I just installed a solar array to charge an existing battery system. Even with full sun my charge controller is not receiving enough voltage to go into charge mode. All the system info is below.

Existing battery bank:

  • 24 volts
  • 600 amp hours

Solar panels:

  • 6 panels wired in 2 series
  • Rated power Pr SW250
  • Maximum power Pmax 250W
  • Maximum power point voltage Vmpp 30.5V
  • Maximum power point current Impp 8.27A
  • Open circuit voltage Voc 37.6V
  • Short circuit current Isc 8.81A

Charge controller:

  • Outback FLEXmax 60
  • Capable of receiving high voltage and converting it to 24+ volts for charging.

In full sun the charge controller is only receiving 18.5 volts from the panels. I would really appreciate suggestions as I can't think of a starting point for troubleshooting.

  • \$\begingroup\$ "wired in 2 series" : are the two strings in parallel by any chance? You are seeing half the quoted voltage. \$\endgroup\$
    – user16324
    Jul 17, 2014 at 19:02

2 Answers 2


As always, instead of just throwing up your hands saying "it's broken", go diagnose it by breaking the system into parts.

Start by disconnecting the panel and seeing if it's putting out the expected voltage in full sun. If so, then attach a known load, like a bank of resistors, and see if you still get the expected voltage with that load. If it doesn't check out, then obviously there something is wrong with your panel.

If that all checks out, then your controller is doing something it shouldn't, or isn't properly matched to this panel. Either it's the wrong controller, it's not proplerly configured for this panel, or it's broken.

Now what of all the things I mentioned above shouldn't have been obvious?


As Olin mentions, test each part of the system separately to confirm things are working as expected.

Assuming nothing is faulty, taking a quick look at the manual, it says that the input voltage must be at least 2V greater than the battery voltage for the Flexmax to come out of sleep mode. If the PV voltage is dropping too far, then I would try either setting the maximum current to a lower value (I believe it mentions this in the manual also, from 5A-80A IIRC) or increasing the number of PV panels.


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