How Do I Size MPPT Charge Controller? - Fiberglass RV


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Old 07-11-2018, 06:06 PM   #1
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How Do I Size MPPT Charge Controller?

I'm looking at Victron MPPT Charge Controllers and can't figure out what size I need. I found the 1st number is maximum pv open voltage, but I don't know what the 2nd number is.

If I have 2 300w 24v solar panels connected to a 12v battery bank, which Victron do I need?

To slightly complicate things, I need to plan for an additional 2 100w (corrected) panels coming into same battery bank. Do I need a 2nd charge controller and since the batteries are lithium, would it be better to get another MPPT controller. Thanks
Link to Victron @ Battle born where they are on sale:

https://battlebornbatteries.com/prod...trollers/mppt/
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Old 07-11-2018, 09:01 PM   #2
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Originally Posted by Huck View Post
I'm looking at Victron MPPT Charge Controllers and can't figure out what size I need. I found the 1st number is maximum pv open voltage, but I don't know what the 2nd number is.

If I have 2 300w 24v solar panels connected to a 12v battery bank, which Victron do I need?

To slightly complicate things, I need to plan for an additional 2 100v panels coming into same battery bank. Do I need a 2nd charge controller and since the batteries are lithium, would it be better to get another MPPT controller. Thanks
Link to Victron @ Battle born where they are on sale:

https://battlebornbatteries.com/prod...trollers/mppt/
The sales reps at Battleborn can fix you up with the right size of controller for your battery system. Just go to the website and click on the "contact us" icon for the ways to get in touch with them. They will have instructions on hooking up the system. They will give you accurate information and answer your questions. That way you can give those of us who also want to know the "official" responses.
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Old 07-11-2018, 09:14 PM   #3
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Huck,

I'm sure it's best to call them. But.......

You wrote (2) 100 v panels will be added. Do you mean (2) 100 watt panels?

If you have a total, eventually, of 800 watts, why can't you just get the next bigger charge controller above 800 watts? Are the panels different voltages?
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Old 07-12-2018, 03:41 AM   #4
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Originally Posted by Raspy View Post
Huck,

I'm sure it's best to call them. But.......

You wrote (2) 100 v panels will be added. Do you mean (2) 100 watt panels?

If you have a total, eventually, of 800 watts, why can't you just get the next bigger charge controller above 800 watts? Are the panels different voltages?
Yes, I meant watts. I will have 1 or 2 100w 12v panels and 2 300w 24v panels. My understanding is I need 1 controller for 100w panels and 1 for 300w panels.

I would get the next bigger charge controller, but I don't understand how they are sized. That's what I am asking.

I know Battle Born would tell me which one(s) I need, but I don't know if they will have the time to explain how they are sized. I want to understand all this. I couldn't find anything on Victron or BB sites that explained it. Same with youtube.
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Old 07-12-2018, 09:50 AM   #5
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Solar Questions Answered

Hello,

Here is a good source of basic solar information:

https://www.solar-electric.com/learning-center

You might be able to put the 12V panels in series and then put that
string in parallel with the 24V panels. The important figure is "Vmp"
which is the voltage which the panel puts out the maximum power.
An MPPT controller adjusts the load to make sure the panel voltage
is at Vmp. If the Vmp total of the series connected 100W panels is close
to the Vmp of the 24V panels all panels could be efficiently hooked
to the same controller.

Good luck with your project.

Larry H
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Old 07-12-2018, 10:10 AM   #6
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The first number in the model number of the Victron controllers is the maximum voltage allowed at the input. The second number is the maximum amps.

You can wire the 24V panels in series, allowing smaller wire or less voltage drop. The two panels would then produce 48V. Systems with many panels often use series/parallel panel groups to keep the voltage under the specifications.

As to the portable panels, I'd go with a separate controller. I wouldn't bother with the expense of a MPPT controller for the portables; just parallel them into a PWM controller. If you wired the rooftop panels in parallel, keeping the system at 24V, and wired the portables in series, producing 24v, in theory, you could run both sets in parallel & use the same MPPT controller for both. Getting the same voltages in real world conditions would be unlikely, and the overall amperage to the batteries would probably be better with two controllers.
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Old 07-12-2018, 11:06 AM   #7
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Thanks guys. It's making sense.
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Old 07-14-2018, 05:25 PM   #8
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This can't be right. Can someone explain where my numbers are wrong?

2 300w 24v panels connected in parallel =

600w @ 24v input to charge controller

a = 600w/24v = 25a (@ 24v)

Since batteries are 12v, amp output from controller will be 25a x 24v/12v = 50a

The reason I think this must be wrong is because this would charge a 100 ah battery in a little over 2 hours.

So the charge controller should be rated more than 32.25v (from spec sheet for my solar panel)v(optimal operating voltage) in and 50a out. Another reason I think this must be wrong is to buy a controller that handles 50a, it looks like it will have at least 100v input.

For example: BlueSolar MPPT 100/50

If this is right, even if I wired the panels in series, this would still max me out on the controller because of the amps out.
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Old 07-16-2018, 02:01 PM   #9
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I found some information on sizing a charge controller and I was almost right. Plus there is an easier way to figure amps. Where I was wrong was I needed to add 25% to total amps for "unexpected current increases".

Here's the easier way to calculate amps.

Take your total w and divide by the smaller of the volts for solar panels or batteries. For example, in my case I have a total of 600w of solar panels and the solar panels are 24v and the battery is 12v.

600w/12v = 50a
Where I calculated 600w/24v*(24v/12v) = 50a

Then you need to add 25% to that number, so in my case I need a charge controller that can handle up to 63 amps.

Another example:

2 100w solar panels and a 12v battery.

200w/12v = 17a + .25(17a) = 21a.
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Old 07-17-2018, 04:18 PM   #10
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I learned some new information today. If the charge controller is a high end controller, then you probably don't have to add the 25%, but if you go with a cheap one, then adding the 25% is a good idea.


I bought up to the next level, so I needed 50a and I bought a controller that supports 60a.
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