Max input voltage for a c-40 controller

rplarryrplarry Solar Expert Posts: 203 ✭✭
I'm trying to help a Friend optimize his solar system. He has a SW4024, a C40 controller, 1920 amp hour 24v absolyte llp battery bank, and a solar array consisting of 24 Kyocera 48 watt, 2.88amp. panels in a fixed south facing roof mount. The panels voltage is listed as 16.2 optimal. The array is wired in 2x12 parallel pairs. The array wiring from the panels to the controller has some expposed connections which show signs of corrosion and needs to be cleaned and put inside of a box. Max amp shown on the c-40 is about 18 amps, which seems a little low for the amount of panels he has. I suspect some losses in the corroded connections.
The battery voltage now only comes up to a little over 26v. I'm wondering if we could wire his array in 3x8 parallel strings to take advantage of the higher voltage and lower the loss in the wiring. After the connections are cleaned of coarse. The max voltage listed for the c-40 is 125v, I'm guessing that is for 48v system so half of that for a 24v would be a max of 62.5v. if so then 3 panels in series should be no problem. Anyone have any other suggestions? I will probably suggest that he cut his battery bank size in half, and maybe if he'll spend the money, try to get him to switch to a MX 60 controller to better charge those big AGM batteries.


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Max input voltage for a c-40 controller

    yes going to a higher voltage will help greatly in addition to cleaning up the wiring. be sure the wiring he has for the length and current passing is of the proper gauge. it will be a necessity to go with an mppt controller to utilize a higher voltage as the mppt controllers are the only ones that downconvert the higher voltages. being it will be an odd 36v input with a 24v output i'd say go with the mx60. the voltage ratings on those pvs are on the low side so it is almost a necessity for that reason to go the odd higher voltage and downconvert with the mx60 if you expect to get much from the pvs.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Max input voltage for a c-40 controller

    The C40 has 150 Volt rated FETs and is spec'ed to 125 Volts BUT when the C40 is charging the array voltage is going to be at the battery voltage

    The C40, as Neil points out is a PWM controller. Basically operation is Bulk mode the input and output are connected so panel voltage = battery voltage, once the Absorb voltage is met, the unit "pwm's" to keep the batteries from going overvoltage. PWM is a very fast on/off , "on" being at battery voltage/ "off" open circuit ... all pwm controllers do this.

    I think the real problem here is you have a 50 kw battery bank, being charged by a 900 watts array ... ( 2.88a * 26V * 12 strings ) = ~900 peak and this would barely keep up over the normal losses of the battery themselfs with no loads. Throw in some real loads an your "friend" will have scrap lead pretty soon to get rid of :roll:

    Going to a Mppt controller would better harvest power as the array can be run at the vmp point ~16 volts  ( 3 volts * 2.88 * 24 ) ~200 watts more when the battery bank in theory BUT for  Typical operating conditions and with panels that put on  less voltage than standard 12V panels ( 17.4 volts is typical ) I think real world, it would be marginal improvement. For 500 dollars one would be better off getting more solar panels for this system ... regardless, C40/Mppt this system needs an appropriate sized charging source
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Max input voltage for a c-40 controller


    Both niel and Solar Guppy / Henry have made good points.

    An 1152 W (STC PV array is too small for a 24 V x 1920 Ah battery bank. The array probably needs to be ~3000 W STC.

    The C40 is a PWM controller, not an MPPT- and/or DC-DC switching model. You can wire the 2 x 12 array into 3 x 8, but the array will operate at essentially the battery voltage. The result will be even less charging current to the batteries.

    The PV modules’ low 16.2 Vmp doesn’t help. As a practical matter in your hot climate, they probably won’t ever operate at the ~28.8 V required (two modules in series) to complete the bulk charging stage.

    An MPPT controller would be a nice addition, but I agree that more PV is needed first. Perhaps a long term solution, allowing for local temperatures, would be an ~1,800 W array operating via an MX60, and the C40 with an optimally configured ~1,200 W array.

    Jim / crewzer
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Max input voltage for a c-40 controller

    wow thanks for catching that guys as i went through it too quickly and assumed that the charge was properly calculated in the first place. that's what i get for assuming. that charge rate maximally is 1.85% and is not good enough to charge your friend's batteries. i concur that at least another kilowatt of pv or 2 if he can do it, but in the meantime your friend better get something into those batteries before they are ruined, that is unless he never used them and those pvs were used as a maintenance charge.
  • rplarryrplarry Solar Expert Posts: 203 ✭✭
    Re: Max input voltage for a c-40 controller

    Actually if these batteries aren't ruined by now then they probably can't be ruined. The batteries are 87 series GNB that aren't made anymore and are probably more than 15 years old. One of my suggestions to him will be cut the battery bank in half, next clean up the wiring and finally if he'll spend the money, to get an MX 60.
    Thanks for the input,
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