I need 1KWh

December 4, 2008 12:51 PM

I need just one portable, reusable, near-silent, non-dangerous kilowatthour.

I am planning on doing some mobile outdoor projection using a laptop and a standard video projector. In total, I expect to need about 500 safe, non-spiky, non-saggy watts for two hours.

I haven't been able to find, despite extensive research, any solutions that don't involve loud, super heavy gasoline generators, or inverters hooked up to the batteries of a running car. Those solutions involve slightly too-large amounts of danger/heavy loads/noise/complexity.

Ideally I'd end up with something that could work on a (heavily loaded) bike, or even man-portable/man-draggable.

People seem to think that car batteries won't work, because they will die from too-rapid discharge and be unusable after a few cycles.

Arrays of supercaps seem like an accident waiting to happen.

Surely some burner has found the local maximum on this question?

posted by felix to technology (15 comments total)

According to this, car batteries have an energy density of 30 Wh/kg, so 33 kg of car batteries would work. Lithium ion or Lithium polymer batteries are better, in the 100 Wh/kg range, but chargine and discharging them might be a little tricky to do safely.

The best way to power the laptop is through its own battery so you probably only need to power the projector.

A 1kW generator is not that heavy and is the usual burner solution to this problem.
posted by pombe at 1:08 PM on December 4, 2008

You need a generator. You really don't want to be lugging that amount of batteries around on a bike.
posted by dunkadunc at 1:11 PM on December 4, 2008

Honda generators are pricy but very slick, light and quiet.
posted by contraption at 1:18 PM on December 4, 2008

Are you certain that you need 500 watts? I mean, have you measured it? The real power consumed by an appliance is often much less than what is on the label.
posted by kc8nod at 1:24 PM on December 4, 2008

Yeah, if you get a gennie, those Honda generators are the way to go.
posted by pombe at 1:45 PM on December 4, 2008

I've been to some open air concerts and they usually hire a not-too-noisy generator which they put some distance away, then run cables from the generator to the main stage.

Would connecting to a car engine and leaving it idling really be that bad if it was some way away?
posted by Mike1024 at 1:48 PM on December 4, 2008

IF you want to do it with batteries and an inverter ( and you could do it that way ), you'll want to look at marine/deep cycle batteries which are designed to be more fully discharged at a lower rate than car batteries. You'll probably need about 2 large marine batteries and a 1kw inverter.

You can buy marine batteries that have a capacity near 1kwh, but they'll last more cycles if you don't completely discharge them every time.
When choosing an inverter, again, overspecify. And beware of potential for fan noise. Most inverters are not very efficient, which is another reason you'll need extra battery capacity.

In the end, a honda generator as suggested above is a lot easier to deal with than building a setup with a couple batteries and an inverter, but it's also noisy.
posted by balberth at 1:48 PM on December 4, 2008

Could you rent a prius?
Bonus - the engine will only come on when the battery charge drops below a threshold, charge up the batteries and then shut off..
posted by Arthur Dent at 2:07 PM on December 4, 2008

Buy three Duracell Powerpack 600s. 28 Ah with a 12V battery for about .33 kWh apiece. They deliver a continuous max of 480W at 120V AC.

Unfortunately, it will require you to unplug everything twice so it's won't be perfectly continuous.
posted by GuyZero at 2:16 PM on December 4, 2008

You might want to track down the guys that do the Dolores Park Movie nights in San Francisco, they do this once a month. I think they might use a small generator, but might have some good advice.
posted by bottlebrushtree at 2:18 PM on December 4, 2008

A generator is the way to go assuming you really do need 1 kWh. If you have a kill-o-watt or similar device you can actually monitor the electricity usage of your projector, and laptop, though that may be able to run on batteries for two hours. If you do plug the laptop in as well make sure the battery is fully charged as charging provides an additional load.

But the easiest thing would be to run an extension cord if you are locating near buildings. Works better if you've got a relatively small number of potential sites that you can check ahead of time with owners.

A last word of caution: if this is going to be done in daylight a standard projector will likely be not nearly bright enough. Be sure to test this.
posted by 6550 at 2:19 PM on December 4, 2008

A laptop and a video projector are not going to use near 500W, unless its a HUGE projector.

After Hurricane Ike, I had a 42" plasma TV, TiVo, refridgerator, DSL modem, wireless router, two fans, two lamps (with CFLs) and a MacBook running off the generator, and the Kill-a-Watt said I only hit around 650W.
posted by mrbill at 3:58 PM on December 4, 2008

A laptop that's not running at 100% CPU usage should need about 50W. A 3000 lumen projector will use about 300W. So 500W is an appropriately conservative spec for the power supply.

Car batteries will easily survive being repeatedly discharged by a third of their rated capacity; it's only deep discharges (half of capacity or more) that kill them quick. A kilowatt hour at mains voltage needs about 1200 watt-hours at battery voltage (to allow for inverter inefficiency). To get that sustainably from car batteries, you'd want 3600 watt-hours or 300 amp-hours of car batteries, which is about five or six biggish batteries, which is bloody heavy.

Marine batteries, on the other hand, are designed for deep discharge. 600W at 12V inverter input is 50 amps, which is too high to be comfortably sustained even by quite a large marine battery; better to use two. So, a couple of 80 amp-hour marine batteries will do the job. Expensive, though, and still quite heavy.

If you use a less-bright projector, and let the laptop run off its own batteries, you might be able to get away with a single marine battery.

A little generator and 50 metres of extension cord probably weighs less than a marine battery.
posted by flabdablet at 4:36 PM on December 4, 2008

mrbill, projectors have big grunty halogen lamps inside them. These are very bright, but not very efficient, and they really do suck down the watts.
posted by flabdablet at 4:40 PM on December 4, 2008

For what it's worth, we used to run our TV off our car back when we didn't have all our solar panels set up. Just start it up every hour or so and you've got it made in the shade, all the KWH your gas tank can deliver.
posted by dunkadunc at 4:58 PM on December 4, 2008