RV Solar Power Set-up Advice Needed

System · January 2007

Hi Everybody,

I've been sifting through this forum all day and have learned a lot. I'm yet another RV'er that loves the comparative seclusion and tranquility of boondocking. If I had only known what I have learned here just today! Oh well nothing I can now do to bring back all of those toasted deep cycle battts over the past 9 years that I thought they had memories and had to be trained to go deep discharge to get full use...

I am upgrading from a 26' travel trailer to a 32' 5th wheel which I want to solarize. When boondocking I and my family typically use about 50 Amps/day, includuing furnace fan in mid-30's overnight weather. My 2 110Ah Group 27's usually lasted about 4 nights before being 80% DOD discharged. I've been replacing them every 3-4 seasons.

How does this sound for the new rig?

Given the larger size of the new rig and the desire to install an inverter I want the new PV system to cover about 100-150 amp/day usage. This rig will be my retirement traveler so I'll be spending more time in it and want to power some more stuff. In keeping with the standard of less than 50% discharge, even with 3 days of minimum sunlight conditions, I figure I need about 500 ah of batteries and about 350-400 watts of solar modules as follows:

2 AGM 8d 245 Ah batteries $1,000
3 KC 130 modules $1,800
about 1,000 Watt Inverter $ 800
Tristar Controller $ 250
Odds & Ends $ 300
Total $4,150

My major question is about the AGM's: Will the Tow Vehicle alternator over charge them, or, conversely, will the AGM's high acceptance rate cause my alternator to overwork and fry? And, if so, could I prevent this by connecting the line carrying the Tow Vehicle alternator (100Amp) charging current through the solar controller to regulate the alternator charge that way?

Any suggestions on my 5th wheel solar set-up ideas, above. Will my batteries be adequate? And any advice about protecting the AGM and/or the alternator, or whether it is even require, would be appreciated. We're very conservative but desire more comforts of home when traveleing for longer periods.

Thank you once again for all of the knowledge you have already provided by way of your volunteer participation here.

niel · January 2007

Re: RV Solar Power Set-up Advice Needed

hi hb phil,
i'm not an rv expert here, but i'll give my 2 cents worth before some of the rv guys chime in. you state that you will use about 100-150ah per day and would like it to go for 3 days. this is 300-450ah for the loads over those 3 days. to not go below 50% dod this must be doubled and would now be 600-900ah in batteries and not the approximate 500ah you came up with. if you can't expand the system with more pvs and batteries then you will have to go with a shortened span of time to go without sun or do more conserving. in any case fit what pvs you can physically fit with some adjustability in order to aim towards the sun better as flat panels produce far less power. another option while stationary is to remotely ground mount a few pvs in addition to those mounted on the rv. two dissadvantages of the ground mounted pvs would be more wire(rv in shade-pvs in sun) and possible theft/damage(more accessable to other people).
in general alternator charging of batteries does not bring batteries to a full charge as the charge voltages are much higher than the alternator's voltage output. some agms do have a slightly lower charge voltage requirement such as the concorde sunxtenders. they require 14.2-14.4v for fully charging the batteries. if the alternator were able to fullfill the voltage requirements in bringing the batteries to a full charge there would be nothing to shut it down from an overcharge. as you gueesed it, automotive charging systems are something less than desirable, but will an alternator deliver a charge? yes, but it generally doesn't reach a full charge. can you put the alternator output through the controller? probably not as most regulation systems(most, but not all mind you) require a higher input voltage. you'd want at least a bulk and a float charging ability no matter the batteries used or the source of the power be it through pvs, an alternator, or both.
i do agree with your choice to use agm batteries as rvs are confined spaces. agms are less likely to cause hydrogen concentrations and agms have a better efficiency in their charging. this also makes them more appealing for use in a home. the main drawback is a higher cost. i'd like to see better warrantees on them as well, but they, in my opinion, refrain from doing so because of a higher sensitivity to overvoltage making them more susceptable to overcharging and damage.
i'm sure others will have some things to say here and could give some of their advice to you as well, but if they don't say now, please ask if you have more questions as it could spir on their input.

SolarJohn · January 2007

Re: RV Solar Power Set-up Advice Needed

NAWS has an Exeltech 1100 Watt sine wave inverter on sale for $601. Sometimes they sell them for even less than that. I have one, and I love it! This cuts $199 off of your estimate.

John

crewzer · January 2007

Re: RV Solar Power Set-up Advice Needed

When boondocking I and my family typically use about 50 Amps/day, includuing furnace fan in mid-30's overnight weather. My 2 110Ah Group 27's usually lasted about 4 nights before being 80% DOD discharged. I've been replacing them every 3-4 seasons.

HB Phil,

That’s pretty impressive! OK… on to the new stuff.

A couple of good rules-of-thumb are to have a battery bank large enough to provide ~3 days of autonomy, and to not discharge your battery bank below ~ 50% SOC. Assuming an average of 125 Ah/day, you may want to consider a 12 V battery bank sized at 125 Ah/day x 3 days / 50% = 750 Ah. That would be three size 8D batteries wired in parallel. This larger bank will also help with winter camping when ambient temperature will reduce useful battery capacity.

Assuming 90% coulombic efficiency for the batteries and energy consumption of 125 Ah/day, the typical requirement would be for a PV array that could deliver at least ~140 Ah. Assuming the equivalent of 4 hours of full Sun in the Winter, you’d need an array rated at 140 Ah / 4 hours = 35 A. Assuming an array rated at 17.2 Vmp STC, the array’s power rating would have to be 35 A Imp x 17.2 Vmp = ~600 W STC.

If the array is “flat” on the RV roof, then the array’s output will suffer, as the low winter Sun will complicate the calculation above. At ~34 degrees N latitude, the winter production factor (“cosine error”) would be ~54%, so you’d need an array with an STC rating at ~1,100 W.

For an array of this size, you may wish to consider an MX60 charge controller instead of the TriStar. The Tristar is a good controller, but the MX’ MPPT feature will increase winter energy production (especially from KC-130 modules), and you could probably get by with a 1,000 W array instead.

Or, stick with the TriStar and the 600 W array and fashion some sort of tilt mechanism that would allow the PV modules to be tilted up towards the Sun. Also, note that flat PV modules tend to get dirty fairly quickly and need to be manually cleaned.

No matter which controller you buy, make sure you also buy its optional remote battery temperature sensor.

Make sure the ~1,000 W inverter is big enough for your planned loads, and that it is a quality true sine wave model like the Exeltech XP-1100. Also note that inverters are ~90% efficient, so that may increase your daily energy requirement.

Tow vehicle alternators are notoriously of little use for charging a large battery bank. For example, your TV’s alternator may be rated at 100 A, but, when warmed up, it might be good for 80 A. Much of that current will go to running the TV’s electricals, and, when the battery voltage reaches the voltage at the end of the wire from the alternator, it will pretty much stop any further charging.

FWIW, my truck's 130 A alternator wasn’t particularly useful in charging the ~180 Ah of batteries originally on my old camper. What I did instead was use the truck’s alternator to run the camper’s electrical loads (i.e., 3-way fridge) while we were underway, and let the PV array charge the batteries with no loads on ‘em. If it’s a sunny day, there’s plenty of sunshine to charge the batteries and plenty of “wind” to cool the PV array!

Finally, there may be a couple of areas in which to reduce energy use, which may allow you to scale back on the system size. One possibility is to look into replacing standard incandescent bulbs in the RV with fluorescent or LED lamps. Another is to consider a radiant propane heater (i.e., Olympian) for heating part of the RV – it doesn’t use a fan. 8-)

See: http://www.uscatalytic.com/uscatalytic/Heaters.cfm
and: http://www.pplmotorhomes.com/parts/rv-furnaces/olympian-catalytic-heaters.htm

HTH,
Jim / crewzer

System2 · January 2007

Re: RV Solar Power Set-up Advice Needed

Thanks Niel, John and Jim now I feel more confident in moving on to start buying this stuff. Looks like I'll be shooting for another 8D AGM and module. That looks like a great deal on the inverter. I'll have to check my new microwave to see if the 1,100W will cover it.

System2 · January 2007

Re: RV Solar Power Set-up Advice Needed

Hate to beat a dead horse.... But let me ask the question about alternator charging of the AGM's in a different way...

Jim in the hypothetical of driving down the road with the PV's charging the batteries along with the alternator - while accepting that the alternator is too weak to defend itself and completely charge the batteries - couldn't the combination of the PV's and the alternator cause an overcharge since only the PV's are regulated by the controller?

Would it hurt to combine the alternator and the PV's input (parallel them) both into the controller so that, at least all current reaching the AGM's is regulated for AGM consumption?

I'm sure you have heard this before here but my wife is aleready freaking out at my estimated cost, above, of solarizing our new rv. Do you guys have any suggestions about how to sneak the cost and the appearance of extra solar module (or two) and an 8D battery by one's wife?

Let me see you figure that one out!

In my energy usage estimate I figured that even on rainy/cloudy days the PV's would be supplying at least some power, so I underestimated the batteries' Ah's capacity needed to make it three days @ 150 Amps per day. Do the PV's shut down completely if they are not in direct sunlight and have only light from cloudy skies?

I would love to be able to adjust/tilt the PV's but am concerned that I would forget and travel with them tilted up. Are the tilting mounts sufficiently strong to withstand at least 70mph winds while traveling tilted up?

BB. · January 2007

Re: RV Solar Power Set-up Advice Needed

You best method to charge from your tow vehicle is to use an inverter to take the 12vdc power, send back as 120 VAC and then use a good quality multi-stage charger to recharge the batteries back on the trailer. There are other options too (like using a 12 to 24 VDC booster) to send power from your tow vehicle back to the trailer--but I can't think of anything cheap and easy.

The standard alternator is just not good for charging deep cycle storage batteries (if you put it deep cycle batteries in parallel with your automotive battery, one or the other will die pretty quickly). You can try bolting another, dedicated, alternator (like a marine model) to your tow vehicle and running welding cable (to manage +/- 100 amps) from the engine to the trailer--but I can't think it would much much economic or ease of use sense.

Given that you would probably need to run the tow vehicle for 3-10 hours (depending on depth of discharge), unless you have really long trips every few days, you (hopefully) will not be running your car/truck that much on your trips.

Lastly, generally a dedicated (good quality) generator will probably be more fuel efficient than trying to run the engine/alternator on your tow vehicle to charge your batteries.

The real answer is, most likely, conservation of power... Changing out anything that uses lots of power (electric fridge, filament lights, 120 VAC appliances running on inverters like stereo, TV, etc.), for low powered alternatives (LED, Boom-Box, propane powered fridge, gas oven instead of microwave, no extra lighting, etc.)... And, if you need the microwave or computer (low powered laptop) for an hour or so--just run the (hopefully quiet) generator then. Use the batteries for quiet time the rest of the day/night.

Also look in magazines for truckers (and probably RV'ers)... You can find 12 vdc crock pots, microwaves and such for those smaller meals--but again, heating with solar/batteries, is usually a bad idea (expensive to store/generate electricity--use propane instead).

-Bill

System2 · January 2007

Re: RV Solar Power Set-up Advice Needed

I guess I didn't phrase my question correctly. I am not interested in charging my AGM's with my Tow Vehicle whatsoever. But since they are in the trailer and will be connected to the Tow Vehicle, I have no choice but to try to regulate whatever voltage or current the Tow Vehicle provides. In fact, I'd love to isolate the AGM's completely from my Tow Vehicle. Alternatively (no pun intended) at least regulate whatever voltage flows from the Tow vehicle alternator to the AGM's. Therefore, I wonder if it is possible to just stick the wires from both the tow vehiclle and the solar array into the solar controller to regulate the current of both to the AGM's. On 2nd thought this may be too much amperage for the controller, the Tristar, rated at 45 Amps I believe.

I will be towing 8-10 hours per day with the AGM's connected to my trucks 100 Amp alternator and to the 23 Amps of the PV's. Just worried that the AGM's may be exposed to too much. I guess the controller would cut off the PV's charge - but what about the truck over the course of those 8-10 hours connected to the AGM's - even at reduced/weak voltages and Amps?

Anyway, as a family who loves boondocking we are very good at conserving power and, as a family, average about 50 amps per day - and that is even when using a furnace in mid-30's night temps. We just turn down the thermostat to 50 degrees. Moreover, I have a generator to do all the charging that I need to do and I don't need the Tow Vehicle for charging my future AGM's, but am stuck with it being hooked into the trailer and connected to the AGM's.

BB. · January 2007

Re: RV Solar Power Set-up Advice Needed

With your 12 volt alternator, you will not be able to charge (or control the charge) through a solar controller. Any solar controller is going to need ~1-2 VDC across it before it will significantly charge the batteries--so your generator would need to output something like 14+1-2 volts = 15-16 volts.

If your alternator put out 16 volts (in the trailer, after voltage drop of the cabling), your vehicle battery would be toast.

However, if you connected your truck wiring directly to the solar panels (to make use of the solar charge controller), you have a very real chance of boiling your own truck battery dry and blowing out your vehicle's electronics--As your solar panels have a real ability to output 17-21 VDC (or more--depending on model and how you have wired them) at 20 amps--more than enough to hose your truck's electrical system. (edit to add: If I understand your thoughts--you would be connecting your panels directly to your truck battery, and the solar panels would be unregulated wrt to your truck wiring system.

I would just check the current and voltage at the trailer battery after they have been charged and you have warmed up your truck--If after checking with a accurate (calibrated) DVM, you find the batteries are getting too much voltage from the truck, you should find the device (either electronic or a mechanical relay) that turns on your trailer aux power (through the trailer's light/power/brake connector) and manually turn it off for long trips (if there is a problem).

Short of actually measuring the voltage on your setup--I would sincerely doubt that you would damage your trailer batteries solely from charging by your tow vehicle's alternator under normal conditions.

-Bill

Brock · January 2007

Re: RV Solar Power Set-up Advice Needed

I will throw in my usual. You could go with only two 8D batteries if you were willing to carry a small genset with you, say a Honda 2000i. It could charge your batteries and run your loads running just a couple of hours a day would do the trick if you had no sunshine. It might also be a way to start with only 2 panels and running the genset an hours a day. Then you could say, gee if we get just two more panels we would only need to run the genset in adverse weather

Jumping back to the TV's alternator. One of the big problems is you need good voltage right at the batteries (14.4) and unless your going to run welding cable back to the RV's batteries it just won't work, heck even #4 you will likely drop .4v or so and then they won’t properly charge. I like to think of it in the way that a vehicles alternator is not meant to properly charge a battery, it is meant to maintain a battery and the only battery it can really do this is the one it is connected to with nice heavy wire.

I have done the inverter / charger trick, that is to run a 1200w mod sine wave inverter to an iota 55 amp charger on the bank and it works well. This also allows you to run a much smaller wire between the two locations and the dedicated iota charger is doing the regulating and work, while the TV's alternator is just trying to maintain the 14v on the starting battery.

System2 · January 2007

Re: RV Solar Power Set-up Advice Needed

BB,

I believe you are right that by hooking the Tow Vehicle (TV) current into the controller along with he PV's I'd actually be hot wiring the PV's straight into to the TV electrical system. You may have just saved me a very major repair bill because I thought it was a pretty good idea! DOH!

I'll just check the current at the trailer to see if it poses any trouble. If so I'll get a battery switch and use two isolated banks: 1 for the trailer running gear (brakes, running lights, slider-outs) and 2; the AGM's for the trailer house systems (lights, fans, water pump, furnace, DC outlet). But the consensus seems to favor that, due to the long run and thin gauge wiring, there really isn't enough current to be a problem. Thanks for your help and warning about hot wiring my PV's with the truck wiring into the controller.

Brock,

I took the plunge and ordered four KC 130's, plus, an Outback MX60. I'll be getting three 8D AGM batteries. That should serve me very well. I like your idea of the truck powering an inverter to, in turn, charge the batteries in the place of the insufficient alternator current. I may do that with a small inverter and small charger dedicated to just the trailer running battery - if I decide to use two isolated banks as I described above. I beleive that there probably won't be a problem letting th e AGM's serve all trailer power needs both running and house stuff.

Along those lines the next big ticket item is the inverter. I was thinking about an inverter/charger: The Magnum MS 2012 2000 Watt Sine wave one. Does any one have any recommendations? One question I have is regarding the Transfer feature of these inverters/chargers that automatically switch from inverting to charging upon feeding it AC.

The question is, does it automatically do the reverse: Automatically start inverting in the absence of AC. I wouldn't want that. For example, I may plug-in to charge the batteries before storing the trailer and forget to turn off the inverter function and drain the batteries.

I definitely want about a 2,000 Watt inverter and the Magnum features a 100Amp charger with AGM-dedicated charging. Seems sweet, except for that one question. I want the auto charging when I plug-in to AC, but, not auto inverting when I unplug the AC. Don't want auto both ways.

Thanks again!

crewzer · January 2007

Re: RV Solar Power Set-up Advice Needed

Phil,

I would love to be able to adjust/tilt the PV's but am concerned that I would forget and travel with them tilted up. Are the tilting mounts sufficiently strong to withstand at least 70mph winds while traveling tilted up?

Sorry, but no.

I took the plunge and ordered four KC 130's, plus, an Outback MX60.

Make sure you also order the MX60’s optional remote battery temperature sensor.

Along those lines the next big ticket item is the inverter. I was thinking about an inverter/charger: The Magnum MS 2012 2000 Watt Sine wave one. Does any one have any recommendations? One question I have is regarding the Transfer feature of these inverters/chargers that automatically switch from inverting to charging upon feeding it AC.

The question is, does it automatically do the reverse: Automatically start inverting in the absence of AC. I wouldn't want that. For example, I may plug-in to charge the batteries before storing the trailer and forget to turn off the inverter function and drain the batteries.

I definitely want about a 2,000 Watt inverter and the Magnum features a 100Amp charger with AGM-dedicated charging. Seems sweet, except for that one question. I want the auto charging when I plug-in to AC, but, not auto inverting when I unplug the AC. Don't want auto both ways.

Don’t know about the Magnums. Another option would be OutBack’s FX2012MT mobile inverter: 12 VDC in; 120 VAC out; 2,000 W; and an 85 ADC charger. Along with a Hub and a Mate, it can be networked with the MX60 controller. USeful features of the Mate are that you can customize and fine-tune the Outback inverter/charger to your particular needs, and you can remotely control the inverter's operation, say the comfort and convenience of inside your RV.

Check the inverter dimensions... the Outback is a physically large inverter, and, considering its specs, I suspect the Magnum is as well.

Here are links to more info:

http://www.outbackpower.com/pdfs_spec/mobile-rv.pdf
http://store.solar-electric.com/fx2012mt.html
http://www.outbackpower.com/pdfs_manuals/Americas%20and%20Mobile%20manual%20rev%2072.pdf
http://www.outbackpower.com/pdfs_manuals/MX60_Manual_6_2.pdf
http://www.outbackpower.com/pdfs_manuals/Mate%20rev%20230.pdf
http://www.outbackpower.com/pdfs_wiring_diagrams/CABIN-RV-BOAT.pdf

HTH,
Jim / crewzer

System2 · January 2007

Re: RV Solar Power Set-up Advice Needed

Jim,

A Magnum Tech Rep stated that the system will automatically go to charging when AC is applied, but, the inverter must be "on" for it to automatically go to the inverter upon the removal of AC. An example is: If the inverter was "on" when AC was applied it goes to charging and automatically goes to inverting upon the removal of AC. If the inverter was off and AC was applied it will charge but will not automatically revert to inverter upon the removal of AC.

I know it sounds simplistic/obvious but I wanted to insert that clarification into this thread in case anyone else had the same question.

Now I just have to find a place to jam all of this stuff into/onto my trailer.

Thanks again for all of your help. I know you aren't salespersons but Northern Arizona got all of my business for all of the above. I checked around: Competitive prices and no tax = great deal for me.

Phil

niel · January 2007

Re: RV Solar Power Set-up Advice Needed

let us know how it works out for you. do watch your charge/discharge rates as i don't believe you will be able to feed the batteries as much as you will draw from them with the 4 kc130s and 3 245ah agms at the prescribed drain rates you've indicated you need. the voltage of the batteries after a few hours of rest will give a general indication of their charge and you should obtain this information from the battery manufacturer if they didn't provide you with it. keeping your resistance losses low will lower your voltage drops with wire that is extra large. you can use the calculator here in the general solar topics area for this purpose. it requires excel or an excel clone program. between 1 and 2% voltage drop would be a good range to shoot for and this will allow for some expansion of your pvs in the future. less than 1% is exponentially expensive to do unless you really plan for more than 2 pvs on any of your future expansions. do not let the voltage drop % go above 3%. do think fuses and disconnects too so that you won't get into a jam.
out of curiousity, what controller are you going with and did you order a battery temperature sensor?

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Niel,

I m going with the Outback MX60. I'm looking at three 8D AGM's but will wait until they are ready to be installed to get the freshest set possible. So here it is:

4 KC130 solar modules
Outback MX60 Controller
Magnum MS2012 Inverter/Charger

Jim has got me interested in the Deka's - maybe I can get 4 of thoe instread of 3 Lifelines. Is there a significant perfomance difference between the Deka and the Lifelines?

We really are fairly conservative and 150 Ah/day would be a high end estimate, specially in fair weather. We have been keeping it at aboput 50Ah/day in the mid-30's temps. Did it about ten days straight this winter.

Phil

crewzer · February 2007

Re: RV Solar Power Set-up Advice Needed

Phil,

The Concorde Lifeline batteries enjoy a very good reputation, but I have no direct experience with 'em. Both Brock and I have Deka AGM batteries. The Deka's cost a lot less, but their published performance specs suggest they won't deliver as many discharge/charge cycles. I have four size 4D's wired in series/parallel for 24 V x 400 Ah. Brock's got eight :-o size 8D's wired in parallel for 12 V x ~2,000 Ah. We're both pretty happy with 'em so far...

Are you located in SoCal? If so, you may want to look into MK brand AGM batteries. MK is a subsidiary of East Penn, which makes the Deka batteries. MK is located in Anaheim, so you may be able to find a competitive local source. See: www.mkbattery.com

Good luck, and I hope you'll come back with some performance reports!
Jim / crewzer

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Jim,

I beginning to feel the influence of the solar/battery power version of the old horsepower adage, "There ain't no replacement for displacement." Only this is a lot cleaner endeavor!

I'm right around the corner from Anaheim. I'm already pricing them, maybe get 4 MK's instead of 3 Lifelines. Of course then I'll want two more panels, and so forth.

David at NWS said that with the Outback MX60 I could wire my panels in serial fashion and the higher voltages would eliminate any wire resistance losses. Would you recommend this?

Phil

niel · February 2007

Re: RV Solar Power Set-up Advice Needed

concorde is also based in california, but stay away from lifeline batteries as the company will not honor the warranty on them in a solar or wind application. the sunxtenders they told me are the batteries for that purpose. crewzer and brock both got the dekas and are pleased with their performance with a lower general cost over something from concorde. either company does make a quality product that i believe you'd be happy with.

BB. · February 2007

Re: RV Solar Power Set-up Advice Needed

Running higher voltage strings does not eliminate resistance, but certainly reduces the amount of power loss due to current flow:

P=I*V=V^2 / R = I^2 * R

In the case of moving from 12 VDC to 24 VDC, the current will be cut in half (because of voltage doubling)... But, in the case of power loss due to wire resistance:

P=I^2R => (I/2)^2 * R = (1/4) (I^2 * R)

So, with the same wire, you cut the power loss by 75%--or you can use smaller gauge wire and save money on the installation (or send the same amount of power a longer distance at a higher voltage with the same loss).

In the case of the MX 60--Using a higher voltage battery bank (plus higher voltage panels strings) allows you to use more panels on the same converter (the MX 60 , up to a certain point, is current limited--so 60 amps at 12 volts = 720 watts max. 60 amps at 24 volts = 1,440 watts max, etc.).

Some converters are a bit less efficient at higher voltages--but, for your case, I would not worry about it and have the system wired to a higher voltage string (making sure that you don't exceed the solar controller's maximum input voltage--and remember that solar panels voltage rises quite a bit in cold weather--can be a real issue if you camp is cold climates--just as low voltage in hot climates could cause you issues with parallel strings hot climates).

For example, a KC130 panel at -4F is about Voc=25.5 volts... If you place more than 5 panels in series, you run the risk of smoking your MX-60 (at -4F).

And at 95F, one panel will only output ~14vdc--not enough to run the MX-60 and charge your batteries on a hot day...

-Bill

crewzer · February 2007

Re: RV Solar Power Set-up Advice Needed

David at NWS said that with the Outback MX60 I could wire my panels in serial fashion and the higher voltages would eliminate any wire resistance losses. Would you recommend this?

Phil,

I would not recommend this (four “12 V” modules in series for a 12 V system application). Such an arrangement would indeed reduce wiring loss (but not eliminate it), but it would also reduce the MX’ operating efficiency.

For your RV application (relatively small system and relatively short array-to-controller wiring), I’d suggest a 24 V array: Wire two pairs of the KC-130 modules each in series, then wire the two pairs together in parallel.

HTH,
Jim / crewzer

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Hi Jim,

What would optimize the the MX60? Would two pairs of PV's, 3 panels in series each, then both pairs wired in parallel to the MX60 work better (more efficiently) than two pairs of two panels? Or, would the higher voltages risk burning out the MX60 in cold conditions? Are two panels in series more efficient than 3 in series? I may be able to squeeze two more panels into the system making it 6 each, KC130's, total.

Why are 4 panels in series less efficient than 2?

I think I'll be getting 4 MK 8D AGM batteries. I know these are made by the same parent company as Deka. Justr wondering how they compare to the Dekas, or, whether it is just a matter of different badging for the same battery?

Thanks again for all of your help

BB. · February 2007

Re: RV Solar Power Set-up Advice Needed

Check the data sheet/user manual for the MX-60. IIRC, the maximum voltage has been increased, by a few volts, to 150 VDC not to exceed. From the current manual (by the way, you got the battery temperature sensor too? RIGHT?):

The maximum open circuit voltage of the PV array must not exceed 150v DC under any conditions or damage to the
controller may occur. Furthermore, the maximum voltage level is recorded in the MX60's computer, allowing this
level to be checked when seeking warranty service.

The MX60 will suspend operation if the PV array’s open circuit voltage exceeds 135v DC. This helps to protect the
controller from damage under very cold conditions. Most PV arrays will warm up significantly once sunlight is
hitting the PV array - which then causes the PV array's voltage to drop and allowing the MX60 to resume operation.

The maximum voltage level is dependant on the brand/model of the solar panel, the number of solar panels connected
in series, and on the temperature of the solar panels. The temperature depends on the system location and other local
climatic conditions.

So, ideally you will want your panels to be between maximum battery bank voltage (at maximum recommended charge voltage--cold bank, equalization--if flooded cell, and hot panels + 2 volts), and the maximum voltage of 135 VDC (maximum operational voltage at solar panel cold voltage of Vmpp--also shall not ever exceed 150 vdc of cold panels and Voc --open circuit voltage).

At this power level, as long as you meet the min/max voltage and current limits for the MX-60, putting the panels in series will save you some copper wiring costs.

You have to pick the temperatures that you want to run the system under... Say you want to run down to -4F and up to 104F...

Voc (at -4F), per panel is 25.59 volts
VmPP (at +104F) per panel is 13.459 volts...

So, one panel, on a very hot day with cool batteries will not meet the minimum voltage of battery charge voltage + 2 volts drop of MX 60 (say ~ 14.5 volts + 2 volts = 16.5 volts minimum panel voltage required).... So, you minimum series connection of panels is 2.

Max power voltage before inverter cuts out is 135 VDC at -4F... So:

Panels max = 135/25.59 volts/volts = 5.2 => 5 panels maximum in series before MX 60 cuts out...
Panel NTE max = 150/25.59 v/v = 5.86 => 5 panels maximum in series to avoid damage to the MX 60.

Number of panel strings in parallel (48 amps recommended maximum input current for controller and 7.39 Imp for the kc 130 panels):

Max number of strings = 48 amps/7.39 amps = 6.5 => 6 strings in parallel max...

So--the random numbers would be 2-5 panels in series and from 1-6 8 strings in parallel maximum. Any of those combinations will work well for you (fix from 8 to 6 strings maximum):

2 panels 1 strings = 2 panels
2 panels 2 strings = 4 panels
2 panels 3 strings = 6 panels
...

3 panels 1 strings = 3 panels
3 panels 2 strings = 6 panels
... etc.

4 panels 1 strings = 4 panels
4 panels 2 strings = 8 panels
... etc. to 8 strings

5 panels 1 strings = 5 panels
6 panels 1 strings = will exceed -4F cold temperature of 150 VDC for MX 60

Lastly, because the output of the MX 60 is 60 amps of current... There is a recommended maximum amount of wattage that the MX 60 can put out... So a 12 volt bank would be 12 volts * 60 amps = 720 watts, 24 volts * 60 amps = 1,440 watts, etc...

From Outback:

Nominal Battery Voltage Recommended Array Size (in watts)
12v 800w
24v 1600w
36v 2400w
48v 3200w
60v 4000w

So, for you system (assuming 12 volts):

800 watts / 130 watt STC panel = 6.15 => around 6 panels maximum for your 12 volt battery bank...

A few things can be adjusted--if your min/max temperatures are not -4F/+104F, you can change the number of panels in the string... And if you have some shadows on your arrays, one (or two) higher voltage strings is probably is probably better than a lower voltage string of 3.

-Bill

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Wow Bill, what an excellent wrap up. It took me several read-throughs before I feel that I really comprehend it.

I doubt that I'll have trouble with -4 temps but can see the panels exceeding 104F very easily - even 120 roof top temps are normal in the SW US. Therefore series wiring is definitely required to keep things charging.

If I am understanding this correctly...and please correct me if I am wrong ...an 800 Watt array (6 KC 130's at about 48 Amps) is the maximum wattage that one MX60 can handle when charging a bank of 12 volt batteries? Or, in other words, any more array wattage would be a waste and I would have to get a 24 volt bank of batteries, or another mx60, to utilize more panel wattage?

This application will be on an rv roof. By necessity the panels will be spread around. Again, correct me if I am wrong, but it is advantageous to have several strings rather than one so that if one gets shaded, the others (if in full sunlight) can still produce at their full capacity. Likewise, as you said, it may be advantageous to have a string with a higher number of panels to keep things flowing under high heat - just as long as it wouldn't exceed 150 vdc at -4f.

I'm printing this whole thread and keeping it with the other solar gear documentation.

I got the RTS for the MX60 yesterday. An RTS comes with the Magnum MS 2012.

This is going to be a lot of fun and will add a whole new and most interesting dimension to my rv'ing/camping experience.

So much for rubbing sticks together to make fire. We'll be firing up the convestion microwave out there. But I think I'll be most thrilled wartching those batteries charge and maxing things out at each site.

BB. · February 2007

Re: RV Solar Power Set-up Advice Needed

Phil,

Note, that I had to change the max number of strings from 8 to 6 in a couple places--I had an editing error earlier and did not catch all of the places the first time...

Yes, ~800 watts is the maximum "raw" panel the MX 60 can make use of... It is because its output is limited to 60 amps x battery bank voltage...

Regarding the shading of series vs parallel strings... I am not the expert here (not that has stopped me before :-D ), but if you have one shaded panel in a string of 6, you will still have the energy of 5 available (minus a diode drop of the shaded panel). If you have two strings of 3, shading one panel will knock out the one string of three (parallel string, only the higher voltage string will supply power to the inverter)... That is my theory, and I am sticking with it.

In the end, preventing shade and tilting your panels towards the sun are going to very significantly increase your total power generation.

Now, for a few variables to ask about... Generally, panels very rarely provide their full output in warm weather. So--it may be possible to bump up the panel count and get more generation with adding another MX 60--but this depends on the ability of the MX 60 to gracefully manage the "excess" current/voltage/power in colder conditions--Others here may be able to answer that question better than I.

Second, I believe I have read elsewhere (here?) that the MX 60 can be programed to support up to 72 amps of output current just fine... And the newer units are rated at 72 amps output. (I used 60 amps because the current manual on the Outback site still says this).

So, you probably have a bit more research to do--but you can run the calculations again using 72 amps, and be able to fully cover your RV with solar panels.

-Bill

PS: Got a few more free minutes right now...

To be clear, if you "over panel" the MX 60 so that you get a full 60/72 amps when hot--you will probably have more panels than the MX 60 can handle in cold weather... I believe, that the MX 60 will simply limit to 60/72 amps of output (assuming you have not exceeded maximum input voltage and the batteries/load are able to accept the full current). It is a "waste" of the extra panels--but if you are not in cold weather that much--it is an operating region you will rarely venture into.

Just for the exercise--same as above but using 72 amps maximum:

48 amps (at 60 amp rating) * 72amp/60amp = 57.6 amps input (educated guess based on 72/60 ratio)

57.6 amps / 7.39 Impp amps per panel = 7.79 strings => 7 strings parallel supported (at least)

800 watts recommended @ 12 volts * 72a/60a = 960 watts of panels recommended maximum (educated guess)

960 watts / 130 watt panels = 7.38 panels => 7 panels maximum...

Now the problem is that you cannot run a 7 panel string without over-voltaging the MX 60, and you cannot divide by two or three strings for an even number of panels (each string must be equal voltage/# of panels).

So--your choice would be to stick with 6 panels (probably in two strings of three)--but (after you ask Outback or your dealer), you would be pretty happy with eight panels (two strings of four) in hot weather. -- assuming that you even want eight panels.

You have other options (like adding a single 24+ volt panel from another vendor with a second, smaller controller)--but you probably need to decide where to go from here.

-Bill

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Bill,

Thanks for those clarifications. I just looked at the MX60 manual which shows that the default Output of 60Amps can be adjusted to 70Amps: "The default charger current limit is 60 Amps and can be changed for circuit breaker or wire size limitations and is adjustable up to 70 Amps for those of you that actually read this manual." The manual recommends using PV arrays of at least twice the voltage of that of the batteries. Also you were correct about the 800Watt recommendation for a 12 volt battery bank. The manual recommends that PV's in cold climates should not be built to exceed 125Volts to keep them from going over the 150V maximum in extreme cold. I got the impression that they prefer a 5 panel/string limitation in very cold climates.

I think I'll be going for 2 strings of three PV's each. The cost of installing a 5.5KW Onan generator into the rv I am ordering would have been $5,500. The alternative energy solution, the solarization of my rv, is now approaching $7,500:

6 KC130 PV's $3,660
Outback MX60 Controller $500
Magnum MS2012 Inverter/Charger with Meter $1500
4 8D AGM Batteries at least $1,800
Misc Parts Breakers, racks, etc. at least $300

If you hear of a murder in Huntington Beach, it will be my own when my wife sees the credit card bill this month. HA!

Phil

crewzer · February 2007

Re: RV Solar Power Set-up Advice Needed

Phil,

Bill’s provided you with a loot of good analysis and recommendations. Your plan to go with two strings of three series PV’s each (a “36 V” array) will certainly work. In fact, that’s how my original system – not much different than what you’re proposing -- used to be set up, and you can read more about my system’s performance on this OutBack thread:

http://www.outbackpower.com/forum/viewtopic.php?t=952

What would optimize the the MX60? Would two pairs of PV's, 3 panels in series each, then both pairs wired in parallel to the MX60 work better (more efficiently) than two pairs of two panels? Or, would the higher voltages risk burning out the MX60 in cold conditions? Are two panels in series more efficient than 3 in series? I may be able to squeeze two more panels into the system making it 6 each, KC130's, total… Why are 4 panels in series less efficient than 2?

Regarding “optimization”, the MX operates most efficiently when the input voltage is just a tad higher than the output voltage. Considering that you’re using KC-130 modules 17.6 Vmp, relatively high) and AGM batteries (14.35 V absorption, relatively low), your MX would probably work “best” configured for a “12 V” / “12 V” system.

However, the total mid-day current from the six modules wired in parallel for “12 V” would be ~44 A, so, as Bill explained, you’d need some big cable for the run from the array to the controller. Also, the horizontally installed array will likely get pretty hot in the summer (poor convection cooling), so the mid-day output voltage will suffer.

My gut feel is that the practical “optimal” solution would be to wire the array for “24 V”, (three parallel pairs of two series modules per pair) and use something like #8 wire between the array and the breakers/fuses and controller.

Either way, there’s no risk of burning out the MX60 in cold conditions. As you’ve discovered, it can handle up to five series “12 V” modules in just about any condition you’d be likely to encounter.

The MX is a “buck” DC-DC converter. As such, the input voltage must be higher than the output voltage. Everything else being equal, such a converter’s efficiency drops as the input voltage increases. Accordingly, four panels in series (high input voltage) would be less efficient than two (low input voltage).

Huntington Beach, eh? I though so when I saw the “HB”. I occasionally do some work over at the NE corner of Bolsa Chica and Bolsa…

Best of luck with your new system!
Jim / crewzer

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Jim,

Nothing beats hands-on experience! That was a great test of the MX60 input/output efficiency too. Between Bill's analysis and your hands on practical use and testing I think we have finally got my little system zeroed-in. Niel was right that with 4 KC130 modules I probably wouldn't be able to "float" the batteries on solely solar power given my intended power usage and I would have had to resort to generator every few days to keep things above 50% discharge. I hate firing that thing up when out boondocking. And, since the whole point of going solar was to eliminate the need of the generator, I've decided to go with 6 modules.

I only have one more question and I promise that will be it! What happens when I apply AC shore power/charger (Magnum MS2012) to an active MX60-controlled solar power charging system? Will the MX60 sense the Magnum charge and go to sleep? Or, will the Magnum sense the solar charge and shut down the AC source charging? Or, alternatively, will the Magnum and MX60 supplement each other. For example when the MX60 snoozes, the Magnum 2012 increases the charge, or, conversely, fry each other?

I guess the best thing would be to turn off the MX60 before applying shore power. However, I know, sooner or later, that I'll forget and plug-in to shore power while the MX60 is still doing it's thing. Will it hurt anything?

Jim, I've got a few acquaintances that work there too. Genuine rocket scientists. Their boys are in the same Cub Scout Pack as my boy.

Phil

PS I noticed that my wife has a container of Prestone Anti-freeze under the sink. Seems strange for Southern California... You don't think that .....Nahhhh!

BB. · February 2007

Re: RV Solar Power Set-up Advice Needed

Generally, you can mix several types of chargers directly to the same batteries without problems for day to day use... The chargers just look at the battery voltage and decide how much current to supply... With several chargers in parallel, the batteries will charge faster... And you have an RTS for each unit, so they should not cause thermal runaway on charging.

With AGM batteries, you don't need to worry about equalization--so you should not have an issue with two chargers trying to equalize too often.

Both the MX 60 and the Inverter/charger are multistage chargers (Inverter: Bulk, Absorb, Float, Equalize, and Battery Saver™)... For camping, you should not have any issues as AGM's usually, in their specs., state that you can hold the charge voltage and the batteries will accept the current needed--safely. You should check the battery specs and confirm that your voltages and charging currents/algorithms are OK (AGM's require different charging algorithms than flooded cell type batteries).

And double check when both chargers are operating to see that nothing strange is happening. Depending on exact set points and accuracy, you may see one charger supplying 80% of the charge current and the other 20%--I am not sure that I would expect 50/50 current sharing (AC Charging + full sun).... Typically, it is virtually impossible to get two or more power supplies to share current by with just set-point voltages. You may choose to set the MX 60 charge voltage at battery requirement and set the the Magnum just 0.1 volt or so lower so that you will primarily use the Sun's power instead of AC power for charging (when both sun and AC are available). Wiring (and charger design) will also affect current "sharing" too--so I would not worry if the the chargers do not "share the load" at all.

For long term storage--AGM batteries do not have much self discharge and you would probably pick the Solar Charger just because it would be a waste of AC energy for maintenance charging (unless the solar panels are covered during storage). I would read the detailed charging algorithms for both chargers and see which meets the long term storage voltage/current requirements
best. You want to avoid both over charging and the possibility of a small load flattening the batteries over a few weeks or months (forgotten light/radio/etc. left on) from discharging the batteries (especially if the AC line cord was accidentally disconnected). Batteries will generally not recover if 100% discharged (and left in that condition for a day or more).

-Bill

System2 · February 2007

Re: RV Solar Power Set-up Advice Needed

Bill,

I was just worried that the controller and the charger and the batteries would get fried. But I guess it makes sense that they will sense what is needed an d only apply that amount that is required. Good tip to adjust the set points slightly differently.

Phil

BB. · February 2007

Re: RV Solar Power Set-up Advice Needed

There is a possibility that two large chargers can supply too much current to a battery and overheat it during charging... Hence one reason for the RTS for each charger. And why I recommended that you check the current(s) to ensure that all looks OK. With some types of battery chemistries, they do want you to limit total charge current and having multiple (large) chargers could cause issues.

There is also the possibility that the chargers oscillate (charger #1 is running, charger #2 comes up and ramps up current, #1 sees high voltage and cuts back, #2 sees lower voltage and steps up curennt more--overshoots then undershoots back down--#1 sees undershoot, then ramps up current--and this continues)... This can also happen when loads are attached--Example and inverter is a constant power device--input voltage drops, takes more current--charger sees voltage drop and ramps up current--inverter sees increased voltage and ramps down current--charger sees voltage overshoot, ramps current back down quickly, inverter sees voltage drop and ramps up current causing more voltage drop, then the whole cycle starts over again...

Oscillations are very real problems with power supplies in electronics (designed to respond very quickly to voltage/current changes). Depending on how chargers are designed--hopefully they have longer cycle times so they don't get into rapid oscillations. By the way, long wires (inductance) would be the enemy here... Twisting the wires (or at least running parallel and tied together) will reduce distributed inductance.

I don't expect a problem as I have not read about anyone having these problems--but it is something to watch for if you have strange issues (like rapidly blinking lights and/or radio noise in certain conditions). Setting the voltage set-points a few tenths of a volt different would probably help fix a problem like this too (if it where to occur).

-Bill

PS: I am a design engineer (not solar) and these are the types of problems that I had to worry about for my job--the chances of this happening are pretty remote and the above is not intended to worry or confuse you... You can ignore the above and only read up/ask questions if anything strange were to happen.

-BB

crewzer · February 2007

Re: RV Solar Power Set-up Advice Needed

Phil,

Our paths have nearly crossed… I was working in HB this past Thursday!

I only have one more question and I promise that will be it! What happens when I apply AC shore power/charger (Magnum MS2012) to an active MX60-controlled solar power charging system? Will the MX60 sense the Magnum charge and go to sleep? Or, will the Magnum sense the solar charge and shut down the AC source charging? Or, alternatively, will the Magnum and MX60 supplement each other. For example when the MX60 snoozes, the Magnum 2012 increases the charge, or, conversely, fry each other?

As Bill indicated, it’s not unusual for multiple chargers to be connected to the same battery bank. There are, however, a few issues to understand. One is that your planned 1,000 Ah battery bank can take bulk charges up to ~300 A, so there’s little possibility of the combination of chargers in the Magnum and the MX frying each other.

Another issue is synchronization. For example, it’s possible that the one charger could be in Absorb mode while the other charger is finished with absorb and is in Float mode. If the Magnum’s charge voltage is higher than the MX’s (i.e., Magnum in Absorb and MX in float), the MX display will read “absorb” (instead of “Absorbing”) to indicate that condition. I’m not sure about how the Magnum’s charger will behave in the reverse condition, but I’m confident that neither the chargers nor the batteries will suffer.

Reinforcing another of Bill’s comments, I strongly suggest you carefully read and program the MX and the Magnum for your AGM batteries. Here are links to the information you’ll need:

http://www.mkbattery.com/images/AGMBatteryCharging.pdf
http://www.mkbattery.com/images/VRLA_TechManual.pdf

Finally, here’s a link to an Outback discussion that includes settings I used for my original 12 V AGM battery configuration:

http://www.outbackpower.com/forum/viewtopic.php?t=848

HTH, and have fun!
Jim / crewzer

RV Solar Power Set-up Advice Needed

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