Solar utility trailer

[theuniverseman]

I have sleep apnea and I like to go camping, not a good combination plus I have a very limited budget so a big comfy RV is way out of the question, so my solution was to purchase a small utility trailer to haul my camping gear and mount my solar panel to. My set up has a 90 watt Monocrystalline Solar Panel and a sealed UPG 75AH battery and a Sunforce 7 Amp Charge controller and a Wagan EL2201 Elite 400 Watt Pure Sine Wave Inverter and my cpap unit uses 90 watts/.75 amps (which if I am not mistaken ought to be a fairly small load for this set up).

Based on the thread I linked below these are the figures I came up with, 90 watts x 5 hours x .50 = 225 watt hours, or 225 wh / 5 hours = 45 watts per hour or up to 16 hours to recharge my battery from 20% to full charge (I need more solar power! well more of a want then a need).

I would like to add a 310 Watt PV plus my current 90 watt for a total of 400 watts, with this set up I would generate 400 watts x 5 hours x .66 = 1320 watt hours. So if my battery is 80% discharged I would need to replace 720 watt hours or 720 wh / 264 watts = just 2.75 hours to a full recharge! Unless of course this would be too much current for my battery.
http://www.solarpaneltalk.com/showth...e-my-Batteries

At any rate here is my project, any advice on my project or corrections to any math errors ect would be great

IMG_3565.JPGIMG_3551.JPGIMG_3555.JPGIMG_3550.JPG

[Sunking]

OK you have made some errors in calculations. Read this thread and it should help you out.

Please allow me to point out some of your errors and how it will affect your design.

Fist step is to determine how much energy you need in a 24 hour period. In your example 90 watts x 5 hours = 450 watt hours.

Immediately we can find the battery size if we know the voltage which I assume you will use 12 volts. Battery AH capacity = [5 x 450 wh]/12 volts = 187.5 AH @ 12 volts.

Ok for the panel wattage needed you need two pieces of information; the type of charge controller, location, and time of year use.

OK you have selected a PWM charge controller vs a MPPT controller. So let's assum you live in Kansas city and use the system year round. So you look up the solar insolation for December which is 3 hours.. So when using a PWM controller you have to account for its low efficiency, and th ebattery charge efficiency. In the case of PWM is 50%. So your panels need to generate 2 x 450 watt hours = 900 watt hours. So from here the formula is Watt Hours / Sun Hours, so 900 watt hours / 3 Sun Hours = 300 watts.

For a MPPT charge controller the factor is 1.5 or 66%. 1.5 x 450 watts = 675 watt hours. So panel wattage = 675 wh / 3 h = 225 watts.

[PNjunction]

... and a Sunforce 7 Amp Charge controller...

The weakest link right now is that Sunforce charge controller, which I'm very familiar with. It is an old-school on/off shunt controller, that is also not temperature compensated. You see them everywhere under a variety of brandnames.

I'd seriously consider using a pwm style controller, such as a Morningstar Sunsaver SS-10-12V or SS-6-12V charge controller. You put a lot of nice effort into that portable setup - but the CC being used now is really not appropriate for serious use.

Have you also thought about what happens if you discharge your battery, and encounter a few days of no sun? You don't want to let a battery sit around discharged for too long. Perhaps something like a Deltran Battery Tender Plus 12V / 5amp smart charger would be next on the list.

The good thing is that your UPG UB-12750 is an AGM, so it can withstand a max charging rate of C/4, (probably a little higher as seen in the docs or on the case) so this should be no problem, and you can even add another panel closely mated in voltage (~10% v) pretty easily for faster charging should you desire, although you'd have to consider beefing up the CC capacity too.

In one of the pics, I think I see the battery inside a battery-case. Be sure you OPEN that while in use.

[PNjunction]

Forgot to mention that if you were to add an additional panel, that would be a parallel connection, not a series connection.

Also, the Sunforce charge controller documentation has you attaching the panel first, and the battery last. This is backwards from the way that most major controllers and battery chargers suggest. Last thing I want is a spark at the battery terminals, and most controllers need to be up and operating first before seeing a panel.

Nice rig btw. It is far better then anything I have ever built for portable use.

[theuniverseman]

Thanks for the advice, I am pleased with how my rig has turned out so far, my aim is to be able to be off the grid for an extended period of time (more than just an overnight campout) so how much power do I need to keep my battery adequately charged (btw I live in southern New Mexico). I seem to be understanding most of the math involved so far but I don’t understand what the 187.5 AH means, I have a 75 AH battery which by my math ought to run my 90 watt cpap for 75 freaking hours (Amp=w/v 90/110 = .8 amps). I am a little confused with what this means for my system, so are you saying that I would need to generate 675 wh in 3 hours in order to recharge the 450 wh used?

Okay as for a charge controller what about the BZ mppt 250 charge controller? http://www.solarhome.org/bzproductsm...ontroller.aspx

As for the battery case it is well vented was there any other reason to remove the top of the battery box?

[Naptown]

Thanks for the advice, I am pleased with how my rig has turned out so far, my aim is to be able to be off the grid for an extended period of time (more than just an overnight campout) so how much power do I need to keep my battery adequately charged (btw I live in southern New Mexico). I seem to be understanding most of the math involved so far but I don’t understand what the 187.5 AH means, I have a 75 AH battery which by my math ought to run my 90 watt cpap for 75 freaking hours (Amp=w/v 90/110 = .8 amps). I am a little confused with what this means for my system, so are you saying that I would need to generate 675 wh in 3 hours in order to recharge the 450 wh used?

Okay as for a charge controller what about the BZ mppt 250 charge controller? http://www.solarhome.org/bzproductsm...ontroller.aspx

As for the battery case it is well vented was there any other reason to remove the top of the battery box?

In red is where you went wrong
You are using a 12 v battery so the volts are 1/10 of your calculations and therefore the amps would be 10x what you calculated or 8 amps + inverter losses.

[bcroe]

Trailer

Based on the number of windshields I have replaced in a million + miles, I
would recommend some kind of flying rock protection while in motion. A
couple panels folding together for traveling with plywood on top comes to mind.
good luck, Bruce

[Sunking]

I am a little confused with what this means for my system, so are you saying that I would need to generate 675 wh in 3 hours in order to recharge the 450 wh used?

Did you read this thread as I suggested. It will explain everything you are overlooking.

The answer to your question is maybe, it depends on what type of charge controller you use, and the number of sun hours you have. If you use a MPPT controller yes the panels have to generate 675 watt hours to replace 450 watt hours. If you use a PWM controller the panels need to generate 900 watt hours to replace 450 watt hours.

The battery is simple once you know your daily load. At 12 volts you need a 200 AH battery.

[theuniverseman]

No I did read it I just did not fully understand it, my calculations were off because I was confused with the 110 voltage produced by the inverter. Thanks Naptown for setting me straight, but I am starting to grasp this stuff, its just a matter of taking into account how everything works as a whole.

[theuniverseman]

I have been crunching some numbers (they actually make sense to me now) and I have some questions about going 24 volt, I just purchased the 75AH UPG battery for $165 and since it was recently purchased hopefully another one from the same company ought to be a match and I would only spend another $165 to get an usable 75AH which at 24v is a total of 1800 wh available or 4 days of reserve.

Eventually I will upgrade to a Morningstar Sunsaver TrackStar 15 Amp MPPT Charge Controller 12V/24V and 310w 36v PV panel (like a Talesun 310 Watt TP672M-310 Solar Panel) but that’s $600 bucks I don’t have right now. For now I can get by with a plug in charger (NOCO Genius G7200) and I can return my 12v 400 watt inverter to amazon and get a 300 watt 24v inverter for an extra $50 more than my current one.

Another concern I have about my project is the effects of vibration on the solar panel, especially one as big as the 310 watt panel is. Do I need to come up with some sort of vibration isolation for it? I was thinking about getting creative with a hacksaw and an old mountain bike frame from a thrift store.

[theuniverseman]

Okay please be patient with me while I figure all of this out, I just realized that I have made some more errors with my battery math, the watt hours is not 1800 wh it is only 900 wh, but at least I know why now. I still need to go 24v however because of the 36v 310 watt PV panel I will eventually get, plus the higher voltage means smaller wiring.