Series connected Solar panels

System

I've read about series connected solar panels being used with MPPT charge controllers to bump the voltage up for the run to the controller. This being done to reduce line losses. and gain other efficiencies of operation. I also have a pretty long run from the panels to the battery bank (60 ft) I can get it down to maybe 40 feet but that is the best I can do.

Can this be done with all solar panels, specifically the Shell SM110s that I am using. I'm thinking about replacing my present C-40 Trace with an MX60. FWIW, my battery bank is 675 AH of T-105s but am looking at upgrading to 4 or 6 L-16HCs. I am looking at adding two more SM110 panels.

I'm also using a DR2412 Trace inverter

My system is currently working well but the batteries are getting long in the tooth and I expect a replacement will probably be need in the next six months to a year. I am going to be adding a Staber Washer and Suncool Evaporative cooler so my consumption baseline will be going up.

Thoughts!

Mike

crewzer

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

System2

Re: Series connected Solar panels

Final total will be 6 panels for 660 watts of collection capacity.

Mike

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

System2

Re: Series connected Solar panels

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.

Three series pairs, then paralleled to the MX 60? All the rest makes good sense and I amy reading your references with interest!! I had not looked at the AGMs much... conventional wisdom is "they are expensive". The operative question being; "What is the cost of one's time servicing a battery bank??" Thanks for the suggestion. Looks like the eight Concorde PVX 2240Ts, (A T105 side battery is a good match for my location and being sealed, only requires physical protecton form short causing accidents. The added cost on up to four PVX 2580 is not onorous.

I think adding panels first is a good place to start so I have adequate solar charging capacity, then batteries, then the MX 60.

Ran all the wiring for my C-40 remote monitor and Trimetric 2020 this weekend. I'm installing DC disconnects and DC overload devices and pre-wiring the installation in the walk in basement of my house, near the AC service panel, so I can move my inverter and battery bank out of the garage in the next few weeks.

Dismembered a derelict camper on my property and hauled half of it away this weekend. Let's not even mention the weed eating!!

Crewzer, Thanks again for your sage advice. The trick seems to be lining each of these elephants up in the right order of movement before the march order is given!!

Mike

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

System2

Re: Series connected Solar panels

Gotcha on the MX60.

AGMs are looking very interesting!

Thanks

Mike