Watts a volt, anyway?

June 5, 2008 2:49 PM

Can anyone help me figure out what kind of power source I need for my outdoor art project?

For an art project, there are a few appliances I want to be able to use outdoors, far from any electrical outlets. These appliances will vary in terms of how much power they require and how long they'll need to be on for.

My first choice would be to use some kind of rechargeable battery so that it runs quietly and has no exhaust. Do things like this exist? How can I determine which model would be appropriate for my needs? (For example running a 700 or 1200 watt fog machine for 30 minutes, or a 200 watt LCD monitor for an hour.)

My second choice would be to somehow connect up to my car, which would invariably be nearby. What kinds of gadgets exist for connecting to my car when it is either running or off, and could I generate enough power this way to run that fog machine or that monitor?

I don't want to use a gas-powered generator. Are there other options I haven't thought of?

posted by xo to technology (10 comments total)

I know very little about electricity, but I do know alot about creating art outdoors.

Things like fog machines are very difficult to effectively use running off any deep cycle battery or any kind of battery for that matter. I also don't think you're going to get good results with any sort of car hook up. Things like monitors with low energy requirements are another story, but with those you run into visibility issues if you're outside in the sun.

I've had success with massive power cables (cumbersome, expensive) and generators (noisy, expensive). You may need to re-think. It's one thing to run a computer off batteries in the middle of nowhere. You want fog? Start thinking smoke cookies, dry ice, that sort of thing.
posted by miles1972 at 3:06 PM on June 5

Yes, you can easily find these. Assuming your fog machine and monitor run on AC, you'll need an inverter to convert Dc to AC. One good brand is Xantrex. The load examples you give should be no problem. Google "power inverter" , "rechargeable battery" and the like.
posted by JimN2TAW at 3:07 PM on June 5

For running something as big as that, unconnected to the wall or a running generator, about your only readily-available choice would be a honking big "Uninterruptable Power Supply" or UPS. You'd plug it into the wall to charge it up, carry it to where you were going to do your exhibition, and let it power the unit.

But they don't tend to have anything like as much energy as you'd need.

The critical number here is in units of "watt-hours". It's the amount of power drawn, measured in watts, multiplied by the time duration, measured in hours. If you run 1200 watts for half an hour, that's 600 watt-hours. This UPS is good for 390 watts for 5.5 minutes, which is about 36 watt-hours, and it costs $180. (Also, it can't provide 1200 watts. Try to draw that much power and its breaker will throw.)

UPS's do exist which can run at the power levels you need, but they're big and expensive and heavy. And they might be difficult to come by because they're not consumer products; they're intended for industrial applications.

Your car can't produce that much excess electricity, either; the alternator isn't designed for that kind of excess load.

Sorry, but your only answers are either a big, long extension cord or a generator. Any other solution will cost you more than you really want to pay.
posted by Class Goat at 3:16 PM on June 5

I used to run a laptop off of a truck battery using an inverter like this one.
posted by muddgirl at 3:29 PM on June 5

(Is this for a Burning-man-type-thing? Consider that a small diesel generator might be much quieter and more environmentally friendly than idling your car to charge up your battery to run an inverter. The generator can be placed 10 yards or so away from your installation, with a sound barrier to dampen the noise)
posted by muddgirl at 3:31 PM on June 5

The cheapest rechargeable batteries for significant power draws are lead-acid (similar to the type used in your car, but you'll want a deep cycle version). You'll also need an inverter to convert the 12 volt DC power from your battery to the 120 volt AC power you are used to plugging things into*. Finally, you'll need a battery charger to charge your 12V battery. It will cost you at least a few hundred to buy these things with a battery big enough to power your fog machine and an inverter that will handle 1200W. Lead-acid batteries are heavy, but your other options (NiCd, NiMH, Li-ion) will be very pricey for your requirements.

You can even buy something that contains the battery, inverter, and charger.

However, you'll need quite a bit of energy to power your 1200W fog machine for 1/2 hour and that unit won't provide enough. Power (watts) is voltage (volts) times current (amps). So to run your 1200W fog machine for 1/2 hour, you'd need 12V at 100A for 1/2 hour (actually the inverter is not 100% efficient, so you'd need somewhere around 115A at 12V, but your battery will actually run higher than 12V, so say roughly 110A). Deep cycle batteries have a amp-hour rating (Ah) and you'll need 55Ah (110A for 1/2 hour). However, when you discharge batteries quickly, you get less energy out of them than if you discharge them slowly, so you'll need to look at the manufacturer's discharge specs to find out how many Ah you'll actually get at your intended discharge rate. Make sure to buy a little more than you need to add a margin of error. You can always use two batteries if you need more power.

*You could find a DC-DC converter to power your LCD from your battery without converting to AC and back to DC, but it probably wouldn't be worth it unless you used it a lot.
posted by ssg at 3:38 PM on June 5 [1 favorite]

Before you buy any batteries or inverters, get an accurate power consumption meter (such as the kill-a-watt) and actually measure the power draw. The wattage ratings that you find on appliances tend to be the maximum, not the typical amount of power they draw. Especially with computer power supplies, the rating is the maximum that it is able to deliver, and is nowhere close to what the typical usage is.

There are several factors that determine which size battery you need to buy. Most batteries are rated in amp-hours, which when multiplied by the battery's nominal voltage will give you watt-hours which is a measure of energy. You also have to take into account that a DC-AC inverter is not 100% efficient, it's more like 70-80%. Additionally, battery capacity is not a constant: the rate of discharge plays a role. In other words, even though 12V at 20 amps for 30 seconds is the same amount of energy as 12V at 2 amps for 5 minutes, it's not the same to the battery. Moreover, sometimes you have to factor in temperature as batteries tend to perform worse if they are very cold, depending on chemistry. (You will almost certainly be dealing with lead-acid gel-cell batteries.)
posted by Rhomboid at 3:43 PM on June 5

After seeing Goat's answer, let me clarify. You should be able to find battery-based power at the levels you want, but it will be expensive and heavy.
posted by JimN2TAW at 3:44 PM on June 5

If your objection to a gas-powered generator is that it's loud, then the solution is to move it away and use an extension cord. If your objection to it is that it isn't "green", then what you need to realize is that all the other solutions proposed here are even worse. AND a hell of a lot more expensive.

The problem with battery technology, whether rechargable or not, is that energy densities just aren't very good. It's a fundamentally difficult problem which probably won't be solved until someone makes a major breakthrough in fuel cell technology.

When it comes to energy density (watts per liter or watts per kilogram) it's damned hard to beat gasoline. A lead-acid battery is good for about 125 thousand joules per kilogram. Lithium batteries can be as much as 1.5 million joules per kilogram. Gasoline tends to run about 45 million joules per kilogram. And it's cheap and readily available.

This generator is 2000 watts, more than you need. It's about $500 and it can run 5 hours on a tank of gas. You could probably find something comparable to rent for $30 per day or even less.

I know that gasoline is evil, OK? But sometimes it really is the best answer.
posted by Class Goat at 4:25 PM on June 5

FYI, to stave off derails: I don't think gasoline is evil, but it's not allowed at my first location so I am looking for alternatives that would be acceptable in all of the places I need to work.
posted by xo at 5:44 PM on June 5