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Old 11-12-2015, 11:34 AM   #31
Join Date: Sep 2015
Posts: 38
A little more about 12V panels vs 24V panels and MPPT vs PWM.

The actual voltage in the spec sheet for a 12V panel will be 17-18V. Remember in order to charge a 12V battery you need at least 15V if the batteries are very cold.

The actual voltage in the spec sheet for a 24V (or HV panel as they are sometimes called) will be 31-35V something like that. Again, in order to charge a 24V battery bank you need about that voltage!

The MPPT controller has the ability to convert 24V to 12V to charge a 12V battery system. That's why they cost more. The hardware required to downstep voltage is not cheap. In addition to this downstep function they also use a more sophisticated circuit to monitor the panel output and choose the optimum voltage/amperage profile during small increments in time. This means if the panel output is changing due to shading or other electrical effects in the circuitry, the controller will always (within the sample period) get the most out of the panels. This "could" result in a potential power gain of, I don't know for sure but you can read the stats each manufacturer provides to see what they say. Maybe 10-20%? And those numbers will be the max you will see.

So let's talk about a 200W system. Watts is volts times amps. So at 12V we could see a max of 16.6 amps. Let's say we gain 20%. Another way to say that is a PWM controller wouldn't deliver 16.6 amps but rather 80% of that or 13.3 amps.

You batteries store energy in amp hours. The way to think about that is if you have a 100 AMP Hour battery you can run an appliance using 1 Amp for 100 hours.

Let's say you run that appliance for 20 hours. You will use 20 amp hours of the 100 amp hours in the battery or 20%. Your battery is at 80% capacity. To charge the battery you will need to put 20 amp hours back in.

Go back to our 16.6 amps and 13.3 amps earlier. In both instances to charge the batteries to 100% will take a bit over an hour at 16.6 and a bit longer than that at 13.3. Not a very long time. Certainly within range of the amount of sun you will get on an average day almost anyplace on the planet at any time of the year.

These are small numbers. In my case in my RV I use about 15% of my 225 amp hour storage everyday. 33.75 amp hours. I have 600W of panels with a max output of 50 amps. In bright sun I charge my batteries in less than hour.

OK, that's not quite true. To understand what really happens you need to understand a couple of things: batteries don't charge linearly and nothing is 100% efficient. So I used 33.75 amp hours but to get that back I might need to generate something like 40 amp hours to account for inefficiencies. The second part is the non-linear thing. As batteries get more and more full they require higher and higher voltages and lower and lower amps. So even though I can generate (in a perfect world) 50 amps the batteries are not accepting that near the end of the charge. An earlier poster said I never see the max out of my panels because my batteries are never really empty. So it doesn't matter how many amps I generate late in the charging cycle it just matters that I'm producing some amps. So back to the MPPT boost as they call it. Late in the charging cycle I don't need boost! I'm not using all the amps I generate anyway! Do I need MPPT? Probably not.

Where do you need MPPT? What's it for? Take large panel arrays on the roof of your house that are feeding the nation's electrical grid. In that case I want to generate as much power as possible. After all they are paying me for it! I can recoup the cost of the MPPT controller.

So for small amp hour requirements charging batteries the MPPT boost may not be worth it. You will rarely use it.

I had one time on my 1000W system where I saw the amps out of the panels into the batteries at 60. Clouds obscured the sun until noon. My batteries were at 70%. They could take the full 60 amps. And they did! And my MPPT controller was getting the max out of the panels. But that lasted for about an hour and a half and as we got more and more amps in we needed less and less and it tapered off fast.

So in our real world if our batteries don't reach full charge until 11:00AM rather than 10:00AM do we really care? It's a guy thing! Of course it matters!

Show Low and Rio Verde, AZ
Klipstr is offline   Reply With Quote
Old 11-12-2015, 07:24 PM   #32
Junior Member
Join Date: Sep 2013
Location: El Paso TX
Posts: 21
Originally Posted by silver View Post
I am looking at that kit. When you say everything did you get the fuses? I did not see those in the description and thought I would need to get separately.
No the fuses and the screws to mount to roof of motor home done come with the kit but everything else was there..

bmaggin1 is offline   Reply With Quote

solar, solar panel

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