I don’t understand where your coming up with 22 to 33.3 Amps (OK I think I do, but I also think you are doing it wrong).

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It’s all about the Watts. Watts is power, Watt/hrs are storage (simplified I know).

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Look at it this way, you have 8ea 100 Watt panels, so a max total possible input of 800 Watts.

If you divide 800W by 48V, you get 16.66 Amps, and 800/72 = 11.11 Amps. But whatever Voltage your panels are putting out, the max Wattage is still 800.

So for me, I look at Watts needed for a given speed and then Watt/hrs left in my pack, to determine how far I can go.

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Another thing to think about is power loss to conversion/heat. In your example using the theoretical max of your solar panels of 800W, your going to loose a little at the controller (The Victron 100/20/48V claims 98% conversion efficiency) so 800x0.98=784. So now your only getting 784W to the battery (assuming no voltage drop from the wires, and there are ALWAYS voltage drop from wires). There is also some loss at the battery itself.

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I can tell you from experience that putting 100W into a battery for an hour does not net you a 100W/hr charge. I have Watt/hr meters at my batteries that count up from zero when using, and back down when charging. When fully charged the count will always be negative (until I zero them). As far as I can tell, my battery loss is around at least 0.5% (small but still relevant). So now you are down to about 780W.

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If you add in the fact that you will also never see the full 100W / panel (unless you are in laboratory conditions) you should always factor in some loss before hand, then later measure when your system is operating.

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You should also be careful when having 2 different batteries that you use independently. You would need to turn the discharged (used) battery off before turning on the charged (un-used) battery. If you connect two batteries with different voltages together, you will probably see some smoke, and need some new wires (unless you have the fuses and circuit breakers needed). This is just a guess of course, because I have never, ever, ever, ever, ever, done such a thing (did I protest too much?).

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And you would also need to separate the charging system from each battery.

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I have my battery banks hooked up to bus bars and use them all at the same time. The solar and shore power chargers AND ALL loads are also connected to these bus bars. The solar charge converters are always on so when the sun is shining all the solar power is being used first. If the batteries are full and the load is small, the solar controllers limit the output to just enough to run the load. If the load increases then the controllers increases the output to match until the load becomes more than the solar is providing (or a cloud passes over) then the load starts drawing from the battery, but only for the excess load above what the controllers are providing.

Then if the load decreases back below the controllers output the excess power from the controllers charge the battery.

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So in your case I would recommend using both batteries until 2/3 discharged, then fire up the generator and charge them back up (boating best practices: 1/3 on the way out, 1/3 for on the way back in and 1/3 in reserve for emergencies).

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But as always, it’s your boat, do it however makes you happy.

“…..or am I way off? 100watts/12volts =8.33 amps x 4 =33.3 .......100watts/18volts= 5.5amps x 4= 22.2amps……”

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You are way off.

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You “add voltage in series” and “add amps in parallel”.

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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.

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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.

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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.

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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.