Solar energy, battery sizing and charging theory, from VSTF
Aug 5, 2013 8:43:04 GMT -8
Sam, eternalnoob, and 1 more like this
Post by cowcharge on Aug 5, 2013 8:43:04 GMT -8
Hey all! Someone suggested that I repost this from the old forum, so here it is! I tweaked it some to make it more clear (I hope) and fix some typos. I also changed all my AC calculations from 120v to 110v, as I think it's more accurate. Sorry 'bout the length (and the metaphors), but I can never seem to explain something with brevity. I try to write so that there are no questions at the end, even for people who have never seen a battery. You never want to hear me try to explain a single joke from a movie, it takes longer than watching the movie would have done. You've been warned...
There's been some chatter here about solar recently, and I've been working on my own design and reading everything I can find, and I spent a couple of years working for a residential solar contractor, so I thought I'd post some basic theoretical principles and guidelines to help those considering adding solar to their campers, and to help with choosing the size of your battery bank, solar or no solar. This isn't all-inclusive (because I'm not Tesla), but it should get you started on the right track.
First of all, I urge people with the budget to afford it to design their system based on their projected electrical needs, rather than picking up a solar panel or a battery and building from there. It is the single most important concept in having no electrical issues when charging is limited to what you can bring with you.
If you think of it in terms of food, given the choice, you wouldn't want to base your diet on a predetermined, limited food supply, because if it isn't enough for your metabolism and activity level, you would either have to limit your activity or face declining health. In terms of expressing it as an equation, that would make your food supply a constant, and your lifestyle the variable answer to the equation, making your activities dependent on the food supply like you're on Survivor. In that case you either must limit your activity to what the calories in your limited food supply can provide, supplement it with a soup kitchen or panhandling, or live with losing weight and hope you don't starve before you beg a meal somewhere. It's much better to base the amount of food you use upon your dietary needs as dictated by your lifestyle, so that your health remains good, yes? Let your lifestyle be the constant, and make the electrical system the variable to be calculated! If you can afford it.
Now if you only camp where there are 100% reliable hookups, then you already have a virtually endless food supply, and can save yourself the tedium of reading this. But if you boondock or the power goes out, proper design can make the difference between an electrically worry-free trip, and trying to decide what appliances you have to use less or stop using in order to make your batteries last the whole trip. And to avoid frequent battery replacement due to repeated over-discharging.
In the case of the battery system of a camper, your "food supply" is the energy stored in your batteries when you get there, and the various sources of charging/operating electricity available: solar, hookups, and generators, and your 'diet" is the electricity you use on a trip. Ideally, for self-sufficient electrical "health", you want to build a system that can "feed" you for your entire camping trip, without forcing you to go to the soup kitchen of a hookup, or to panhandle from your generator. And to have leftovers in the fridge for that impulsive midnight snack.
I'm not including wind power in this post, because the "crop yield" varies wildly, and your neighbors won't much like the smell of it cooking (it's noisy, and pretty expensive, and can be dangerous if not supported properly, or designed to handle high winds when they pop up). But small, battery-charging wind works MUCH better than grid-scale wind in the right location, and could certainly be useful in the right place. It's just tricky to use a "portable" wind system, especially getting it over the trees (and sturdily mounted) to get good, steady winds to make it worth the money.
Of course, if you can't afford a complete solar diet (or you don't have the roof space without spoiling that beautiful canned-ham profile), you use the solar you can afford to supply what it can, and then make up the difference with a generator, or a jump from the truck.
I'll start with just 12v DC appliances for the basics, and pretend we're boondocking with no battery charging capability at all, to determine the size of the battery bank I'd need for a three-day trip.
The key math you need is one simple equation: Watts = Amps x Volts. If a 12v light constantly requires 2 amps to run (as the old bulbs in my '76 2250 do), that makes it a 24-watt light (2A x 12V = 24w). An 800-watt, 110 volt microwave requires 7.27 amps AC to run (800w / 10v = 7.27a). That equation and some simple arithmetic will allow you to design your whole 12v system.
Every electrical appliance sold in the U.S. has (or had) a data plate or sticker that outlines its maximum electrical draw. I've included a link to a google pic of one here. As you can see on the left side, it draws 3.2 amps, and I believe it says 90 volts (3.2a x 90v =288 watts).
I've read that most modern appliances such as TVs draw much less than their dataplates say they will, but it's smart to plan as if it will use the whole 3.2 amps, in order to err on the side of having extra battery capacity (good for rainy days). The most accurate method would be to put an ammeter on the positive cable close to the battery while it runs. The data plate may be labeled in watts or amps or both. The number we need is amps, so if it is labeled in watts, just divide that by the voltage to get the amps.
Some appliances with motors, like Skil saws, compressor refrigerators (residential or "dorm" fridges, as opposed to motorless absorption fridges like the Dometics and Norcolds that came in the old trailers) and A/C units, can briefly draw much more current when they start up, called "surge" draw (it is why generators and inverters have both continuous and surge power ratings) but that is mostly of concern to us when trying not to overload our fuses and breakers, generators, or inverters (don't start them all at the same time, and don't start up a big-motor appliance when you're already near the limit of your fuses), not so much when thinking of batteries and solar panels, because it is only a brief surge while the motor gets up to speed.
Deep cycle batteries are rated in amp-hours of storage capacity. An 80-ah battery can supply 80 amps for 1 hour before it's totally drained. Or 1 amp for 80 hours, or 40 amps for two hours, and so on. So, using that one equation, W = A x V, the number you need to find is the amps drawn by each appliance. Then you multiply the amps by the number of hours you will use that appliance every day to get the amp-hours you need in capacity to feed that appliance every day. To use the old 12v, 2 amp incandescent ceiling light bulbs in my camper as an example (making them 24-watt bulbs [2a x 12v = 24w], or perhaps they are technically lower wattage bulbs that just draw part of the 2a due to wire losses from resistance, rusty old light fixtures or wire length, but either way they actually draw 2a at the battery according to my ammeter), if I want to run one of them for six hours every night, I need 12 amp-hours (ah) of supply each night for that one bulb (2a x 6hr = 12ah). And on a 3-day trip with no battery charging capability, I would need 36ah of battery capacity to run just that one light (12ah/day x 3 days =36ah/trip).
Let's say my water pump draws 5 amps, and I use it for ten minutes a day for a shower, and perhaps another ten minutes for all the other uses, like washing dishes, cooking and filling my drinking glass. That is 5a for 20 minutes per day, or .33 hours. 5a x .33h = 1.67ah/day. For the same three-day trip, that makes 5ah/trip (1.67ah/day x 3days =5ah/trip). So to run one light bulb and my water pump for a three-day trip with no battery charging capability, I would need 41 ah (36 + 5). Batteries don't like to be discharged below 50% (they only have so many deep discharges available in their lives before they fail, and not using more than 20% is even better for their life expectancy), so you need to double the amp-hours of demand, when sizing your battery bank. In the example above, if I want my batteries to last more than a year, I would need to buy at least an 82 amp-hour battery. The demand can add up quickly when you add stuff like fans or 110v appliances running through an inverter, so building a battery bank that can handle a long trip or high daily demand without charging gets expensive. And heavy.
Some appliances like forced-air furnaces and refrigerators are more tricky to assess, because they don't run all the time, but have instead what's called a duty cycle. For example, by not-really-paying-close-attention observation, my 4-amp furnace blower runs about 1/3-1/2 of the time on a Maine winter night, depending on how cold it is. It would be more accurate for me to time it to see what its duty cycle is at various temperatures. But if I do my calculations based on it running at a higher duty cycle than I know it uses and size my batteries to accommodate that usage, it would give me a safety margin in battery capacity, which never hurts.
How often a refrigerator turns on and off depends on the ambient temperature, the temperature setting of the fridge control, the amount of food inside, and how often and for how long you open the door, so it would be very tricky to estimate its duty cycle accurately. You could sit there on a hot day and time how often the motor turns on and how long it runs, while the kids run in and out grabbing sodas, in order to get a ballpark figure. Maybe you'd find it runs for 10 minutes 4 times every hour, making its duty cycle .67hr ([4cycles x 10min] / 60min/hr = .67hr, or a 67% duty cycle) But again, it's safer to think of it as running constantly for our calculations in order to have a built-in safety margin in battery capacity. You could conceivably end up wasting money on batteries too large for your needs if you overestimated the usage of too many appliances, but for one fridge or a furnace it probably won't be that bad. It's a judgement call that you have to make in consultation with your wallet. Hey, I can't do everything for you.
In order to accurately assess your total electrical diet, you need to add up all the amp-hour requirements for every appliance aboard your camper, just like I did with the light and water pump. If you can't find a data plate, then put an ammeter on the + battery cable while only that appliance is running and find out what it draws. It's more accurate to do that anyway, even if there is a plate on the appliance, because measuring at the battery will include the current needed to get the electricity down the wire to the appliance as well as that actually used by the appliance. Wire losses don't really affect 120v stuff much (they affect 12v stuff greatly), but accuracy is its own reward, so it's still better to measure than to assume the data plate is correct. Plus you may learn that they use a lot less than the plates say they do, allowing you to relax a bit on the battery size. Or maybe you'll find out they draw a lot more than it says on the plate, meaning the appliance is getting worn out, or maybe that you have wires that are too small or too long, or a bad ground that is causing too much resistance. Or that mice have hooked up an alternator to your motor to run their home theater.
Once you've added up all your appliances and estimated the time each day that you will use them, you are now able to calculate the size of the battery bank and solar system needed for your trips.
Again, batteries die an early, tragic, horribly painful, drawn-out death if discharged below 50% very often, so size your battery bank at twice the size of your between-charges electrical diet.
A few notes on battery types and their lifespan:
Lead/acid batteries work by a chemical reaction of sulfuric acid reacting with a lead oxide plate (the positive plate or cathode) and a lead plate (the negative plate or anode), turning both plates partially into lead sulfate. The sulfuric acid electrolyte becomes less acid and more water as the battery discharges. The more discharged they become, the more lead sulfate is deposited on the plates, and the more diluted the acid electrolyte becomes. When you recharge them, the sulfate on the plates is converted back to sulfuric acid in the water, and the plates return to lead and lead oxide. The more often and more deeply you discharge batteries, the more lead sulfate is deposited on the plates, and the less of it gets converted back into acid when recharging. This is what eventually kills most batteries, the increasing inability to reverse the chemical reaction through charging, called sulfation. The buildup of sulfate on the plates clogs the porous plates, reducing surface area and inhibiting the reaction, as well as reducing the amount of lead and acid available to generate electricity. And charging, especially overcharging, "boils" off water, making checking water levels a required item of maintenance every month or so. This boiling off is actually the water being broken down into oxygen and flammable hydrogen, which is why wet batteries need to be vented outside, and why sparky electrical components like charge controllers should not be in the battery compartment. Batteries can also release lead sulfide gas, which can corrode electronics.
There are three main types of wet-cell lead-acid, "flooded" batteries. I'm not including gel or AGM batteries here, because of their cost. They have their advantages and disadvantages, but cost keeps them out of my camper (along with beautiful young women). The three wet-cell types are car batteries, marine/deep cycle batteries, and true deep cycle batteries like those used in golf carts, wheelchairs, Hoverrounds, and forklifts.
Car batteries are inadequate for campers. They are designed with thin, porous lead plates in order to maximize the surface area for the chemical reaction, to allow them to both send and receive large amounts of current quickly. It takes a large surge of current to turn over a cold engine, but it is of very short duration, hence the larger surface area. But the batteries are never supposed to get discharged deeply, and are also recharged quickly by the high-amp alternator (a stock '94 Ford van has a 95-amp alternator). Leaving your lights on all day or cranking the stereo at the beach will shorten the life of a car battery greatly, as will using it in a camper, unless it's for a bare-bones, one-light-and-a-radio type of system that is recharged every day. What happens is the thin plates get eaten away by the extended chemical reaction and covered in lead sulfate, leaving not enough surface to convert the lead sulfate properly back to lead, which sulfate ends up flaking off and dropping down to the bottom of the battery case, where it eventually piles up and shorts out the plates, killing one or more cells. They will work, for a while, but I wouldn't use car batteries in my camper unless that's all I could afford.
"Marine/deep cycle" batteries are a little better, but are still mainly designed to start engines and be recharged quickly by an alternator, so again they are not really suitable for camper use. They are more robust than car batteries, but will still die an early death if repeatedly discharged deeply. Again, if that's all my budget allowed or my minimum system required, ok, but it wouldn't be my first choice.
True deep cycle batteries like those used in golf carts are made with much heavier lead plates because they are not expected to have to put out all their energy at once to start an engine. They are larger and heavier. These are the batteries you want in your camper, if you want the most efficient, longest-lasting battery bank for your money. Two 6v golf cart batteries wired in series to make 12v will last years longer than either of the other two types, and don't cost much more than car batteries unless you buy AGM batteries. I just bought two Duracell EGC2 6v 230ah batteries at Sam's Club yesterday for $216 (I was charged an extra 10% because I'm not a member, I used a day pass). While one trojan 230ah 12v AGM battery sells online for $565. Mine are of the GC2 form factor (not much bigger than a truck battery), and weigh 63 pounds each, so I can (kinda) easily move them, unlike the two 25-year old, ex-residential solar, 113-pound L-16s they replaced (A lot of fun dragging them 200 yards across the tundra yesterday in a plastic toboggan and lifting them into the truck, then out of the truck and into a shopping cart). When combining two 6v batteries in series, the voltage doubles but the ah capacity stays the same as one battery, so my total capacity is still 230ah. If you use multiple 12v batteries in parallel, then the ah capacities are combined (2, 80 ah 12v batteries in parallel = 160 ah).
If a battery is rated in Cold Cranking Amps (CCA), it is an engine-starting battery, either car or marine, as that is a measure of its ability to instantly convert that stored chemical energy into massive amps. That also means it will not give you the long life you should expect out of a camper battery. A true deep cycle battery will be rated in amp-hours, not cranking amps, as that is a measure of the size of its storage capacity, not its 0-60 horsepower.
Deep cycle batteries are marathon runners, good for the long slow haul, while car and marine batteries are sprinters, and get tired quickly.
Just to make sure that horse is really dead, whatever kind of battery you get, the longest life comes from NEVER discharging them below 50%. So when you figure your electrical needs in amp-hours, double it (at least) to get the battery bank size you need to survive between charges. Again, if you can afford the price and weight of a bigger bank, 20% maximum discharge is even better and will give you years of life.
My new 230ah bank, assuming a full charge and allowed to only discharge 50%, therefore using up no more than 115ah before recharging, would allow me to run my 4a furnace (assuming constant running at a 50% duty cycle, a conservative estimate unless it's wicked cold out, especially once I finish insulating) along with a 2a light bulb on 24/7, for 28.75 hrs before recharging ( 115ah / {[4a x .5 duty cycle] + [2a x 1.0 duty cycle]} = 28.75hr. So I could go a full day in a blizzard (covering my solar panel with snow), with it 75 degrees inside, with a light to read by, without going outside to start the generator, and I could probably still listen to the radio, watch a lil' telly and use the water pump without hurting my batteries. Gives me a nice sense of security for $216. And that's heating a 22.5' trailer in Maine, where it was 14f on Christmas Eve 2011 when I camped out. Once I get LED lights I'll be able to keep ALL the lights on during that period. Think what you guys in a little Compact, with a fanless Wave heater, could do with all that power... Surround sound, anyone?
110 volts and inverters:
When you start plugging 110 volt appliances into an inverter and running them off batteries, your consumption goes way up. It takes approximately 9.2 amps DC from the batteries to create 1 amp AC through an inverter. That is because, aside from inefficiency losses, the wattage going into the converter from the batteries equals the wattage going out of the inverter to the 110v appliances, and lower voltage for the same wattage means more amps. So a 1500 watt AC hair dryer also needs the same 1500 watts, but in DC @12v, plus a little for losses in the inverter, coming out of the batteries. From our power formula, 1500 watts AC @110v = 13.6 amps AC. And the same 1500 watts @ 12v DC = 125 amps DC out of your batteries (ouch)! 125a DC/13.6a AC = a conversion factor of 9.2.
So to calculate how many amps DC you need to produce the amps DC required by a 110v appliance, multiply by 9.2. A 45-watt LED tv like the ones I saw at Sam's yesterday (it's awesome how energy efficient they're getting) would need .41 amps AC to run (45w / 110v =.41a). From an inverter, that means we would need 3.76 amps DC from our batteries (.41 x 9.2), almost as much as my furnace blower or two 12v light bulbs. A 60-watt incandescent light bulb, 5 amps DC (60w / 110v AC x 9.2 = 5a DC). My batteries' available 115ah would be used up in 55 minutes drying Rapunzel's hair (115ah capacity /125a draw =.92 hours, or 55 minutes). So be stingy with the high-amp AC appliances, which are those that use motors and/or heating/cooling elements. And use 12v lighting, especially LEDs, which take hardly any juice.
Now, finally, on to solar and recharging:
When boondocking you need to feed those batteries if you don't want them to starve to death. If you want to stay out indefinitely, then you need to put as many amp-hours back in the batteries every day as you use. In fact, because of inefficiencies, you need to put back more than you use just to break even. The easiest ways are through a generator or solar panels. As I said earlier, I'm not considering wind, as in most camper situations it's impractical and potentially dangerous. You could drag the camper into town and plug in somewhere, but again, not very practical. You can wire up the truck to charge the batteries off the alternator in an emergency, but that's very fuel-inefficient and noisy, although it's great while towing, and I have a battery isolator switch in my truck for that purpose (any auto parts store has them, and they must be matched to the amp output of your alternator).
It takes a long time to fully charge batteries, in part because typical converters don't charge at a high enough voltage to really pack the batteries full of amps. From what I've been reading, a wet-cell golf cart battery likes to be charged at 14.8v (despite what the converter manufacturers say), even higher in cold weather, and most commonly-accepted-as-good converters only put out 14.4v, and they fairly quickly drop down to 13.6 or so, unless you force them to stay in bulk mode. The amps also drop over time, so the closer you get to charged, the slower it charges. The higher the charging voltage, the faster the amps get pushed into the battery. Too high though, and you'll boil all the water out of them.
One guy I've been reading, who thinks almost every RV manufacturer and solar installer is incompetent (who's been full-timing with his wife for ten years without a generator and with only rare hookups, who isn't selling anything, whose writing rings scientifically true to me, and who spends a lot of time helping people fix their solar installations so they work), says that basically you can never get your batteries fully charged using a converter unless it's plugged in 24/7 with no loads. He says the idiot lights in manufacturer's solar installations say that the batteries are charged when they're at the same state that his are after three days of no sun on his panels, and that it's no wonder you see all these "state of the art" trailers covered with panels yet running their generators all day, with their owners complaining they can't go a whole evening on the batteries. The manufacturers and solar installers yell at him and call him a crackpot, but he says that they're either ignorant or are only in it to sell more solar panels... He asks why, when you go to an "expert's" shop, that his solar-equipped rig is always plugged into an outlet. I'm fond of crackpots, myself.
Anyway, for calculation purposes, let's say my camper's appetite per day is 100ah. Since I only have 115ah to play with, that means I have to recharge every day or switch to 18th-century mode and use candles. You can typically get about 6 hours per day of good, full-blast sun up North in the summer. 6 hours into 100ah equals 16.67 amps I need to push into the batteries for those 6 hours to keep up. That means I need 246 watts coming out of my charge controller (16.67a x 14.8v = 246.7w). This is assuming I have a good, adjustable controller that I can set to charge at 14.8v. That means I need at least 250 watts of 12v solar panels on my roof, assuming no losses, which there always are. Those losses can be minimized by using much larger wires and shorter wire runs from the panels than those used by typical installers. And the proper wire size will get you more power than an expensive MPPT charge controller, says the crackpot. He says his system is efficient because he uses 6-gauge wire from panels to controller with only 12 feet of run, and 4-gauge from the controller to the batteries. Again, he full-times and doesn't own a generator, and runs power tools, a Mr. Coffee, a TV and a sewing machine, and doesn't have to worry until it's cloudy for three days, so he must be doing something right. Another big power killer that he sees from the "pro" installers is shadowing of the panels. He's got some pictures on his site of installations where there are only tiny shadows from the corner of a fridge or roof vent, that cut the panels' output by a third, or half , or even 90%. One little leaf on your solar panel basically means you'll never get your batteries charged.
I just bought a big, 220-watt, 12-volt residential panel from my ex-boss, which, if used with the right controller and wires, should let me shove about 15 amps into my batteries while the sun shines. Assuming the 100ah usage and 6 hours of sun, that's going to leave me about 10ah short every day, meaning I'll have to find another 10ah somewhere, either through running my little 800-watt generator for an hour or so, or hooking up to the truck. I'm going to build a motorized tracker though, so perhaps that will make up the difference. Panels mounted flat on your roof lose a lot of power due to the bad angle at which the sun hits them. The crackpot's system is tiltable for elevation but not sun-tracking, so while he has the best absorption angle vertically, early in the morning and late in the afternoon his panels aren't generating what they could, because the sun is striking them from a side angle. Most RVs seem to use multiple small panels, so perhaps he thinks it would be too expensive and complex to make them all track the sun. My plan is to use two large panels, which will make it easier to build trackers.
Figuring all this out is a lot like the scenes in Apollo 13 where Gary Sinise is in the simulator, looking for every available amp to keep the real spacecraft running long enough to get back to Earth. Well, that is, other than the whole "they'll die if we don't get it right" thing. We won't die if we get it wrong, we'll just be sitting in the cold and dark, bored to death.
I plan to full-time in my camper, so ideally I want to get to the point where I don't need a generator, like the crackpot. You folks with canned-ham Compacts have smaller, curved roofs with a sexy profile that you probably don't want to spoil with panel mounts, where on my boxy '76 2250 I have a big flat roof, with enough room for another panel of the same size (65" x 37" IIRC), when I can afford to add another one. You would probably have to use multiple smaller panels to get the kind of charging capacity I will have, or use a portable ground mount. Mounting panels off the camper has its own disadvantages, like long wire runs and the possibility of theft. More 'n' likely, if you add solar to a compact, it will be a supplement, not the mainstay of your charging system, just because of space and complexity.
So what it comes down to is this: Calculate your usage per day for all appliances (don't forget "phantom" loads, many appliances like TVs use electricity as long as they're plugged in, even when turned off, so disconnect stuff from your inverter when you're not using it). Then multiply it by the number of days in your longest boondocking trip, and if you want to camp without charging at all, get a battery bank that is twice the size of that total trip usage figure. If you can charge every day, get a battery bank that is twice the size of your DAILY usage. For totally relying on solar in sunny weather, you need enough panels to generate enough amps over six hours (your usable solar time will vary with latitude, weather patterns and panel orientation) to replace your daily usage. That is with no reserve for rainy weather. Going whole-hog is installing enough panels and batteries to replace more than your daily usage, so you can stay ahead of your diet and have some reserve for rainy days, like the crackpot's system. Any gaps can be made up with a generator or jumper cables, but ideally your solar keeps you charged. I hope this clarified things a bit instead of making them more confusing, and that people will find it useful.
If anyone has anything to add or correct, feel free!
The crackpot's site: handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/
There's been some chatter here about solar recently, and I've been working on my own design and reading everything I can find, and I spent a couple of years working for a residential solar contractor, so I thought I'd post some basic theoretical principles and guidelines to help those considering adding solar to their campers, and to help with choosing the size of your battery bank, solar or no solar. This isn't all-inclusive (because I'm not Tesla), but it should get you started on the right track.
First of all, I urge people with the budget to afford it to design their system based on their projected electrical needs, rather than picking up a solar panel or a battery and building from there. It is the single most important concept in having no electrical issues when charging is limited to what you can bring with you.
If you think of it in terms of food, given the choice, you wouldn't want to base your diet on a predetermined, limited food supply, because if it isn't enough for your metabolism and activity level, you would either have to limit your activity or face declining health. In terms of expressing it as an equation, that would make your food supply a constant, and your lifestyle the variable answer to the equation, making your activities dependent on the food supply like you're on Survivor. In that case you either must limit your activity to what the calories in your limited food supply can provide, supplement it with a soup kitchen or panhandling, or live with losing weight and hope you don't starve before you beg a meal somewhere. It's much better to base the amount of food you use upon your dietary needs as dictated by your lifestyle, so that your health remains good, yes? Let your lifestyle be the constant, and make the electrical system the variable to be calculated! If you can afford it.
Now if you only camp where there are 100% reliable hookups, then you already have a virtually endless food supply, and can save yourself the tedium of reading this. But if you boondock or the power goes out, proper design can make the difference between an electrically worry-free trip, and trying to decide what appliances you have to use less or stop using in order to make your batteries last the whole trip. And to avoid frequent battery replacement due to repeated over-discharging.
In the case of the battery system of a camper, your "food supply" is the energy stored in your batteries when you get there, and the various sources of charging/operating electricity available: solar, hookups, and generators, and your 'diet" is the electricity you use on a trip. Ideally, for self-sufficient electrical "health", you want to build a system that can "feed" you for your entire camping trip, without forcing you to go to the soup kitchen of a hookup, or to panhandle from your generator. And to have leftovers in the fridge for that impulsive midnight snack.
I'm not including wind power in this post, because the "crop yield" varies wildly, and your neighbors won't much like the smell of it cooking (it's noisy, and pretty expensive, and can be dangerous if not supported properly, or designed to handle high winds when they pop up). But small, battery-charging wind works MUCH better than grid-scale wind in the right location, and could certainly be useful in the right place. It's just tricky to use a "portable" wind system, especially getting it over the trees (and sturdily mounted) to get good, steady winds to make it worth the money.
Of course, if you can't afford a complete solar diet (or you don't have the roof space without spoiling that beautiful canned-ham profile), you use the solar you can afford to supply what it can, and then make up the difference with a generator, or a jump from the truck.
I'll start with just 12v DC appliances for the basics, and pretend we're boondocking with no battery charging capability at all, to determine the size of the battery bank I'd need for a three-day trip.
The key math you need is one simple equation: Watts = Amps x Volts. If a 12v light constantly requires 2 amps to run (as the old bulbs in my '76 2250 do), that makes it a 24-watt light (2A x 12V = 24w). An 800-watt, 110 volt microwave requires 7.27 amps AC to run (800w / 10v = 7.27a). That equation and some simple arithmetic will allow you to design your whole 12v system.
Every electrical appliance sold in the U.S. has (or had) a data plate or sticker that outlines its maximum electrical draw. I've included a link to a google pic of one here. As you can see on the left side, it draws 3.2 amps, and I believe it says 90 volts (3.2a x 90v =288 watts).
I've read that most modern appliances such as TVs draw much less than their dataplates say they will, but it's smart to plan as if it will use the whole 3.2 amps, in order to err on the side of having extra battery capacity (good for rainy days). The most accurate method would be to put an ammeter on the positive cable close to the battery while it runs. The data plate may be labeled in watts or amps or both. The number we need is amps, so if it is labeled in watts, just divide that by the voltage to get the amps.
Some appliances with motors, like Skil saws, compressor refrigerators (residential or "dorm" fridges, as opposed to motorless absorption fridges like the Dometics and Norcolds that came in the old trailers) and A/C units, can briefly draw much more current when they start up, called "surge" draw (it is why generators and inverters have both continuous and surge power ratings) but that is mostly of concern to us when trying not to overload our fuses and breakers, generators, or inverters (don't start them all at the same time, and don't start up a big-motor appliance when you're already near the limit of your fuses), not so much when thinking of batteries and solar panels, because it is only a brief surge while the motor gets up to speed.
Deep cycle batteries are rated in amp-hours of storage capacity. An 80-ah battery can supply 80 amps for 1 hour before it's totally drained. Or 1 amp for 80 hours, or 40 amps for two hours, and so on. So, using that one equation, W = A x V, the number you need to find is the amps drawn by each appliance. Then you multiply the amps by the number of hours you will use that appliance every day to get the amp-hours you need in capacity to feed that appliance every day. To use the old 12v, 2 amp incandescent ceiling light bulbs in my camper as an example (making them 24-watt bulbs [2a x 12v = 24w], or perhaps they are technically lower wattage bulbs that just draw part of the 2a due to wire losses from resistance, rusty old light fixtures or wire length, but either way they actually draw 2a at the battery according to my ammeter), if I want to run one of them for six hours every night, I need 12 amp-hours (ah) of supply each night for that one bulb (2a x 6hr = 12ah). And on a 3-day trip with no battery charging capability, I would need 36ah of battery capacity to run just that one light (12ah/day x 3 days =36ah/trip).
Let's say my water pump draws 5 amps, and I use it for ten minutes a day for a shower, and perhaps another ten minutes for all the other uses, like washing dishes, cooking and filling my drinking glass. That is 5a for 20 minutes per day, or .33 hours. 5a x .33h = 1.67ah/day. For the same three-day trip, that makes 5ah/trip (1.67ah/day x 3days =5ah/trip). So to run one light bulb and my water pump for a three-day trip with no battery charging capability, I would need 41 ah (36 + 5). Batteries don't like to be discharged below 50% (they only have so many deep discharges available in their lives before they fail, and not using more than 20% is even better for their life expectancy), so you need to double the amp-hours of demand, when sizing your battery bank. In the example above, if I want my batteries to last more than a year, I would need to buy at least an 82 amp-hour battery. The demand can add up quickly when you add stuff like fans or 110v appliances running through an inverter, so building a battery bank that can handle a long trip or high daily demand without charging gets expensive. And heavy.
Some appliances like forced-air furnaces and refrigerators are more tricky to assess, because they don't run all the time, but have instead what's called a duty cycle. For example, by not-really-paying-close-attention observation, my 4-amp furnace blower runs about 1/3-1/2 of the time on a Maine winter night, depending on how cold it is. It would be more accurate for me to time it to see what its duty cycle is at various temperatures. But if I do my calculations based on it running at a higher duty cycle than I know it uses and size my batteries to accommodate that usage, it would give me a safety margin in battery capacity, which never hurts.
How often a refrigerator turns on and off depends on the ambient temperature, the temperature setting of the fridge control, the amount of food inside, and how often and for how long you open the door, so it would be very tricky to estimate its duty cycle accurately. You could sit there on a hot day and time how often the motor turns on and how long it runs, while the kids run in and out grabbing sodas, in order to get a ballpark figure. Maybe you'd find it runs for 10 minutes 4 times every hour, making its duty cycle .67hr ([4cycles x 10min] / 60min/hr = .67hr, or a 67% duty cycle) But again, it's safer to think of it as running constantly for our calculations in order to have a built-in safety margin in battery capacity. You could conceivably end up wasting money on batteries too large for your needs if you overestimated the usage of too many appliances, but for one fridge or a furnace it probably won't be that bad. It's a judgement call that you have to make in consultation with your wallet. Hey, I can't do everything for you.
In order to accurately assess your total electrical diet, you need to add up all the amp-hour requirements for every appliance aboard your camper, just like I did with the light and water pump. If you can't find a data plate, then put an ammeter on the + battery cable while only that appliance is running and find out what it draws. It's more accurate to do that anyway, even if there is a plate on the appliance, because measuring at the battery will include the current needed to get the electricity down the wire to the appliance as well as that actually used by the appliance. Wire losses don't really affect 120v stuff much (they affect 12v stuff greatly), but accuracy is its own reward, so it's still better to measure than to assume the data plate is correct. Plus you may learn that they use a lot less than the plates say they do, allowing you to relax a bit on the battery size. Or maybe you'll find out they draw a lot more than it says on the plate, meaning the appliance is getting worn out, or maybe that you have wires that are too small or too long, or a bad ground that is causing too much resistance. Or that mice have hooked up an alternator to your motor to run their home theater.
Once you've added up all your appliances and estimated the time each day that you will use them, you are now able to calculate the size of the battery bank and solar system needed for your trips.
Again, batteries die an early, tragic, horribly painful, drawn-out death if discharged below 50% very often, so size your battery bank at twice the size of your between-charges electrical diet.
A few notes on battery types and their lifespan:
Lead/acid batteries work by a chemical reaction of sulfuric acid reacting with a lead oxide plate (the positive plate or cathode) and a lead plate (the negative plate or anode), turning both plates partially into lead sulfate. The sulfuric acid electrolyte becomes less acid and more water as the battery discharges. The more discharged they become, the more lead sulfate is deposited on the plates, and the more diluted the acid electrolyte becomes. When you recharge them, the sulfate on the plates is converted back to sulfuric acid in the water, and the plates return to lead and lead oxide. The more often and more deeply you discharge batteries, the more lead sulfate is deposited on the plates, and the less of it gets converted back into acid when recharging. This is what eventually kills most batteries, the increasing inability to reverse the chemical reaction through charging, called sulfation. The buildup of sulfate on the plates clogs the porous plates, reducing surface area and inhibiting the reaction, as well as reducing the amount of lead and acid available to generate electricity. And charging, especially overcharging, "boils" off water, making checking water levels a required item of maintenance every month or so. This boiling off is actually the water being broken down into oxygen and flammable hydrogen, which is why wet batteries need to be vented outside, and why sparky electrical components like charge controllers should not be in the battery compartment. Batteries can also release lead sulfide gas, which can corrode electronics.
There are three main types of wet-cell lead-acid, "flooded" batteries. I'm not including gel or AGM batteries here, because of their cost. They have their advantages and disadvantages, but cost keeps them out of my camper (along with beautiful young women). The three wet-cell types are car batteries, marine/deep cycle batteries, and true deep cycle batteries like those used in golf carts, wheelchairs, Hoverrounds, and forklifts.
Car batteries are inadequate for campers. They are designed with thin, porous lead plates in order to maximize the surface area for the chemical reaction, to allow them to both send and receive large amounts of current quickly. It takes a large surge of current to turn over a cold engine, but it is of very short duration, hence the larger surface area. But the batteries are never supposed to get discharged deeply, and are also recharged quickly by the high-amp alternator (a stock '94 Ford van has a 95-amp alternator). Leaving your lights on all day or cranking the stereo at the beach will shorten the life of a car battery greatly, as will using it in a camper, unless it's for a bare-bones, one-light-and-a-radio type of system that is recharged every day. What happens is the thin plates get eaten away by the extended chemical reaction and covered in lead sulfate, leaving not enough surface to convert the lead sulfate properly back to lead, which sulfate ends up flaking off and dropping down to the bottom of the battery case, where it eventually piles up and shorts out the plates, killing one or more cells. They will work, for a while, but I wouldn't use car batteries in my camper unless that's all I could afford.
"Marine/deep cycle" batteries are a little better, but are still mainly designed to start engines and be recharged quickly by an alternator, so again they are not really suitable for camper use. They are more robust than car batteries, but will still die an early death if repeatedly discharged deeply. Again, if that's all my budget allowed or my minimum system required, ok, but it wouldn't be my first choice.
True deep cycle batteries like those used in golf carts are made with much heavier lead plates because they are not expected to have to put out all their energy at once to start an engine. They are larger and heavier. These are the batteries you want in your camper, if you want the most efficient, longest-lasting battery bank for your money. Two 6v golf cart batteries wired in series to make 12v will last years longer than either of the other two types, and don't cost much more than car batteries unless you buy AGM batteries. I just bought two Duracell EGC2 6v 230ah batteries at Sam's Club yesterday for $216 (I was charged an extra 10% because I'm not a member, I used a day pass). While one trojan 230ah 12v AGM battery sells online for $565. Mine are of the GC2 form factor (not much bigger than a truck battery), and weigh 63 pounds each, so I can (kinda) easily move them, unlike the two 25-year old, ex-residential solar, 113-pound L-16s they replaced (A lot of fun dragging them 200 yards across the tundra yesterday in a plastic toboggan and lifting them into the truck, then out of the truck and into a shopping cart). When combining two 6v batteries in series, the voltage doubles but the ah capacity stays the same as one battery, so my total capacity is still 230ah. If you use multiple 12v batteries in parallel, then the ah capacities are combined (2, 80 ah 12v batteries in parallel = 160 ah).
If a battery is rated in Cold Cranking Amps (CCA), it is an engine-starting battery, either car or marine, as that is a measure of its ability to instantly convert that stored chemical energy into massive amps. That also means it will not give you the long life you should expect out of a camper battery. A true deep cycle battery will be rated in amp-hours, not cranking amps, as that is a measure of the size of its storage capacity, not its 0-60 horsepower.
Deep cycle batteries are marathon runners, good for the long slow haul, while car and marine batteries are sprinters, and get tired quickly.
Just to make sure that horse is really dead, whatever kind of battery you get, the longest life comes from NEVER discharging them below 50%. So when you figure your electrical needs in amp-hours, double it (at least) to get the battery bank size you need to survive between charges. Again, if you can afford the price and weight of a bigger bank, 20% maximum discharge is even better and will give you years of life.
My new 230ah bank, assuming a full charge and allowed to only discharge 50%, therefore using up no more than 115ah before recharging, would allow me to run my 4a furnace (assuming constant running at a 50% duty cycle, a conservative estimate unless it's wicked cold out, especially once I finish insulating) along with a 2a light bulb on 24/7, for 28.75 hrs before recharging ( 115ah / {[4a x .5 duty cycle] + [2a x 1.0 duty cycle]} = 28.75hr. So I could go a full day in a blizzard (covering my solar panel with snow), with it 75 degrees inside, with a light to read by, without going outside to start the generator, and I could probably still listen to the radio, watch a lil' telly and use the water pump without hurting my batteries. Gives me a nice sense of security for $216. And that's heating a 22.5' trailer in Maine, where it was 14f on Christmas Eve 2011 when I camped out. Once I get LED lights I'll be able to keep ALL the lights on during that period. Think what you guys in a little Compact, with a fanless Wave heater, could do with all that power... Surround sound, anyone?
110 volts and inverters:
When you start plugging 110 volt appliances into an inverter and running them off batteries, your consumption goes way up. It takes approximately 9.2 amps DC from the batteries to create 1 amp AC through an inverter. That is because, aside from inefficiency losses, the wattage going into the converter from the batteries equals the wattage going out of the inverter to the 110v appliances, and lower voltage for the same wattage means more amps. So a 1500 watt AC hair dryer also needs the same 1500 watts, but in DC @12v, plus a little for losses in the inverter, coming out of the batteries. From our power formula, 1500 watts AC @110v = 13.6 amps AC. And the same 1500 watts @ 12v DC = 125 amps DC out of your batteries (ouch)! 125a DC/13.6a AC = a conversion factor of 9.2.
So to calculate how many amps DC you need to produce the amps DC required by a 110v appliance, multiply by 9.2. A 45-watt LED tv like the ones I saw at Sam's yesterday (it's awesome how energy efficient they're getting) would need .41 amps AC to run (45w / 110v =.41a). From an inverter, that means we would need 3.76 amps DC from our batteries (.41 x 9.2), almost as much as my furnace blower or two 12v light bulbs. A 60-watt incandescent light bulb, 5 amps DC (60w / 110v AC x 9.2 = 5a DC). My batteries' available 115ah would be used up in 55 minutes drying Rapunzel's hair (115ah capacity /125a draw =.92 hours, or 55 minutes). So be stingy with the high-amp AC appliances, which are those that use motors and/or heating/cooling elements. And use 12v lighting, especially LEDs, which take hardly any juice.
Now, finally, on to solar and recharging:
When boondocking you need to feed those batteries if you don't want them to starve to death. If you want to stay out indefinitely, then you need to put as many amp-hours back in the batteries every day as you use. In fact, because of inefficiencies, you need to put back more than you use just to break even. The easiest ways are through a generator or solar panels. As I said earlier, I'm not considering wind, as in most camper situations it's impractical and potentially dangerous. You could drag the camper into town and plug in somewhere, but again, not very practical. You can wire up the truck to charge the batteries off the alternator in an emergency, but that's very fuel-inefficient and noisy, although it's great while towing, and I have a battery isolator switch in my truck for that purpose (any auto parts store has them, and they must be matched to the amp output of your alternator).
It takes a long time to fully charge batteries, in part because typical converters don't charge at a high enough voltage to really pack the batteries full of amps. From what I've been reading, a wet-cell golf cart battery likes to be charged at 14.8v (despite what the converter manufacturers say), even higher in cold weather, and most commonly-accepted-as-good converters only put out 14.4v, and they fairly quickly drop down to 13.6 or so, unless you force them to stay in bulk mode. The amps also drop over time, so the closer you get to charged, the slower it charges. The higher the charging voltage, the faster the amps get pushed into the battery. Too high though, and you'll boil all the water out of them.
One guy I've been reading, who thinks almost every RV manufacturer and solar installer is incompetent (who's been full-timing with his wife for ten years without a generator and with only rare hookups, who isn't selling anything, whose writing rings scientifically true to me, and who spends a lot of time helping people fix their solar installations so they work), says that basically you can never get your batteries fully charged using a converter unless it's plugged in 24/7 with no loads. He says the idiot lights in manufacturer's solar installations say that the batteries are charged when they're at the same state that his are after three days of no sun on his panels, and that it's no wonder you see all these "state of the art" trailers covered with panels yet running their generators all day, with their owners complaining they can't go a whole evening on the batteries. The manufacturers and solar installers yell at him and call him a crackpot, but he says that they're either ignorant or are only in it to sell more solar panels... He asks why, when you go to an "expert's" shop, that his solar-equipped rig is always plugged into an outlet. I'm fond of crackpots, myself.
Anyway, for calculation purposes, let's say my camper's appetite per day is 100ah. Since I only have 115ah to play with, that means I have to recharge every day or switch to 18th-century mode and use candles. You can typically get about 6 hours per day of good, full-blast sun up North in the summer. 6 hours into 100ah equals 16.67 amps I need to push into the batteries for those 6 hours to keep up. That means I need 246 watts coming out of my charge controller (16.67a x 14.8v = 246.7w). This is assuming I have a good, adjustable controller that I can set to charge at 14.8v. That means I need at least 250 watts of 12v solar panels on my roof, assuming no losses, which there always are. Those losses can be minimized by using much larger wires and shorter wire runs from the panels than those used by typical installers. And the proper wire size will get you more power than an expensive MPPT charge controller, says the crackpot. He says his system is efficient because he uses 6-gauge wire from panels to controller with only 12 feet of run, and 4-gauge from the controller to the batteries. Again, he full-times and doesn't own a generator, and runs power tools, a Mr. Coffee, a TV and a sewing machine, and doesn't have to worry until it's cloudy for three days, so he must be doing something right. Another big power killer that he sees from the "pro" installers is shadowing of the panels. He's got some pictures on his site of installations where there are only tiny shadows from the corner of a fridge or roof vent, that cut the panels' output by a third, or half , or even 90%. One little leaf on your solar panel basically means you'll never get your batteries charged.
I just bought a big, 220-watt, 12-volt residential panel from my ex-boss, which, if used with the right controller and wires, should let me shove about 15 amps into my batteries while the sun shines. Assuming the 100ah usage and 6 hours of sun, that's going to leave me about 10ah short every day, meaning I'll have to find another 10ah somewhere, either through running my little 800-watt generator for an hour or so, or hooking up to the truck. I'm going to build a motorized tracker though, so perhaps that will make up the difference. Panels mounted flat on your roof lose a lot of power due to the bad angle at which the sun hits them. The crackpot's system is tiltable for elevation but not sun-tracking, so while he has the best absorption angle vertically, early in the morning and late in the afternoon his panels aren't generating what they could, because the sun is striking them from a side angle. Most RVs seem to use multiple small panels, so perhaps he thinks it would be too expensive and complex to make them all track the sun. My plan is to use two large panels, which will make it easier to build trackers.
Figuring all this out is a lot like the scenes in Apollo 13 where Gary Sinise is in the simulator, looking for every available amp to keep the real spacecraft running long enough to get back to Earth. Well, that is, other than the whole "they'll die if we don't get it right" thing. We won't die if we get it wrong, we'll just be sitting in the cold and dark, bored to death.
I plan to full-time in my camper, so ideally I want to get to the point where I don't need a generator, like the crackpot. You folks with canned-ham Compacts have smaller, curved roofs with a sexy profile that you probably don't want to spoil with panel mounts, where on my boxy '76 2250 I have a big flat roof, with enough room for another panel of the same size (65" x 37" IIRC), when I can afford to add another one. You would probably have to use multiple smaller panels to get the kind of charging capacity I will have, or use a portable ground mount. Mounting panels off the camper has its own disadvantages, like long wire runs and the possibility of theft. More 'n' likely, if you add solar to a compact, it will be a supplement, not the mainstay of your charging system, just because of space and complexity.
So what it comes down to is this: Calculate your usage per day for all appliances (don't forget "phantom" loads, many appliances like TVs use electricity as long as they're plugged in, even when turned off, so disconnect stuff from your inverter when you're not using it). Then multiply it by the number of days in your longest boondocking trip, and if you want to camp without charging at all, get a battery bank that is twice the size of that total trip usage figure. If you can charge every day, get a battery bank that is twice the size of your DAILY usage. For totally relying on solar in sunny weather, you need enough panels to generate enough amps over six hours (your usable solar time will vary with latitude, weather patterns and panel orientation) to replace your daily usage. That is with no reserve for rainy weather. Going whole-hog is installing enough panels and batteries to replace more than your daily usage, so you can stay ahead of your diet and have some reserve for rainy days, like the crackpot's system. Any gaps can be made up with a generator or jumper cables, but ideally your solar keeps you charged. I hope this clarified things a bit instead of making them more confusing, and that people will find it useful.
If anyone has anything to add or correct, feel free!The crackpot's site: handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/
