outback mx60 question

Re: outback mx60 question

Cam,

Your array’s STC specs for four modules wired in series are 25.7 Vmp x 4 = 102.8 Vmp x 4.8 A Imp. In real world conditions, you should see ~85 V x 4.8 A (~400 W) around local solar noon on a sunny, clear, and cool day with the controller operating in MPPT (Bulk) mode. Array voltage will be a bit higher in the winter, and somewhat lower in the summer.

Your array’s STC Voc spec is 32 V x 4 = 128 V. While that’s well below the MX60 absolute input voltage limit of 150 V, it’s not all that far below the operational limit of ~141 V. In fact, with but a 10% cushion above the array’s STC spec of 128 Voc, exposing the array to morning temperature below 32 F might cause the MX60 to briefly suspend operation.

Thin wire will reduce array voltage at the controller, but not array current. 12 AWG wire is not uncommon for small PV arrays, especially when the array voltage is high and the array current is low.

Low array current is usually the result of haze, air pollution, or high humidity; poorly aligned modules (wrong tilt angle and/or azimuth angle); module shading (tree branches, overhead electrical wires, etc.); or dirty modules (bird souvenirs, leaves, dirt and grime, etc.).

I’d suggest confirming that your array is well aligned, shade-free, and clean. Good alignment for this time of year is a tilt angle (up from horizontal) equal to local latitude, and facing south.

Note that the MX60’s input current display is a calculated value, and not a measured value. My recollection is that’s its accuracy at low values can be suspect.

As you know, The MX60 controller’s active MPPT function does not operate when the controller’s output current is < 5 A. This info is contained in the manual. Instead, the MX60 sets the array voltage at the array Voc times the % setting in the "U-Pick” menu.

The OutBack factory default "U-Pick" setting is 77%. I found early on that I could increase bulk stage “< 5A” output current by increasing the “U-Pick” % setting to something in the 85% range. You might want to go into your MX60, check the setting, and experiment with different values.

Another factor in your system (high array voltage, relatively low power) is that the controller may be operating at less than 90% efficiency. Check page 66 of the MX’s latest manual.

In summary, check your array for alignment, shading, and dirt/debris, and try fiddling with the “U-Pick” % setting to optimize the array’s voltage vs. current intersection and increase output current.

HTH,
Jim / crewzer

Re: outback mx60 question

Cam,

The 5.5 A Isc reading is encouraging, as is the report about your clean array’s good alignment and lack of shading.

Adding a second string of modules should result in different behavior and performance from the controller. It will spend much more of its time in MPPT (bulk) mode operating at >5 A output current, which will allow it to operate in its semi-active MPPT mode. Operating efficiency should improve as well.

It’s worth noting that PV array’s rarely deliver their STC rated power. Among other parameters, STC specs are based on PV module cell temperature of 25 C (77 F), which translates into an ambient temperature of ~ -10 C, or ~ 14 F. Since PV module voltage drops as cell temperature increases, modules operating in “normal” ambient temperatures deliver significantly lower voltage. A typical voltage coefficient of temperature is in the range between -0.4% / C and -0.5% / C.

Using the Evergreen 180 W module as a typical and convenient example, here are its STC specs:

Power: 180W
Voc: 32.6 V
Vmp: 25.9 V
Isc: 7.78 A
Imp: 6.95 A

And, here are its “Normal Operating Cell Temperature” (NOCT) Conditions (800 w/m^2, 20 C ambient) specs:

Power: 129 W (-28.3 %)
Voc: 29.8 V (-8.6 %)
Vmp: 23.3 V (-10 %)
Isc: 6.20 (-20.3 %)
Imp: 5.53 (-20.4 %)

The voltage reduction is primarily due to the higher ambient temperature. The current reduction (~ -20%) is primarily due to the reduced insolation (also -20%). So, as your ambient temperatures increase this spring and summer, expect for array voltage to suffer accordingly. This is normal behavior, and it’s not an indication of a problem with your charge controller, regardless of manufacturer or model.

Good luck with your experiments and tweaking, and I hope you’ll post additional observations and results. Additional entertaining reading and technical resources are available over at OutBack’s forum. See: www.outbackpower.com/forum

HTH,
Jim / crewzer