How to Solar Power Your Casita
Posted 11 May 2009 - 01:40 PM
I am interested in the solar power article. I would like to purchase something this month before going out on a month long trip. Thanks.
Posted 13 May 2009 - 10:08 AM
The standard advice is to start by figuring out how many amp hours you use, and explanations of the subsequent process can be found on the AM Solar and RV Solar Electric websites, probably among others. The bottom line is that ultimately what you can use is going to depend on your battery(ies). A rule of thumb is one battery and a 65 watt solar panel per person. Eventually more power usage will necessitate more batteries.
For a lot of Casita owners we are talking about one or two people making due with one battery. Some of us are too conservative re: towing safety issues to put something heavy like a second battery or a generator on the tongue, and some of us are unable or unwilling to keep moving a battery between a TV and the ground next to the trailer.
So the battery question then comes down to whether you are going to live with the battery you already have, or if you are going to spend time and money acquiring one that will give you the most amp-hours possible and still fit in the Casita battery cave. However many amp-hours you supposedly have, you need to think in terms of using no more than about 20% before recharging, if you want to keep the battery alive as long as possible. This may require installing more energy efficient LED-type lights, substituting a catalytic heater, finding ways to limit the use of your water pump, and so forth.
The refrigerator is a whole topic in itself. Suffice it to say here that if most of your Casita time is spent travelling from place to place, and you hesitate to run the frig on propane while towing, putting your solar panel on the roof, though more complex, potentially expensive, and less efficient, is probably going to be your best option.
Okay so you have an estimate of how much electricity you can use on a daily basis. This may exceed what you actually do use, but for myself I wouldn't want to necessarily be confined to my current usage. Still, you can start small and buy additional solar panels if needed. The maximum solar energy that can be accommodated with a single battery is about 125-130 watts. If you don't want to start that big you probably want to be able to reach that capacity with just 2 or 3 panels, so start at 40-65 watts. Partly the decision will rest upon the expense and how much size and weight is reasonable for you in one panel.
The next question you probably want to ask is how much sunlight can you expect usually to get. If there will be trees, remember that even a little shade on a panel will reduce the energy output quite substantially. So you need to think in terms of getting pretty much all that you can between 10 am and 2 pm. In open areas, an additional 4 hours will probably give you half again as much as those 4 hours do.
Say you have a 100 amp-hour battery and want to be able to recharge it in one day, figuring that it may rain tomorrow. Take those 20 routinely available amp-hours (which could last a really frugal electricity user 2,3 or more days) and divide by 4 hours, and you thus figure that you need a solar panel that produces a 5 amp current. The amps your panel can produce is the watts divided by the volts. Panels in our size range often have about 17 volts, in which case you would need an 85 watt panel, or two 40 watt panels hooked together in parallel (positive to positive and negative to negative). Hooking up panels in series (negative to positive) will give you more volts, not more amps.
For the most part you probably want to consider your battery full at 14.2 volts. The length and guage of the wire used to connect the panel to the battery will determine how much voltage is lost in transit. A wire loss chart at solarseller.com will help you figure out what guage wire you need depending on the distance at which you want to be able to place your panel, and how many volts you can afford to lose over that distance. If you use 10 or 12 guage wire you may be able to go up to 100 feet and still have some voltage to spare, unless your panel is smaller than 40 watts and 16 volts. (Wire of that guage is not of neglible cost.)
To protect the battery from damage caused by overcharging you also need a charge controller. An MPPT controller can convert some (10-30%) of the excess voltage from your PV panel into increased amperage and is therefore probably worth the increased cost. Expect prices in the $80-$240 range dependent on the maximum current (amperage) it can handle, and how much information the display panel gives you. A 22 amp controller is probably of excessive capacity. If your PV panel(s) will give you 5 amps, you probably want at least a size 7 to accommodate the boost from excess voltage.
Posted 13 May 2009 - 03:48 PM
An excellent posting! I think those "energy inventories" are pretty lame. After all, how many hours (minutes) does one run the water pump per day. Etc. So I think your analysis is just great.
Only one slight issue: the amperage you need has to do with the 12V level. If you are charging your 12V battery at 5A that is 60 watts. After one hour, you have gained 5 ampere-hours (in 12V battery language). So, if you have a 105 amp-hour deep discharge battery, say, you would need to charge for 21 hours at the 5A level.
I think you were referring things to the higher panel output voltage, but I think the thing to do is refer everything to the 12V trailer battery level.
Great going. I'll continue to monitor for your insights here---as well as to hear how your first trip pans out with solar.
Edited by Art Davis, 13 May 2009 - 03:50 PM.
Posted 27 May 2009 - 04:26 PM
Okay, I have just had an experience with that $88.00 charge convertor we have been discussing on this thread.
First of all, I typically disconnect my trailer battery---but reconnect it once per month to recharge, after which I disconnect it again.
I always follow the recommended procedure of disconnecting the negative termnal of the battery, then the positive terminal. That way, if I accidentally make contact between either terminal of the battery and the trailer frame, there won't be sparks. Then, when I am ready to charge the battery, I always connect the terminals in reverse: first the positive, then the negative.
Now I have had the solar charge controller wired into my system, hiding out in the bowels of my trailer. I just went through the procedure of connecting the trailer battery to recharge it---and fried the charge controller because I connected the positive before the negative. Never gave it a thought, just did my normal maintenance. However, the problem announced itself with a strong odor of burning insulation. I popped open the controller box and saw that there was a chip melted down into a slag heap.
Now this is normal maintenance procedure for recharging a trailer battery on a trailer that is being stored---and I wind up frying my charge controller by doing so. What a prima donna unit!
I am telling this story here so that you won't do as I did and buy this unit. I think I am just out by almost ninety bucks---and still have to buy a decent charge controller.
P.S. Rich---I think you have one of these. Is your memory that good that you always recall this connection sequence, one that violates the rules of normal battery maintenance?
P.P.S. John Drake at solarseller.com has bluntly refused to deal with the problem. Suggests I send it to the mfr, who seems willing enough to repair it (probably for a fee), but I won't be stuck with this thing. So---I'm looking for another charge controller. Any advice from anyone?
Edited by Art Davis, 28 May 2009 - 06:51 AM.
Posted 27 May 2009 - 09:35 PM
Anyway, re: Art's problem, I don't routinely disconnect my battery. Can't, actually, since it just about totally fills the battery cave, (had to remove the carrying strap in order to get it stuffed in), and is too heavy for me to move. But I've heard others say they use a battery switch, and have wondered if such a device would provide the best method to insure that my water heater does not get turned on accidentally when the trailer is not in use, but is entered briefly for one reason or another.
And now I'm wondering if use of a battery switch would also protect the charge controller from the problem that Art has just experienced.
The PT charge controller instructions are to hook it up to the PV panel before hooking it up to the battery, but I'm not doing that because I have the charge controller wired directly on to my now inaccessible AGM battery terminals. (In fact, the ring connectors added just enough height to the terminals that it became necessary to file a notch at the top of the battery cave opening in order to get the terminals through it.) I've got the charge controller velcroed onto the battery, and the recommended 2 feet of 12 g extension cord connected to the charge controller, leaving just barely enough room to get the battery cave door closed and locked. The other 23 feet of extension cord is wired to the PV panel, so I just plug it in. There are no temporarly loose wires that can touch things they shouldn't, and no call to remember what connects to what, and when. I just plug it in. What remains is figuring out a strain relief mechanism to prevent the cord and the charge controller from getting accidentally yanked apart.
The first non-rainy day on our recent excursion I plugged in the panel and moved it about every 2 hours to best catch the sun, and the battery was almost fully re-charged when the sky got cloudy late in the day. Maybe 2 hours after that I checked the inside voltage meter which is inserted into the DC outlet, and was alarmed to find the battery almost fully discharged. So I thought maybe, without direct sunlight, current had been running from the battery to the PV panel, which would be a real pain in the ass. Sometime later, after we had the Honda generator running, we discovered the frig was on DC. Don't know how that happened, and at first wondered if plugging the trailer into an ac source caused the frig to switch. I don't recall that hooking up to ac ever caused the frig to switch off propane before, but this was our first use of the trailer this year, last year was our first, and we didn't get to use it as extensively as we had planned then.
Anyway, for now I am assuming that we somehow bumped into and thereby inadvertantly switched the frig over to DC, long before we started the generator, and it was the frig on DC that ran the battery down. Could this just be wishful thinking? And what do people do to prevent an accidental switch of the frig to DC?
My remaining question is this: the instructions that came with the PV panel said that it has to be grounded. Is this necessary with a single 65 watt panel? If so, what is the best way to get it grounded?
Posted 28 May 2009 - 03:58 PM
I see no reason the panel, itself, has to be grounded. Grounding (in an electrical power context) is merely a saftey issue. The panel puts out less than about twenty volts, so there is no hazard.
Perhaps someone else with more experience in specialized situations can comment, but I do not ground mine---and see no reason to do so.