"…..or am I way off? 100watts/12volts =8.33 amps x 4 =33.3 .......100watts/18volts= 5.5amps x 4= 22.2amps……"
You are way off.
You "add voltage in series" and "add amps in parallel".
So your 4x100watt 12 to18V panels get you 48 to 72 Volts at 5.5 to 8.33 Amps. Then times 2 (for your two strings in parallel) gives your solar array a max output of 72 Volts at 16.66 amps with a minimum of 0 Volts and 0 amps (like at night). 16.66 Amps is the max even though it would only be 16.66 at 48 Volts, it is still the max Amps. And the only reason to know the max amps is to size the wire from the panels to the controller.
So whatever controller you use needs to be able to handle this max Voltage output as the controllers input. So you could probably get by with a 100 Volt controller as your panel array is set up now.
On Solar controllers the listed Voltage is the max input but the Amps listed are the max output. Your Max output Amps is determined by max Watts of combined solar panel output divided by battery pack voltage. In your case 800W/48v=16.66A (a battery depleted all the way down to 40V would get you 20A). So a 20A output controller would work for you.
So it would seem that you need a 100v 20A solar controller. You must now make sure that the one you use has capability to output for 48V batteries (I have one 100V30A controller that will only charge 12V or 24V batteries, but its OK cause I have several 12V and a couple 24V battery packs).
It just so happens that Victron makes a 100V/20A@48V controller. I have a Victron 150V/35A@48V. I also have a BougeRV 150/40@48V and I am very happy with all my controllers, none of them have failed me yet.
Just realize that all the rated Watts, Volts, and Amps of any solar panels are THEORITICAL maximums. If you were to see an actual 100 Watts from a 100 Watt panel, you might be the first. I have some 440's that I have never seen more than around 375 Watts. And if the voltage drops on your panel, the amps will usually drop also because its from a lack of sun. So you will never see 8.33 Amps at12V or the full 5.5 Amps at 18V, but that's what you use to size the controller.
If the voltage from your solar panels drop below the minimum needed to charge it will simply stop charging.
A good solar controller will regulate the output voltage to whatever is best to charge the batteries connected to it, this is why usually you have to hook up the batteries to the controller first, so it can sense the voltage of the batteries it is trying to charge, but you will have to set the chemistry type and maybe the settings for your particular battery pack.
Any controller will use some of the voltage and/or amps from the solar panels for operation, so there will be some voltage drop across the controller. This is normal and at least one reason that actual voltage input needs to be more than planned voltage output. This is known as a voltage step-down converter. I don't know of any SOLAR battery charger/converters that will step up AND down. They might be out there somewhere, but I don't know of any, I have only ever seen solar step-down charger/converters. Now if you were charging a higher Voltage battery with a lower voltage battery, you could use a DC to D/C step-up converter, but those only go up, not up and down.
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