Series connected Solar panels

Re: Series connected Solar panels

Mike,

How many PV modules will you have in total once you add the "two more"?

More later...
Jim / crewzer

Re: Series connected Solar panels

Mike,

You could indeed wire your six PV modules for 24 V nominal in a 2 X 3 array, connect the array to an MX-60 and then charge a 12 V battery bank. This is more of a DC-to-DC step-down conversion function than an MPPT function, as some MPPT controllers do not have this large step-down capability.

Everything else being equal, doubling the array’s voltage and halving its current to the controller will reduce the power loss in the array-to-controller cabling by 75% (P=I2R). As you suggest, this is an important consideration for systems involving high current and/or long cabling runs. Note that a 60 foot cable run generally includes 120 feet of wire.

Your chosen PV modules are very good candidates for connection to an MPPT controller. Their fairly high STC Vmp (17.5 V) should provide some “extra” voltage that could be converted to additional charge current, even on a hot day, and their correspondingly low STC Imp (6.3 A) will help minimize power loss in the array-to-controller cabling.

Note, however, that while the MX-60 is a great controller, it’s not 100% efficient. Based on your proposed configuration, my experiments indicate that the controller would run at ~95% efficiency around mid-day (Your lower input voltage and output current should result in better conversion efficiency).

Accordingly, any combination of loss reduction in the array-to-controller wiring plus MPPT benefit would have to exceed ~5% to realize any net gain from the MX-60. My estimate is that you may see an overall net energy gain in the 5% to 10% range should you upgrade to the MX-60. Your climate will have an effect on that potential gain, as there’s less "extra voltage" available in hot climates due to the modules’ reduced power output.

You’ll have to decide whether investing in the MX-60 makes sense to you. Six of the 100 W modules will pretty much max out the C40, so, if you plan to expand again, you’ll need another controller anyway. You may find this old thread to be useful in your deliberations: http://www.wind-sun.com/smf/index.php?topic=1034.0

Since you’re also looking at upgrading your battery bank, let me recommend that you consider AGM batteries instead of flooded-cell types. Properly configured, AGM’s require much less attention and maintenance (no watering, no equalization), and their recharging efficiency (W out / W in) is much better: ~89% vs. ~78% for flooded-cell types.

HTH,
Jim / crewzer

Re: Series connected Solar panels

Mike,

Three series pairs, then paralleled to the MX 60?

Yes, that's it: Wire two modules into a series-pair (24 V nominal), build three series-pairs, and then connect the three pairs in parallel. Based on Shell SM110-12P specs, your array's STC spec will be 35.0 Vmp x 18.9 A Imp.

Note that the C40 cannot convert the nominal 24 V array to nominal 12 V to charge the batteries, so you'll have to wire the array for 12 V now (all six modules wired in parallel, or 1 x 6) and then rewire the array for 24 V when you buy the MX.

Other sources for AGM batteries in GC2 (i.e., T-105) and 8D sizes are Deka / East Penn, MK (a division of Deka) and Discover.

Have fun!
Jim / crewzer