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I need to use a projector for a presentation (about 30 mins, but I'll need to make it about 3 times), but there will not be any wall outlets available. I thought about using an inverter and a 12/24v battery (auto?), but I'd like to know if there are any easier options, or if I'm missing something in my original idea. The projector is a BenQ MS612ST, it has/needs a power supply of 100-240 VAC and consumes 275W. Can anyone help? Thanks so much in advance!

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Consumer hardware question; not electronic design. Perhaps try the superuser.com StackExchange site, which is about computers in general: hardware and software. (The idea I'm grasping at here is that the projector is connected to your computer and you want to make it more mobile). – Kaz Jan 17 '13 at 1:34
@Kaz - I don't think this question would really fit on superuser. The OP is basically asking "What can I build that can power a bunch of appliances in a place without available wall power". It seems that, assuming the OP wants to build something, rather then buy something, it would fit here fine. – Connor Wolf Jan 17 '13 at 2:43
There used to be a commercial product from Galaxy Audio called the Far Outlet that would be ideal for an application like this, but unfortunately, it was discontinued some years ago. I've long wondered why a replacement product never appeared. – Dave Tweed Jan 17 '13 at 5:11

A 280W load will drain a car battery rather quickly. That's something like having seven headlights on. Three repeats of 30 minutes? You're lucky if you get through one. The capacity you will see on a fast discharge rate will fall short of the theoretical Ampere-hours capacity of the battery.

I would borrow or rent a portable generator that runs on fossil fuels.

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It depends if the projector uses mains voltage directly (EG to power a 240v lamp) or if it actually uses a mains adapter to drop the supply to low-voltage DC. An inverter is not 100% efficient, and then you would have some more loss through the mains adapter too... but if by some bit of luck your projector ran directly from the 12v battery (you might still want to use a DC regulator but that will be more efficient) you might just swing it.

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What you are describing is essentially a UPS. You can go to APC's website and browse their products to get an idea of how feasible this is. It looks like the smallest unit they have which would have a chance of running your load for 90 minutes is the APC Smart-UPS 2200VA (about $1000 list price), but given manufacturer's tendency to overstate performance, you probably want to go bigger.

As you can see, not cheap or small. Even if you build it yourself, you can't overcome the physical limitation of battery energy density.

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