And it is used in most (all?) type MPPT charge controllers. They simply do not have the ability to raise the voltage from the solar array and charge the battery bank.

    So, back to your needs... You have a 48 volt battery bank. With a solar array, the typical input voltage range for Outback would be around:
    • 70<Vmp-array<~110 VDC

    The 70 volt minimum (~Vmp-array = 17.5 volts minimum for a 12 volt battery bank) is based on the fact that solar panels are not ideal current sources... The Vmp of the solar panels varies with temperature... The higher the operating temperature of the solar panel, the lower its output Vmp (voltage maximum power point) is...

    When all the numbers are crunched (and the fact that cooler batteries require higher charging voltages, equalizing batteries requires higher charging voltage, add in voltage drop for wiring and across the charge controller), Vmp-array needs to be > ~70 volts minimum.

    The maximum voltage for an Outback charge controller is ~140 VDC (operational?) and ~150 VDC maximum (as always, check the manual for exact numbers--I do not work for Outback) before damage/voiding the warranty. This is the Voc (voltage open circuit of the array in very cold weather (Voc rises with cold solar panels) with no current flowing--such as a fully charge battery bank)... Again, working the numbers, this typically works out to a Vmp-array maximum of around 100-110 VDC or so (depends on how cold it gets in your location).

    Most charge controller manufactures have a web program that lets you calculate the Vmp/Voc range for your array. Here is Outback's string sizing too.

    So--Since you have custom solar panels, you need to figure out their Voc/Vmp/Imp/Isc ratings at standard temperature and solar conditions. And plug them into the calculator to figure out your 3/4/5/6 etc. series string configuration for your array.

    Note, the typical "optimum" Vmp-array is around 2x the battery voltage--But do not worry about that--Just size the array to give you a workable Vmp-array that does is in the operational range of the controller--And uses as many panels as you have (you may run 3-5 panels in series--or can only run 4 in series maximum and the 5th will be unused).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BajaSunBajaSun Solar Expert Posts: 72 ✭✭✭✭
    Re: 24V array to charge a 48V battery bank. Possible or not?

    Thanx guys.
    When I checked the panel specs and found the Vmpp to be 35.2V I decided NOT to hook all five panels up in series. Even though down here at Latitude 22 we're hot most of the time I just felt those parameters were too close to the OB specs. At one time though, I do believe another person had them hooked up this way!
    I decide on two sets of two panels panels in series and then paralled to the CC. I have already on another panel to series with my remaining one and should then Benin pretty good shape.

    Thanx again.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 24V array to charge a 48V battery bank. Possible or not?
    BajaSun wrote: »
    Thanx guys.
    When I checked the panel specs and found the Vmpp to be 35.2V I decided NOT to hook all five panels up in series. Even though down here at Latitude 22 we're hot most of the time I just felt those parameters were too close to the OB specs. At one time though, I do believe another person had them hooked up this way!
    I decide on two sets of two panels panels in series and then paralled to the CC. I have already on another panel to series with my remaining one and should then Benin pretty good shape.

    Thanx again.

    It's even worse than you think: 5 * 35.2 = 176, which is instant blow-out of the FM. But the number that really matters is the Voc, which is going to be about 44 on those panels: 5 * 44 = 220. That would be too much for anything but a MidNite Classic 250 or Xantrex XW 600 MPPT 80. And that's without any amplifying effect of cold temps!

    The 2 by 2 configuration is your best (only) choice at this point. If you get another of the same panels you can do three parallel strings of two, but each string must then have a fuse or breaker to protect against the other two back-feeding too much current to it should something go wrong.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 24V array to charge a 48V battery bank. Possible or not?

    i can guarantee that if somebody else did that that they failed to tell you it blew out their controller as coot is 100% right and you are lucky to not have listened to that other person said, if that is what they said. it is the voc total in a string you need to look at for max voltage to the controller. hot temps just won't make these voltages higher than they are listed for in the specs at 25 degrees c or 77 degrees f.
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