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WARNING - this little gem of an essay is rather technical. If you can't tolerate the technicalities, don't try to run on batteries. It's not for the technically challenged.
If you have a loud sound system, a refrigerator, a lot of lights, or any other high wattage electric load, you will probably find that batteries do not provide enough power. Running a generator will be your cheapest and simplest option.
A lot of camps have more modest power requirements. They should consider using batteries as their power source. A good rule of thumb is that, if you need to draw less than 1200 watt hours per day, a battery is the best option.
"Right," you say. "What's a Watt hour?" We're getting technical already!
For each device you plan to run from the battery, multiply the power it needs (in watts) by the time it will be in use (in hours). Then add up the watt hours for all of the devices that you will use, to get the total. Example - A lamp that draws 60 watts from the battery, running four hours a day, requires (60 watts) x (4 hours per day) = 240 watt hours per day. So you can run exactly five of them on one 1200 watt hour battery.
Beware! If you use a DC-to-AC inverter with your battery, you need to multiply the power requirement by 1.4, because the inverter wastes a lot of power. This issue is discussed in more detail below.
Here's the deal. WalMart and other warehouse type stores carry something called a deep cycle marine battery. They come in various sizes, and are rated according to their capacity in Amp hours. (Yep, that's Amps of current times hours of usage.) A typical battery costs $50, weighs 70 pounds (31kg), and has a 100 Amp hour capacity. Since it produces 12 volts, a 100 Amp hour battery holds (12 Volts) x (100 Amps) = 1200 Watt hours of energy. If you are willing to fully recharge your battery each day, that means you can draw 1200 Watt hours per day.
You might want to buy two batteries. That way, you can be recharging one while you run your camp off the other one.
You need to think about the fact that batteries produce DC (direct current), not the AC that most appliances use. There are two possible solutions to this little annoyance. You could buy appliances that are designed to run off a 12 Volt car battery. Or you can buy an inverter, which converts 12 Volt DC into 120 Volts AC.
J.C. Whitney sells lots of cheap 12 Volt DC accessories for cars, trucks, and recreational vehicles. There's an astonishing variety of stuff available, including incandescent and fluorescent lamps, strobes, searchlights, UV lights, radios, audio power amplifiers, fans, air conditioners, refrigerators, pumps, motors, horns, sirens, and a gizmo that plays the theme to The Godfather at ear-splitting levels. There must be half a dozen places that sell such things by mail order. You can also find a more limited selection in any car parts store. It's worth noting that the stuff that's designed for 12 Volt DC use is usually very power efficient. That matters, because it's a pain in the butt to charge the battery, so you want to use as little juice as possible. In particular, I recommend using fluorescent lighting instead of incandescent wherever possible, because you get the same amount of light with only 25% as much power! If you prefer incandescent, use a halogen lamp, which is less efficient than fluorescent but more efficient than a standard light bulb.
If you can't or don't want to run your camp on 12 Volt DC, you need to buy an inverter. Find one that takes a 12 Volt input and converts it to a 120 Volt AC output, and has a standard AC outlet. (It turns out that's what most of them do.) Next, decide how many Watts of peak power you need.
In the example I used above, the five 60 Watt bulbs draw 5 x (60 Watts) = 300 Watts of power, if they are all on at the same time. I recommend that you double your peak power requirement, so in this example you would buy a 600 Watt inverter. You can get a 300 Watt inverter for $65 and a 600 Watt inverter for $90. You won't be sorry you bought the higher capacity unit. Running an inverter at its full capacity is a recipe for disaster.
How do you recharge your battery? One way is to charge it from your car's alternator. You can use jumper cables to hook the deep cycle battery up in parallel with the car's battery while the car's engine is running (just like you're giving the car a jump start). But that will draw a lot of power from the car's battery very quickly, and maybe damage it. A safer but more annoying alternative is to disconnect the battery clamps from the car's battery and then connect them to the deep cycle battery. There's still some danger involved here. When you charge a battery, it releases a fair amount of hydrogen gas, which is highly explosive. So keep the battery outside in a breeze while it's charging - if you don't want your car to explode!
Why bother with a separate deep cycle battery? Your car comes equipped with both a charger and a battery. You could connect an inverter to the car's battery posts, and let the engine idle for a few hours per day to keep the battery charged. This is temptingly simple. It also has a huge drawback. Remember I recommended that you buy a deep cycle battery? It's different than a standard car battery. It doesn't mind if you run it completely empty over and over. A car battery minds a lot. It will die within a week with that kind of treatment, and you will have to get your car towed back to civilization at an astronomical cost. If you are willing to get a Voltmeter to monitor the Voltage of your car's battery, and recharge it whenever it gets below 12.5 Volts, you will be safe. But you won't get to use all of the battery's capacity that way. A typical car battery has a 60 Amp hour rating, and you need to recharge it when you've used 60% of its ultimate capacity. That means you only get 60 x 0.6 = 36 Amps of usable capacity, which is one third of the usable capacity of a deep cycle battery.
Here's a swell idea, but it might be cracked. You could replace your car battery with a deep cycle battery. I don't know anyone who's tried this, but if it fits in the battery compartment, it ought to be able to run both your car and your camp with minimum fuss. Try it at home first, just in case it's a terrible idea. If it works, let us know. If it doesn't, we're really sorry you ruined your car, but don't say we didn't warn you. (Maybe you should try it on a rental car instead. "Hey, this car won't start, get me another one.")
Oh, and if you run the deep cycle battery completely empty, how are you going to start the car to recharge it? (If you were really slick, you could add in a gizmo that would make the battery cut out when it still had just enough juice to start the car.)
One thing, though, about the section on running your camp with batteries:
On the suggestion that someone might want to replace their regular car battery with a deep-cycle type (so as to not have to carry both).
This is bad in the same way that deep-discharging a car battery is bad.
Deep-cycle batteries are designed to be slowly discharged, and the excess heat generated within them when being hit with a huge load (ie. the car's starter) will damage them rather quickly. They'll work, at least as much as a car battery will work for running christmas lights and a stereo, but will fail pretty fast (leaving you stuck).
Electric wheelchair batteries have the deep-discharge characteristics that you might want for running a theme camp. They have a fairly large capacity, can be chained together (of course), and can often be found pretty cheap used. If you're in the habit of charging your batteries with your car, it's not usually necessary to remove the battery that's already in the car. In fact, if you get an isolator (available from any RV (motor home) dealer), you can set it up so you don't even have to disconnect them. Additionally, if you get a sealed battery, you don't have to worry about hydrogen floating about...
If you think it's worth the bother, you can read lots more about deep-cycle vs. car batteries in the archives of the type2 mailing list (a VW bus discussion group, at www.type2.com, archives at vw.i405.com). Just search for "aux battery" - VW campers often have a second battery to run electrical bits (lights, fridge) while parked.
- Ian Baker <ian@sonic.net>
Because of the danger and inconvenience of charging your deep cycle battery from your car's alternator, you should consider the second method of recharging it. Buy a battery charger from the same place that sold you the battery. A good battery charger will cost you about $50. It converts 120 Volt AC to 12 Volt DC. You will need to find someone who is willing to let you use their generator to power your battery charger.
Pay attention to the charging current the charger can put out. A typical unit puts out 20 Amps, and can therefore fully recharge your 100 Amp hour battery in about five hours. A wimpy charger will only put out 8 Amps, and would take 12 hours to recharge your battery.
When you are recharging your battery, you will probably want to disconnect all of your appliances from it. When your battery has been recharged, and you plug them all back in, you need to pay attention to the polarity of the battery. 120 Volt AC doesn't care much about polarity. But 12 Volt DC cares a lot. Most inverters, for example, become scrap iron when you connect them to the battery with the wrong polarity. The best way to ensure that the polarity is correct is to use standard 12 Volt cigarette lighter sockets and plugs. Make sure their Amperage ratings are high enough for your inverter. You can also rely on colour-coding (red is plus, black is minus, usually), and pray that clueless folk don't take it upon themselves to reconnect things the wrong way. But remember, shit happens, especially in the desert. So a better alternative is to buy an inverter that doesn't care about the polarity. If you find one, let us know so we can tell everyone else.
Another concern with inverters is heat. They are typically about 70% efficient. That means that if your inverter is supplying 700 Watts to the load, it is actually drawing 1000 Watts from the battery! What happens to the other 300 Watts? It gets turned into heat inside the inverter. Big inverters therefore need to have a good way to get rid of a lot of heat. They often have a heat sink, and some of the bigger ones also have a built-in fan. In the desert, where the ambient air temperature can reach 100 degrees F, you will definitely want a fan. Don't rely on the breeze, because there might not be one. And if your inverter overheats, it will revert to scrap iron. Inverters have a strong desire to become scrap iron. Don't encourage them.
Fans are good for keeping things cool. They are also, unfortunately, good for sucking playa dust into the sensitive electronics inside. Playa dust is a mixture of clay and alkali. The clay won't do much damage, because it's soft and non-abrasive. But the alkali is potentially corrosive, especially if it gets damp. If you wash it out thoroughly with hose water and let it dry in the sun before putting it in storage, your inverter might actually still work next year.
Using a deep-cycle battery as your car battery is indeed a swell idea, but as Ian Baker warns, a regular deep cycle battery doesn't cut it. You have to use a "Deep cycle starting/cranking battery", which is optimized for both deep cycle and starting. I have used this type of battery in my cars, for years, and they have given perfect service. I mean cranking a Chevy V-8 engine in frigid Michigan winters.
They seem tougher than regular car batteries by quite a bit, and last longer... the "30 month" deep cycle batteries last me 70+ months, whereas the "72 month" car batteries tend to last me 30 months :-) I don't think the battery itself would cause any problem for modern high tech cars, but modern car electronics can be a bit sensitive to Voltage spikes from big external chargers or cheap inverters that are electrically noisy. So I'd be careful about accessories.
Yes, you could drain the battery down so far you couldn't start your engine. It wouldn't damage the deep cycle battery but could be a pain to deal with. I'd say keep another battery around to jump off. Batteries will "bounce back" a little if you unplug all the load for a few hours, often enough to get a start. That's worth a try if you have time to kill.
I think the very best deep-cycle-only batteries are "Edison Cells" aka nickel-iron - the old standby for running railroad signals. Good energy density, they last 20 years or more, less toxic, won't freeze, thousands of recharges - but they're always wet cells, they can't put out enough surge current to start an engine or run an electric car, and they have different charge rates and voltages (more like a Ni-Cad).
Lastly I want to say I think a generator and a deep cycle battery would be a really nice combination, if properly set up. You could run your generator just once or twice a day to top off the batteries... and have power all the time.
You've probably noticed by now that running on batteries is a lot of work to set up, and a lot of work to operate. Why bother? Well, just remember that generators suck too. They are big, expensive, noisy, stinky, and sometimes cranky. Once you get used to running on batteries, you may get to like them.
NOTE - Please dispose of your deep cycle batteries by recycling them, not by throwing them in the trash or in the woods. They are mostly lead, which turns sweet happy children into morons. The world doesn't need any more morons. Really.
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