solar question

[silvereagle1964]

recently purchased '64 eagle. coach has an additional alternator (24v) and a vanier 24v 3600w inverter.
No house batteries at this time. I was told the system was installed to run the 110v a/c on the road.
I want to install house batteries and 24v solar panels as the coach is all 110v. except for water pump,
fantastic fan, and toilet and we like to dry camp a lot.  I went online to a number of solar places that do
rv's and none even showed 24v panels for sale. 24v panels seem to be the lowest per watt price.  24v
mttp controller and remote panel is only about $60.00 more than 12v. Any input on the use of 24v.
system, or scratch the alternater and inverter I already have and start new?

thanks, mark reid
64 silvereagle 006670
53 gm-4104-252

[bobofthenorth]

Stick with the 24 volt plan, you won't regret it.  You'll be able to use smaller wires to carry the same watts which will make feeding the inverter easier and it will also make wiring your solar system easier.  Personally I would look for identical panels and wire them in series to an appropriate MPPT controller, for the same reason, it will let you use dramatically smaller wire sizes.  There's lots of internet chatter about whether MPPT or PWM controllers are "better" but for me the logistics of installation wire size make the MPPT and series installation the only way to go.

[TomC]

Just a comment-it has always made me scratch my head-all trucks and buses in Europe are running 24v. I don't understand why we run 12v. Both my transit bus and Kenworth truck are 12v-with the exception of the 24v starter on my Kenworth-but that's old technology. Good Luck, TomC

[Iceni John]

PV panels intended for grid-tie use, such as the 255W Sharps I have, are the least expensive per watt  -  I paid only $0.81 from Solartec in Northridge CA.   These panels are not "24V" per se  -  their full-power voltage is about 30, not enough to charge a 24V bank of batteries, but they are intended to be connected in series to their charge controller(s).   Bear in mind that most decent MPPT controllers have a high voltage input limit of 150V, meaning no more than four of these GT panels in series.   However, one other thing to consider  -  MPPT controllers are most efficient when stepping down the least difference in voltage:  if their input voltage from the panels is more than twice their output voltage to the batteries they will produce more heat and their efficiency drops significantly.

The amount of panels is determined by the Ah of batteries you'll have, and the amount of batteries is determined by the loads, so that's where you need to start your calculations  -  how many Ah of DC and AC will you use in a day?   The generally-accepted derating for PV-to-batteries is 0.77, and for PV-to-AC is 0.51, i.e. 1,000W of PV will supply about 770W of battery charging power or 510W of AC power, after factoring in losses from cables, charge controllers and inverters.   Keep your cables short and fat, crimp all your lug connections "gas-tight", use good breakers on the DC side (Carling Type C are the best), and buy decent charge controllers and inverters.

FYI, for my 12V house system I have eight 255W panels mounted in individually-tiltable support frames so they can lay down at 22 degrees below horizontal or be raised up to 45 degrees above horizontal.   Each row of four panels is connected in parallel through individual 12A fuses in their combiner box, then feed through 4AWG cables inside the roof ribs down to the charge controllers in the luggage bay.   I have two Morningstar TS-MPPT-60 controllers, each one with a 50A input breaker and an 80A output breaker, and each controller will feed its own bank of four golfcart batteries wired in series and parallel for 450A at 12V.   The two banks of batteries are combined through two Cole-Hersee 250A Schottky diode battery isolators so one bank cannot back-feed into the other, and their combined (but electrically separate) outputs feed the DC load center (a Square D QO 6-breaker panel) and the two inverters, a Cotek SK2000 sine-wave and a smaller sine-wave just for the fridger.   I'll also have an 80A 120VAC battery charger for those rare days I cannot keep my batteries charged from solar  -  I've decided on a Cotek CX1280 because its Power Factor is way better than the usual Iota and similar non-PFC chargers.   This charger will easily run off my emergencies-only Champion 3500W generator that I've converted to propane.   I can also switch the house batteries to the engine starter if the start batteries are low, but I think I'll also have a small separate PV system just for the starting batteries using a small PWM controller like the Morningstar SunSaver Duo.   It's been a lot of work to get to this stage, but the prospect of having free electricity for as long as I have the bus is appealing.   (OK, I know it's not really free, but it's better than running a generator all the time!)

HTH, John

[RJ]

Mark -

Chris & Cherie of Technomadia have done a lot of work, and written it up, about using solar.  They recently updated their blog, so you might want to surf around over there for more info, too.  www.technomadia.com is the link.

FWIW & HTH. . .

[bobofthenorth]

Just a comment-it has always made me scratch my head-all trucks and buses in Europe are running 24v. I don't understand why we run 12v. Both my transit bus and Kenworth truck are 12v-with the exception of the 24v starter on my Kenworth-but that's old technology. Good Luck, TomC

Thread drift alert!!  But a big AMEN to this comment.  If 6 volt was a complete disaster then 12 volts is maybe only half a disaster but it is incredibly stupid.  For a while there was a rumour running around the auto industry that they would switch to 48 volts which made a lot of sense to me but obviously it never happened.  If I was starting clean I'd run an all 24 volt system.  I know the naysayers will immediately point out how much more expensive the components are but - IMHO - that is small beans in the big picture.  12 volt & its attendant high amperage creates a ton of problems, even in relatively low current situations like clearance lighting.  It made a HUGE difference when we switched all our clearance lights to LED but the difference wasn't because the lights were LEDs, it was because the current draw went down to the point where our old wiring could easily handle it.

[silvereagle1964]

so far from your posts to my solar question I learned that using the mttp controllers allows for the use of higher voltage output panels. Now the batteries, I found 240 a/h 12v batteries, 250 lbs ea. footprint is slightly larger
than 6v golf cart batteries, but about twice as tall. which would be the best?

[bobofthenorth]

Battery capacity is all about the weight.  I'd make the decision based on the space available and the battery format that allowed me to cram the most weight into that space. 

[Iceni John]

If you're using tall batteries like L-16s, it's best to use charge controllers or chargers that have a useful Equalization voltage (as long as they're not AGMs!) to help stir up the electrolyte to avoid stratification, otherwise the SG readings from the top won't be representative of the entire electrolyte.   Any good charge controller should be adjustable to various charging parameters for different types and voltages of batteries.

John

[silvereagle1964]

the vanner 24-3600c has a timed equalizing feature so I should be ok there. 
I would like to put four of the 12v batteries I saw, that would give me
480 a/h @ 24v. That would be 1,000 lb. don't know if the small bay on the right side
behind the rear axles would hold that or more?  You eagle fans need to chime in
on this please.

[HB of CJ]

You will NOT be able to run the A/C with solar panels.  You would need at least 10x more panels than the roof would mount.  You will be able to somewhat recharge from other DC usage using roof top mounted PV panels.  You can use hot weather 12vdc PV panels in series to get the needed high 24 vdc needed to let the PV panels work on the roof of the Bus Conversion during hot weather.  The voltage will most likely need limiting some times so as not to hurt the inverter.

But ... usually roof mounted PV panels and Bus Conversions do not work that well.  The panels must be in direct sunlight to work.  The aforementioned Bus Conversion needs to be in dark shade to stay cool, unless you are Vulcans, who would not need the A/C to begin with.  Good luck.  Now if you can figure out some good way to remote the panels in the sun while the Bus Conversion remains in the shade, then that may work OK.   Still will need LOTS of panels for the A/C.

HB of CJ (old coot)   The higher the PV voltage, yep, the smaller and cheaper the cable like already said before me better.  Hope this helps.

[silvereagle1964]

I don't expect to run the central air from the solar panels, if air is needed i'll need to
plug in or run the generator.  I am hoping that between the solar and the additional
24v alternator I will be able to have a/c if needed while driving.

[Dlsnow]

FWIW - we dry camped exclusively for two years (in the desert) and DID run our AC unit (coleman mack 3 13,500) off of our solar system.  We never plugged in and never ran (dont even have) a generator.

Keep in mind we are very active so when i say we used the AC figure it may have had between 2 and 6 hrs of use daily.

use the basic math available and know what your efficiencies and losses are especially on the inverter.

our system was recieving about 6.5Kw daily in Yuma, Tucson... during the winter season

[Iceni John]

FWIW - we dry camped exclusively for two years (in the desert) and DID run our AC unit (coleman mack 3 13,500) off of our solar system.  We never plugged in and never ran (dont even have) a generator.

That's good to know.   I've always assumed that I would also be able to run A/C from my solar panels, so your experience bears that out.   With 2,040W of panels I am hoping to run a small (6,000BTU ?) window A/C in the bedroom, and/or a larger (12 to 15K ?) mini-split in the front of the bus.   Either way, I want to run them without drawing down the batteries, so I'll still have full charge by sundown to last me through the night.   My two charge controllers can pump up to 120A into the batteries or to the inverter, so I hope that's enough for the A/Cs' running loads, and the batteries can help supply their starting surges.

John 

[Dlsnow]

Make sure to size your inverter main breaker. I had to move mine to a 150amp (dc rated) from an 80 to accommodate the startup load.

Dave


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