Yes, I've been thinking about that, in that the panels are rated at 12 volts but on a sunny day, they roll out an average of 18 volts. That's the dilemma. With each panel putting out 18 volts, I'll have more voltage (and less amperage) with 22amps/hr vs 33.3amps if the panels are only putting out 12 volts each. The system would then be putting out 72 volts rather than the 48 volts to charge a bank of batteries. I agree that a charging rate above 48 volts.....up to 54, according to some sources, is desirable, to charge a 48 volt bank.
The answer to the dilemma is, as you mentioned, to probably get a 150 volt controller with a 20 (or more likely 40 amp) potential output. If the panels drop down to 12 or 13 volt output, there would be an increase in amperage, right? That could bring the amperage up to 33.3 amps and the controller would have to handle it.....or am I way off? 100watts/12volts =8.33 amps x 4 =33.3 .......100watts/18volts= 5.5amps x 4= 22.2amps. Question #2 is if the controller can handle the increased incoming voltage of say 72 volts and the batteries gobble up the outgoing 22.2 amps, how can we get that extra few volts to add to the 48 volts needed by the batteries to be effectively charged? Can the controller recognize that and feed the batteries 54 volts to charge? I have a Xantrex C60 model controller I use on the wind generator but it will only accept 12 or 24 volts up to 55 volts input but can regulate up to 60 amps. It isn't suited for this and I'll need another controller to do what you suggest. Any recommendations?
" I have 8 100watt/12 volt panels ……"
I have seen some 100 Watt panels advertised as 12V, but most actually put out somewhere between 21 and 26 "open Volts" (just measured with a meter and not connected to anything). If there is no data plate on your panels that tells you the open voltage, then you need to measure it because it is important to size your controller.
For example if your cells actually put out 26 open volts, and you had 4 in series, then a 100 Volt controller wouldn't be big enough.
But here's the thing, if they only put out an actual 12V open volt, then 4 in a series would not be enough to charge a 48V battery pack, even a lead acid, you would need a minimum output of 51.8 to charge lead acid (and it would take a loooong time).
So if your 4 panel strings are capable of around 100V, then you probably gonna want a 150V 20A controller (20A because 800W/48v=16.66A).
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