Need advice on setting up Solar for my 5th wheel

Re: Need advice on setting up Solar for my 5th wheel

Welcome to the forum Jim.

Well last questions first and all that ...

It is possible to find panels for $1 per Watt or even less. But they are usually large "GT style" panels and you may have a hard time fitting them to your roof. You'd be looking at things over 3 feet wide and 5 feet long each there.

With an RV application space becomes an over-riding issue. You can't put just any panel up there, it has to fit. So you're going to have to measure the free areas, plot it out on paper, and do a lot of thinking before you find the maximum you can get.

You will probably find an advantage in monocrystaline panels as they take up less area per Watt, being a couple percent more efficient. They are also more expensive and not usually available in smaller sizes.

If you've got six GC2's that's three parallel battery strings on what is most assuredly a 12 Volt system. That can be a problem if not wired correctly. This is another area where replacing them with fewer, higher-capacity batteries can be advantageous even though it costs more per Watt hour.

The bank you have now is about 660 Amp hours @ 12 Volts. Ideally you would have 1000+ Watts and an 80 Amp MPPT charge controller for that. Chances are you can't fit 1000 Watts on the roof. Maybe half that? Which means you won't get full recharging ability from the panels, only extended battery time. You may also want to see if you can drop that third string. You'd be going from 3.9 kW hours @ 50% DOD to 2.6 kW hours @ 50% DOD. Depending on how much power you draw from the generators and how much you can get 'directly' from whatever solar you install that may be enough. Normally we tell people to measure their loads with a Kill-A-Watt meter, but that's not always easy to do in an RV because some significant loads can be DC and there's not always a handy plug to use for the AC items. You could run the whole thing from generator for a while and measure the power coming from that to the RV, but it may not be that accurate either (battery draw, converter - if equipped - usage, etc).

Re: Need advice on setting up Solar for my 5th wheel

OzzJim wrote: »

Thank you for responding quickly,
I had to 'Google' DOD... Newby..... I understand now. I am not against replacing the batteries, but would prefer not to for financial reasons. Bottom line is that I want to do this right, just have to sell another Air conditioner
What is the highest output compact panel I can get? Also you mentioned dropping the 3rd string... couldn't Google that.. Batteries maybe? I see the problems associated with a large battery bank, great to have the extension of power available, but the recharging is a problem on solar then. Need to have the 3 stage recharge needed for them. I am like the kids nowadays: I want it all... NOW
What type and different makes of batteries would you suggest, so I can see if they available here in The K.C. MO. area?
Thanks for your time helping me.
Ozz

I can't help you with what's available in Kansas City, Missouri; I live quite a long ways away in a different country. Sometimes very different. Brands that I have seen work well: Crown, East Penn/Deka/US Battery (three different labels but the same battery), Trojan (pricey). Also, believe it or not, some people have got good value out of 'warehouse' GC2's from places like Costco.

Yes, DOD is Depth Of Discharge. We have a whole glossary of terms, but no one ever looks at it. http://forum.solar-electric.com/showthread.php?6136-Glossary

Three parallel battery strings can be trouble. Not will be; can be. http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power If you can use fewer batteries and less total capacity it's better all around.

You are still faced with the limitation of what will fit on the roof. Panel brands don't matter much in terms of output because they all use the same basic technologies. Monocrystaline gives the highest output for a given area, polycrystaline (now being called "multicrystaline' just to confuse people) comes in second, and thin film amorphous panels a distant third.

Some RVers solve the space problem with 'portable' panels set up in the sun while they park in the shade. These have issues too: easier to steal, easier to damage, long wire runs to deal with (trip hazard, damage hazard, Voltage drop). But it can work to have a few hundred extra Watts out there.

Re: Need advice on setting up Solar for my 5th wheel

Wiring diagram: that should work fairly well. Best is for equal lengths off each battery pair to bus bars, with a fuse for safety on each pair. Like the attached diagram, except the battery strings would be only two in series instead of eight.

Rolls Surrette batteries are expensive and in my opinion not worth the money. There have been a lot of problems reported with them of late. BTW companies do make 6 Volt batteries of slightly higher capacity that take up a similar amount of space. I'm using some that are 232, Trojan makes 225's (and other sizes), and Crown has listings up to 240 Amp hours. If your batteries are still good I wouldn't think about replacing them with something else; wait 'til they need replacing.

The number of panels has nothing to do with 3 stage charging: that is a function of the charge controller and almost every single one will do it. The number of panels also has nothing to do with if there's enough power to charge or not: it's the Watts they add up to and the specifications.

For example: to get 66 Amps peak current @ 12 Volts for your battery bank you'd need either 1155 Watts of 'standard' panels (Vmp around 17.5) on a PWM controller (two in fact, as they only go up to 60 Amp) or 1028 Watts on an 80 Amp MPPT controller. In the latter case they can be any kind of panel so long as the array Vmp is above 17.5 (minimum needed for charging a 12 Volt system): the controller will down-convert the higher Voltage into greater charging Amps.

Why does this make a difference? Here's a system design I was working on yesterday for someone, comparing two ways of getting the same amount of charging power:

Three 130 Watt panels @ $285 each = $855 (390 Watts)
plus Xantrex C35 controller $85 = $940
22 Amps output for two GC2's

Two 190 Watt panels @ $218 each = $436 (380 Watts)
plus MidNite Kid 30 Amp MPPT controller @ $285 = $721
24 Amps output for two GC2's


As you can see the higher Wattage panels are cheaper per Watt so when you get near 400 Watts using an MPPT controller becomes less expensive. With even larger panels the difference is more dramatic.

A 130 Watt poly panel is 58" x 26". A 260 Watt mono panel is 66" x 40". Do a little math and you see that two of poly panels have the same power but take up 3016 square inches whereas the one mono panel is 2640 square inches. 376 square inches or 12% less space for the same power. But you may not be able to fit large panels to your roof.

Re: Need advice on setting up Solar for my 5th wheel

vtmaps wrote: »

The wires that run from the battery bank to the inverter are labeled in your diagram as being "equal length cables". Those two cables do NOT need to be equal length. If you can make one of them shorter it will be to your benefit.

--vtMaps

Just to clarify:
The wires from inverter to battery bank do not need to be equal length just as vtMaps says.
The wires between the batteries in each battery string do need to be equal length.
The wires paralleling the battery strings do need to be equal length.

Thus the string on top has zero extra positive wires and two extra negatives, the string in the middle has one extra positive and one extra negative, and the string on the bottom has two extra positives and zero extra negatives: each string has the same number of (equal length) extra wires.

Re: Need advice on setting up Solar for my 5th wheel

OzzJim wrote: »

I measured my RV Roof, I can put 5 SolarWorld Sunmodule 315 Watt Monocrystalline Panels on it, giving me 1720 watts, would that get me where I need to be with my six batteries? I am also changing out my RV reefer with an energy star household refrigerator, as it is 7 years old, and I want to switch to the electric... or will that be too much on the system?
Thanks.

Those are some of the largest panels there are. If you can fit five of them you can fit just about anything.

You do not want to use five of this type of panel. The reason being they would all be in parallel due to the odd number. That will require a combiner box box capable of taking all five panels; a six position box.

But it does appear you have a lot of roof space available. If you can fit 1720 Watts you can get 110 Amps of charge current on a 12 Volt system. That would also mean a need for two charge controllers.

For two parallel strings of 225 Amp hours @ 12 Volts (450 total) you only need 700-ish Watts and one MPPT controller.

Down-size the panel choice a bit and save yourself some money and grief. You should be able to fit four 240 Watt panels (960 Watts), for example. No combiner needed if you run two parallel strings of two in series (although the higher Vmp of the arrau would be slightly less efficient). If there are going to be a lot of protrusions possibly casting shadows on the panel, then put them all in parallel on a four position box.

Re: Need advice on setting up Solar for my 5th wheel

Last question first.

There's a difference between an RV 'frige and a residential unit, and not just size. RV types are usually propane with 120 VAC back-up for the heater. If you shift to residential you lose the propane ability, although that's not necessarily a bad thing. But it does mean that all your refrigeration requirements must be met with electric power. You have just about enough capacity to do that. Given the difference in design and the huge number of different refrigerators in general it's hard to say if the all-electric would be more efficient than propane. The off-grid rule-of-thumb is the longer you stay out there, the more worthwhile the investment in electric over propane.

You really do not need a huge array for the battery bank set at 450 Amp hours @ 12 Volts. You will have some fall-off due to less-than-perfect insolation that RV's suffer from (can't always control the panel angle to the sun). Think of it like this: if you max out a single 80 Amp controller on a 12 Volt system you'd only have 1250 Watts of array. More than that and you'd need another controller. Even at one you could be producing more Amps than the batteries can safely handle: 80 into 450 is 17% charge rate and would really stress them.

It used to be possible to recommend panel brands. Now manufacturers come and go like the weather. They're all made pretty much the same, so as long as they meet specs and certification and have a sop towards warranty they're all the same.

Re: Need advice on setting up Solar for my 5th wheel

OzzJim wrote: »

I must be missing something on the AH. You said my battery bank was 450 AH, but with 6 @ 225 wouldn't that be a total of 1350?
But, you state that an 80 amp controller will calculate that I have only a 1250 watt array, the max my batteries can safety charge.
I am learning here, so be gentle. We are all experts in our field, hard to know everything about everything
So.. what do I need for my batteries, the 1250 watt array?
What do you suggest I buy for the 1250 if so? How many of what size, and which controller.
Thanks for your help, hope I am not sucking all the oxygen out of the room with my questions.

Six 6 Volt batteries? I thought you had four. Six is a bigger number.

Two in series per string to get 12 Volts. Amp hours does not go up; only the Voltage does.
Three such strings in parallel = 3 * 225 or 675 Amp hours @ 12 Volts.

Ideally to recharge that you'd want a peak current of 67.5 Amps. The formula for sizing an array on an MPPT controller is: Amps * nominal Voltage / efficiency factor. In this case that would be 67.5 * 12 / 0.77 (typically) = 1052 Watts.

The most an 80 Amp MPPT controller can handle at 12 Volts is the same formula only at its maximum current: 80 * 12 / 0.77 = 1247 Watts. (Sometimes I round numbers. Usually upwards.) You could go with that maximum size on 675 Amp hours @ 12 Volts and achieve a peak charge rate of just under 12%. It wouldn't hurt anything.

Most people have very similar questions about their systems. Often I'm answering two or more similar questions at the same time and get confused as to the subtle differences.