PV Size vs Battery Bank Size

[Chris.Z]

Good morning, afternoon or whatever it is when you're reading this! 

I bought a house a couple years ago that needed to be partially gutted. I did the only logical thing, as a controls tech, and took advantage of the situation by running control cables everywhere (light switches, recessed lights, smoke detectors, doorbell, network devices, etc.) and control/monitor everything from an Automation Direct Do-More PLC. I also have a Raspberry Pi 3 running AdvancedHMI that allows me to monitor my DC bus voltage and current draw and the status of all the lights in my house. Everything currently powered by four 12V 35Ah batteries wired in parallel that are being maintained by a NOCO Genius G7200 Smart Battery Charger. The long-term goal has always been to go solar.

In reading about the Classic 150, I realized that I don't necessarily have to have a 12V PV array. That provides an obvious advantage in overcoming the voltage drop from the array to the charge controller. What are the other advantages? Should I also consider switching to a 24 or 48V battery bank? The drawback there is that most of my devices are 12V, but that's easily worked around. 

Thank you,
Chris

[ClassicCrazy]

Yes the advantage to wiring your PV at a higher voltage is that you reduce the size of the PV wire going to the Classic. The best practice is to keep the string voltage from PV  not too high above your battery voltage because then the Classic will not have to do as much work to drop the voltage and it will run cooler.

As far as changing to a higher battery voltage I think the main advantage to that is if you have a 120v inverter then it is like the Classic - the closer the battery voltage is to the inverter voltage then the inverter will run cooler - but also if you have a large inverter a higher battery voltage really cuts down on the wire size from the batteries to the inverter . My 3500 watt inverter uses 4/0 wire size from a 24v battery - that is large and more expensive wire and harder to work with too.

So if you already run all your loads on 12v I don't really see any advantage to going to a higher battery voltage except for one reason - when you put batteries in series instead of parallel you eliminate a lot of balancing issues that you can get since in series all the batteries get the same current running through them. You need to take good care of wiring 4 batteries in parallel to make sure that the current is equal in all of them. 

And if you run a higher voltage then you would need a DC to DC converter to drop back down for your 12v loads - though Samlex makes some good efficient relatively inexpensive DC to DC converters to do that .

There is a string calculator on the Midnite webpage that you can play around with various configurations of PV in series to a Classic and then you can also find voltage drop calculators online to see how the wire size works out for your setup.

Larry