Best 12 volt camper configuration

You have really screwed your self. With any MPPT controller you should wire the panels to operate high of voltage as possible. With 8 battery panels 4 in series, in parallel with the same. On a 12 volt system using the Kid controller maximum efficiency on a 12 volt battery system is running a panel Voc of 90 volts. But where you really went wrong is with 12 volts and a 30 amp controller. At 500 watts input to a 12 volt battery will generate up to 40 amps. You can certainly use 510 watts input, but you will loose 110 watts doing so.

Does not mean all is lost as you can always move up to 24 volts and utilize all your panel power because at 240 volt is 20 amps with 500 watts input.

I strongly disagree. The outback and midnite controllers (the ones I am most familiar with), become less efficient, make more heat, and have lower output amp capacity when the input voltage is very much higher than the output voltage.

Optimal input voltage for a 12 volt battery is about 24 volts... that allows for some voltage sag due to hot panels, and gives a few volts headroom to the controller.

Of course, if there is a large distance between the panels and the controller, you may wish to trade off controller efficiency in order to save on cable cost.

--mapmaker

Page 39 in the KID manual shows you the efficiency graph. Optimum efficiency for 12 volt battery is 70 to 90 volts.

Pretty much the same for all Midnite Solar controllers power charts

Over on the Midnite forum, Bob Gudgel has indicated that producing efficiency charts for their controllers is an expensive and complex undertaking and is not high on their priority list. He also stated that the Classic efficiency curves were similar to the Outback efficiency curves. Some of those Outback efficiency curves have been posted here:

As you can see, lower input voltage raises efficiency.

Also read comments by vtMaps and halfcrazy:


Sunking, The links you provided are power charts, they are NOT efficiency charts (although you can infer some efficiency data from them at their limits of performance).

They show that the maximum output current (and therefore output power) is reduced as input voltage rises.

For example: the kid power graph shows that for a 12 volt battery an input voltage of up to 70 volts will yield a 30 amp output. As the input voltage goes up (to 90 volts or higher) the output current drops. The reason for this is reduced efficiency. Reduced efficiency means more heat production in the controller. Since the kid is already producing all the heat it can handle (with a 70 volt input and a 30 amp output), if you reduce efficiency (by raising the input voltage) it cannot handle the extra heat caused by reduced efficiency, and its output is throttled back.

This analysis has been 'blessed' over on the Midnite forum by their engineers. This analysis only applies to controllers operating at their maximum output, and that is because the efficiency data is inferred from the power charts which only have maximum output data.

By the way, the Midnite controllers are capable of operating at their maximum rated specifications, and they will protect themselves from over-heating damage, but in my opinion, it is not good conservative design to plan on operating them this way for long stretches of time. Here is a quote from Robin Gudgel:
--mapmaker

So where are the Midnite Solar Graphs?

I don't care about Outback, I am familiar with Outback efficiency curves. On a 24 volt battery system Outback is most efficient with 34 volt input @ 98.5% efficiency, and at 70 volts input is 97% efficient. However that is not the full picture. You have to include wiring losses and cost into the equation. That is where the higher voltages wins out most of the time unless the distances are extremely short. .

As I mentioned, Midnite has chosen not to produce efficiency graphs. However, the midnite engineers were the same engineers who designed the outback MX controllers, and they claim the midnite efficiency is similar to the outback efficiency.

I certainly agree that efficiency is not the full picture.

In many systems with moderate distances between the combiner and the controller, raising the string voltage only transfers the inefficiency from the cable to the controller. IMO, it is better to make heat in the cable than in the controller.

This is an important consideration when you are pushing the power limits of the controller... a little extra heat in the controller is not so important when you are only running the controller at half power.

--mapmaker

EDIT: Opticalmike, hope that we haven't derailed your thread to much, but I see that you've already had your questions answered over at the Midnite forum and at the wind-sun (NAWS) forum.

Well if it is like Outback running 24 volt battery there is not enough difference in efficiency to be significant between 98.5 and 97% on 34 and 70 volt input respectively. Running 34 volts means parallel panels which means more combiners, fuses, and larger wiring. I will gladly let the controller eat 15 more watts out of 1000 every day. With both looking at 4% total voltage loss will cost me much less money and less complexity.

Because my charge controller is a foot from the combiner Box and another foot to the battery bank and the panels are less than 20 ft away on #10 multi strand wire, I'm inclined to go with the parallel config. Pushing nearly 30 amps of juice into that kid.

You are aware 500 watts is the maximum power input limit on a 12 volt battery right, and optimum Voc = 37 volts. Read page 17 of you rmanual.

I can't find anything in the manual about optimum VOC being 37 volts. The power charts show input voltages as low as 18 volts for a 12 volt battery. Midnite's online string sizing tool allows even lower voltages.

--mapmaker

I have one of the Beta Kids. It has a life time warranty. That, along with the publicly noted statement from Midnite staff:

Link to post on the Midnite Forum.

Mario, at Midnite, has stated the same thing. I figure if anyone knows these controllers, they certainly do.

So, I am going to test this. I am going to attach 600 or 700 watts to my Kid. Considering we are in rainy season here now, I doubt very seriously it will "overload" the Kid anyway. Most days, I imagine I will be lucky to get 1/2 the rated array's output. But, at least I will have more power available on cloudy days, than I would if I simply limited the array to 3 modules.

Did you read page 17 in the manual? Have you looked at the wiring diagrams on pages 34 to 37? Have you looked at the programming flow chart diagrams and noticed input voltages they use in their examples?

The kid was designed to use 230 to 250 watt grid tied panels (60 cells modules). Can you use more expensive (2 x cost per watt) battery panels in parallel? Yes if you want to throw away money using expensive panels, combiners, over current protection, complicate the system, and loose the MPPT advantages it was designed for.

If you were the manufacture and stated you can use battery panels on a public forum, would you tell the customer on a public forum who has battery panels they missed the boat on the design? That is what we are talking about, design with optimum performance with lowest cost possible.

So the OP wants to use 85 watt panels to make 500 to 700 watts and wire them in parallel? Is that what you would do? That is going to take 5 to 8 panel meaning 5 to 8 over current protection devices, many connections, feeder cable, and a combiner. IMHO that is just plain silly, more than doubling the cost, and a nightmare of wiring. If I had to work with it I would use the panels in pairs or threes in series to at least cut down on cost and minimize input current. For petes sake if he uses 700 watt input he gets clipped to 500 watts. Well guess what? He could have save a bundle of cash and just used a cheap $30 PWM controller where 30 amps in = 30 amps out. Effectively the design just eliminated the MPPT controller altogether. I guess that is what you are recommending he do?

Yes, of course I read it... three times. I was trying to figure out where you got the notion that 90 or 37 volts Voc was optimal.

What I find (on page 17) is:
I don't see the word "optimal", I see the word "common". And I certainly agree with you that using commonly available panels makes economic sense.

This conversation began with a discussion of input voltage vs efficiency. My understanding of these Midnite buck converters is that the less down conversion they have to perform, the more efficient they are. If panels were commonly and inexpensively available with a Voc of 32 volts, I suspect they would be more optimal than 37 Voc panels for charging a 12 volt battery.

--mapmaker

Point I am trying to make here the difference in efficiency from 19 volt to 37 volt panels is less than 1% or no different from what I can ascertain. The difference between 37 and 70 volts is 1.5%. We are not talking much difference and huge money savings can be had using higher voltage and smaller wire.


Then there is the complexity of parallel panels require fuses and combiners not to mention the expense. Lastly battery panels cost twice as much as Grid Tied panels. I can do the same with just 2 = 250 watt panels in series using 16 AWG wire. With 85 watt panels twice the cost in panels racking for 6 panels, six fuses, larger wire, combiner. It is much easier to take a little loss in the controller than try to make up for it with much large wire and hardware.

That is all I am trying to pint out. Sure if I were the manufacture and someone said I have 12 volt 85 watt battery panels in parallel I would say you can make it work. Ask them off-line if they recommend that and I bet you get a answer.