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RV and Cabin Systems

Table of Contents:
Solar panels
Regulators (charge controllers)
Fuses, wiring, lights
Battery capacity chart
12-volt wiring sizing chart

The information found here is applicable to small cabins,boats,RVs and car camping and comes from ten years of running an alternative energy business, a year's worth of one to two month camping/installing trips to Baja, two years of sailing Central America, South America and the Caribbean, and two years of living off the grid in the Santa Cruz mountains; yet these are only my experiences and opinions. I hope to help you eliminate some of the trial and error I went through,
but at the same time can only touch the surface in this How-To solar primer. Everyone will have different power needs; everything from powering a transistor radio to powering a whole rock band and a single light bulb to a small village. So take what you can use here, buy what you can afford, and live within its limitations. In solar, we talk of the components in the following way: First we have the solar panels and then what's known as balance of system components. The balance of system consists of batteries, inverters, charge controllers, voltage meters, and generators. Of all these, the batteries are the real heart of the system and in terms of use the hardest to understand. You generally can't kill a solar panel except by shattering the glass and even then it still produces some power, but batteries are killed often times within a few months by the novice. With care, they can last four to ten years. Think of your first system as training wheels; be prepared to make mistakes and be prepared to shell out some money to get the gear. If your budget won't allow, then buy candles and forget it. In this technology you get what you pay for; cheap inverters and batteries are built to sell not for real use. Buy quality! Define what you want to do. If you just want a few lights in camp or power for the radio, then maybe a 12-volt DC system is all you need. But if you want to power devices from home that run on AC, then you need an inverter, and if the devices are sensitive loads (like audio equipment, laser printers, and certain quirky electronics) then you probably need a sine wave inverter, which will cost two to three times as much as the modified sine wave inverters. Figure out how much power you need. Use these ohms law formulas: amps = watts / volts
volts = watts / amps

Convert all the loads you want to run into amps. If it's a 12-voltdc device, it's already rated in amps. The charging source should have the amps ratings on the back of it as well . AC devices are usually expressed in watts,but the conversion doesn’t take into account that motorized loads will draw 2-3 times more during startup which means inverters will need to be sized for the surge . (A 500-watt blender ,full,will surge to 1000-1500 watts momentarily,a inverter capable of 1500 watt surges and appropriate fuse would be needed) Divide the watts by 12 volts, since that's the voltage the inverter runs on (500 / 12 = about 42 amps), and then multiply that by 1.15 to factor in inverter inefficiency (42 * 1.15 = 48.3). Now multiply the loads you want to run by the number of hours you want to run them (let's say a half hour every day: 48.3 * .5 = 24.15), which gives you the amp hours you need. Your battery should be rated at least twice the size of your needs (so we need a 50 amp hour battery, minimum). Finally, figure on a means of charging the battery that will supply that many amp hours back into the system every day.

Another example: we want to run a radio that draws 4 amps at 12 volts, and we plan on using it 3 hours every day, and we want to run 2 lights that draw 2 amps each for 4 hours each night. We have

4 amps * 3 hours      = 12 amp hours, and
(2 amps * 4 hours) * 2 = 16 amp hours.
                   Total 28
We need a battery with a capacity of at least 58 amp hours. We must replace 28 amps every day, and a solar day will be
around 4-6 hours of charging time, so we need a solar panel that will put out 28 amps in 6 hours, which is about 4 and
a half amps per hour. The wattage required is the output voltage of the solar panel (usually 17.1 volts) times our 4.5 amps,
which is roughly 80 watts (round up to be safe). Solar panels are price- compared by looking at dollars per watt, and small
panels sell for anywhere from $7 to $16 per watt, and larger 75 to 100 watt panels average $5.50 per watt. Estimate then
that an 80 watt panel will cost $5.50 per watt or $440. If you are saying ouch, then don't despair, because you have two

1. You can leave out some of the loads and live with a smaller system, or

2. If mobile or boat,you can use your own alternator to supplement some of the charging.

I would not recommend cheap AC generators as an option because they are time bombs, noisy, and for charging DC will only supply a few amps of DC output unless coupled to a large AC charger. TOP

Solar panels
There are a few different types of panels to choose from, so choose carefully as some are a better value. There are mono- and poly-crystallized panels composed of between 30 and 36 cells (round or square black discs mounted behind the glass). There are thin film(Triple junction)amorphous panels where the light- sensitive material is deposited on non- tempered glass or sometimes stainless steel in the case of flexible panels. Thin film is 30% less efficient than crystalline panels, and they degrade rapidly. They have the advantage of being producible in smaller sizes than crystalline panels and are common in cheap solar recharging products. For true portability, nothing can beat tedlar coated crystalline panels that are on aluminum plate with the cells mounted to the aluminum and covered with shatter proof plastic. In terms of power output, an important concern is voltage. The panel's peak output voltage is determined by the number of cells connected in series. To charge a 12-volt battery to 14.7 charge volts, one needs to have a panel that puts out at least 16.9 volts peak output. That number multiplied by the maximum current gives the wattage of the panel. Fewer cells = lower output voltage and works only in cooler climates. As cell temperature increases, the panel voltage drops; any fewer than 36 cells wont' get the voltage high enough to do a full charge. Stay away from self- regulating panels, as their fewer cells limit their voltage; when they heat up, they're too low to charge to the max. Be sure to always use a separate regulator to automatically prevent the opposite problem; overcharging. The addition of an ammeter coming off the panel will show amps going to the battery, and a voltmeter will show the battery charging voltage, hopefully as high as 14.5 to 14.8 volts. An hour or two at these voltages is considered a full charge. TOP

Your car or boats alternator supplies 35-100 amps and typically tops off the starting battery in the first ten minutes of running. After that it could easily supply power to a second deep cycle auxiliary battery. The easiest way to do that is to come off the positive of the starting battery and go through a solenoid or electrically controlled gate that passes power on to a second battery only when the engine's running, thereby preventing drain down of the starting battery when the engine is off. They are simple systems to install, work well, and a solar panel is easy to add for those times when you don't anticipate running the engine. I suggest such a system unless you don’t plan on using the engine at all, in which case you are looking at what's known as a "stand alone system" or a solar- only charging system.Warning: Some alternators are too wimpy for heavy charging . Check their temperature by touching the case,too hot to keep a hand on is a sign of overload. TOP

Regulators (charge controllers)
The best small regulators are pulse width modulated or PWM controlled. Look for that feature no matter what size regulator you’re using. Sizes are based on the maximum amps the regulator can handle; allow for future expansion when deciding on a size. If you had a 60-watt panel that put out 3.5 amps, you'd be looking at least for a 6-amp regulator to be on the safe side. Be sure to add a fuse between the panel and the regulator, if the unit doesn't have one. The better regulators come with temperature compensation and ideally with some means to equalize the batteries. TOP

The least expensive decent meter is the handheld multimeter. After that, a panel mounted digital will be the least costly. Do not buy or use the dial type or analog meters, as they have terrible accuracy. Look at the battery capacity chart voltage figures below and imagine trying to get 0.1 volt accuracy with an analog meter -- impossible! TOP

An ammeter is useful to measure current output from the solar panel. Put it on the battery side of the regulator. The simple analog types are fine. They come in 0-5, 0-10, 0-20, 0-30, and 0-60 amps. Pick one that's just above your maximum output. Fuses,Wiring & Lights:Fuses can be purchased at electronics, auto parts, and marine stores, at RV shops, and of course mail order. Class T fuses are generally used between inverters and batteries, if the inverter is 500 watts or greater. While fuses are easy to find, good holders are harder to come by.. Match fuses to loads by fusing to 25% to 50% more than the maximum load. Use flexible wire that is sized for the current and the length of run. This is very important. If there is any doubt, use oversize wire. Keep wire runs short to limit voltage drops. Most alternative energy catalogs have wire sizing charts in the back. The most efficient and expensive lights are white LEDs. Next in efficiency are compact fluorescent lights. They come in 12-volt DC, but the AC ones are the most common and least expensive. Next are halogens which put out a very white light and make great spotlights. Standard incandescents are the least efficient and have a short lifespan. Reflectors can be used to add to any light's effectiveness. TOP

Fuses, wiring, lights
Fuses can be purchased at electronics, auto parts, and marine stores, at RV shops, and of course mail order. Class F fuses are generally used between inverters and batteries, if the inverter is 500 watts or greater. A type R hardware store fuse could be used. If you drill 5/16- inch holes in the blades coming off its ends, you can bolt the fuse directly to the battery. Otherwise a holder is necessary. While fuses are easy to find, good holders are harder to come by. Mail order may be the best bet. Match fuses to loads by fusing to 25% to 50% more than the maximum load.

Use flexible wire that is sized for the current and the length of run. This is very important. If there is any doubt, use oversize wire. Keep wire runs short to limit voltage drops. Most alternative energy catalogs have wire sizing charts in the back.

The most efficient and expensive lights are white LEDs. A good compromise is compact fluorescent lights. They come in 12-volt DC, but the AC ones are the most common and least expensive. Next are halogens which put out a very white light and make great spotlights. Standard incandescents are the least efficient and have a short lifespan. Reflectors can add to the light's effectiveness. TOP

Inverters are the devices that take the battery power (DC) and convert it to 120 volts AC or household power, which is what most tools and appliances run on. Inversion is done one of two ways: 1) transformers, 2) high speed switching circuits. The quality of the output is defined as: Modified square wave and Sine wave, which is what grid power runs in. The modified square wave inverters are less expensive and more efficient, but the sine wave inverters have a cleaner output which allows one to run certain delicate electronics and play audio equipment without the hum associated with modified square wave inverters. For running compact fluorescent lights, tools, laptop computers, and most appliances, the modifieds are fine. The cheap small inverters are all modified square wave, and use switches to make the power. More expensive but far superior for running motorized loads are the transformer type inverters. These start at 500 watts and go up to 5,000-watt sizes and come with optional battery chargers built in. The Trace UX series are real work horses in the 500W and 1100W sizes. Stat Power builds good small cigarette lighter plug-in switcher type inverters. "You get what you pay for," and it applies to all the equipment in an alternative energy system, especially inverters. Make sure to put the appropriate fuse between the battery and inverter. TOP

Batteries are the oldest equipment in the system, and even with many newer battery technologies the plain old flooded lead-acid deep cycle battery is hard to beat for weight, cost, availability, and endurance. Buy quality if you want longevity. The purer the lead used, the better the battery. The best are Rolls & Surrette, with Trojan and Deka building good mid-range batteries, followed by U.S. Battery, with Exide being cheapest and least durable. TOP

What makes a deep cycle battery vs. a car starting battery? The number and thickness of lead plates and the amount of space below the plates. A car battery needs lots of up front power to turn the motor over and only gets run down 10%.It will not last through repeated deep discharge. A deep cycle battery is rated in amp hours at 80% of its capacity and has fewer and thicker plates. For longevity, only cycle them down 50%. If the battery is rated 80 amp hours, to play it safe only withdraw 50% of that or 40 amp hours. So with this battery for example, a 4 amp load should be shut off after 10 hours of use (4 amps x 10 hours = 40 amp hours, the maximum safe discharge). There is an easy way to determine remaining capacity; with an amp hour meter that counts the credits and debits to your battery, and spits out a percent at the push of a button. The second cheapest alternative is to use a voltmeter to read the voltage when the battery is resting (see chart below); no charge/discharge occurring for at least an hour. For example, 100% = 12.7V, 75% = 12.4V, 50% = 12.2V, 25% = 12.0V, and discharged = 11.90V.

To use these figures, one must have an accurate digital voltmeter. Throw away any dial type (analog) voltmeters, as they are worthless as battery monitors. A cheap digital meter costs $30 to $60. Are your batteries worth it? If not, then at least spend $10 on a cheap hydrometer and put up with the mess of dipping it into all those holes and measuring the specific gravity of the cells. Specific gravity readings will read more or less like this: 100% = 1.265, 75% = 1.225, 50% = 1.190, 25% = 1.155, discharged = 1.120. Till now, I've only talked of flooded batteries. Of course, there are ni-cads, nickle- metal- hydrides, gelled electrolyte lead acids, and A.G.M. or absorbed glass mat lead acid batteries.

If you are only looking for 10 to 50 amp hour batteries to run a tape player or some small load, then these might be good possibilities, but be forewarned they are all more expensive technologies and more complicated to maintain correctly. I've seen enough gel batteries hit the trash to know they are problematic. They absolutely will not tolerate being charged beyond 14.2V. The subject of batteries is far too wide for this primer, so I'm only focusing on flooded batteries. Monitoring can be a slippery deal, almost like checking the air pressure in your tires without coming to a stop.

A few hints: we've already discussed reading a battery at rest, but what if it's being charged while you're reading it? The voltage will be driven higher as the battery is being charged and around 14.5 volts depending on the temperature, the acid will start to bubble. A little bubbling is good, but a strong bubbling is burning off the water, and most regulators cut back the charging to prevent this. Typically most charge controllers (or regulators as they are also called) will sense battery or ambient temperature and compensate -- colder / more voltage and warmer / less voltage. So here's your key: when you see the battery voltage peak out in the upper 14s then you have more or less charged the battery fully and can consider the battery at 100%. Below that is real "seat of the pants" gauging.

On the other side of the coin, if the battery is being discharged as you monitor it then the numbers will read lower. There's no sliding scale to apply "seat of the pants," so try to stick with reading resting voltage and hopefully see that your batteries are getting up to 14.5 - 14.8 every day or two. Battery size: The most common sizes are group 24/80 amp hours and group 27/105 amp hours. I have an 80 amp hour battery as an auxiliary in my truck that supplies my 800 watt inverter. If I'm running large loads then I start the engine and let the alternator pour some current in to keep the volts high to give the inverter more grunt. My alternator charges both the auxiliary and starting batteries simultaneously while I drive.

When I camp for days at a time, I usually bring along a solar panel to supplement the charging. I use to have the bigger 105 amp hour battery, but felt that I couldn't justify the extra weight for the occasional camping trip so went smaller. The battery must match the inverter to some extent; the larger the inverter and load on it, the larger the battery needed. For larger systems, I'd recommend 6-volt golf cart batteries or L16s hooked up in series. Even a 600 watt inverter running at maximum output will draw the voltage down quickly on a group 27 battery. When the inverter sees lower voltages, its output drops in proportion. Even if you had a 1000 watt inverter and it was coupled to a small battery, you would only see 1000 watts for a few minutes. So don't scrimp on capacity! TOP


    Please do not use your car's starting battery for anything but starting the car. Get a deep cycle battery that's isolated from the starting battery. Bolt it down securely and fuse it.

    If you plan to use the horribly inefficient portable electric ice boxes with the cigarette lighter plugs, add ice to them to cut down run time and always keep them in the shade. They will kill your battery within a few days without supplemental charging.

    Wind generators:. They only work when there's lots of consistent wind, and they need hefty towers to get above ground level.They are not practical for camping but have their place on boats and in cabin systems that are proven wind sites.

    Most inverters (except the more expensive sine wave ones) produce noticeable hum in audio equipment and sometimes won't start cheaper compact fluorescent lights. Buy only quality brands because the cheap ones are inefficient, unreliable, and noisy time bombs.

    Secure solar panels. They will break if bashed hard enough. Make sure the wind cant flip them if used portably, and place the wires so people don't trip over them. TOP

Battery capacity chart

Percentage of Charge 12V Battery Voltage 24V Battery Voltage Specific Gravity


12.70 25.40 1.265


12.64 25.25 1.257


12.58 25.16 1.249


12.52 25.04 1.241


12.46 24.92 1.233


12.40 27.80 1.225


12.36 24.72 1.218


12.32 24.64 1.211


12.28 24.56 1.204


12.24 24.48 1.197


12.20 24.40 1.190


12.16 24.32 1.183


12.12 24.24 1.176


12.08 24.16 1.169


12.04 24.08 1.162


12.00 24.00 1.155


11.98 23.96 1.148


11.96 23.92 1.141


11.94 23.88 1.134


11.92 23.84 1.127
Discharged 11.90 23.80 1.120

Specific gravity values can vary + or - 0.015 points of the specified values. This table is for the Trojan L-15 battery in a static condition, no charging or discharging occurring, at 77 degrees F. Discharging or charging will vary these voltages substantially. Source: Trojan Battery Company TOP

12-volt wiring sizing chart
The following chart gives the maximum distance one-way in feet of various gauge two- conductor copper wire from power source to load for 2% voltage drop in a 12V system. Do not exceed the 2% drop for wire between PV modules and batteries. A 4 to 5 percent loss is acceptable between batteries and lighting circuits in most cases. To allow for a 4% loss, double the lengths given in the chart.

Amps #14 #12 #10 #8 #6 #4 #2 #1/0 #2/0 #4/0
1 45 70 115 180 290 456 720 - - -
2 22.5 35 57.5 90 145 228 360 580 720 1060
4 10 17.5 27.5 45 72.5 114 180 290 360 580
6 7.5 12 17.5 30 47.5 75 120 193 243 380
8 5.5 8.5 15 22.5 35.5 57 90 145 180 290
10 4.5 7 12 18 28.5 45.5 72.5 115 145 230
15 3 4.5 7 12 19 30 48 76.5 96 150
20 2 3.5 5.5 9 14.5 22.5 36 57.5 72.5 116
25 1.8 2.8 4.5 7 11.5 18 29 46 58 92
30 1.5 2.4 3.5 6 9.5 15 27 38.5 48.5 77
40 - - 2.8 4.5 7 11.5 18 29 36 56
50 - - 2.3 3.6 5.5 9 14.5 23 29 46
100 - - - - 2.9 4.6 7.2 11.5 14.5 23
150 - - - - - - 4.8 7.7 9.7 15
200 - - - - - - 3.6 5.8 7.3 11



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