batteries and projectors – bioskop consumer test

A Report from the Bioskop R+D dept.
Although we’ve been thinking about it for a while, Institution was the first bioskop event where we experimented running projectors off batteries. The site was underground – so using a generator was going to cause serious fume and noise problems.

Our research lead us on a trail through several options – here’s the science, as they say in the shampoo ad…

1. First we tried out a 12v deep cycle battery, borrowed from a friend who lives on a boat.
This is the kind of battery you get in a caravan (or boat), but not a car.
We then put it through a 300 watt inverter to convert it to mains (120 v) power for the projector. This was the first problem we ran into… even though the projector bulb is only 150 watts, inverters have problems with inductive loads. Basically anything with an electric motor in it needs a lot of power to get it going… so our guess was the fan in the projector was causing problems. We were going to need a more powerful inverter – maybe 800 watts? but these aren’t cheap.

2. Rather than getting a bigger inverter – Why not get a smaller projector? The site was going to be pitch black so having a super-bright projector wasn’t a priority.
We knew that friends who run a bicycle-powered cinema had a pico projector… it turned out that they also had their own rechargeable 12 volt battery. This was a relief because the deep cycle battery was heavy and also… a little scary – considering there would be a lot of water and metal where we were going.
Another benefit of the pico was it didn’t need an inverter because it runs off 18 volts. All it needed was a step up transformer to pump the 12 volts up to 18 volts.

This set-up was looking pretty promising – but a nagging question had got lodged in our minds…
The internet seemed to be saying NO. Skin is a good enough resistor to stop the current produced by a 12 volt battery stopping a human heart.  On the other hand wet skin is a much better electrical conductor.

A few days before the event we did a recce to the site with our equipment – taking the 12 volt battery along in a water tight Peli case.

hmmm… we were getting bad vibes. 12 volts might not be enough to seriously hurt anyone but…  Electricity, lots of metal surfaces, lots of water and twenty people stumbling around in the dark was feeling like a bad combination.  We seemed to be unwittingly producing a Chris Burden scenario.

3. The pico projector we were borrowing ran off an external power source, but there are picos that can run off  their own internal batteries.  ( The batteries are about the same size as the ones in mobile phones – no risk of electrocution!)

These internal battery projectors cost about £250, and we wouldn’t recommend them because currently they’re  really dim, resolution isn’t great, and the batteries don’t last much longer than an hour.  The only reason to buy one really is if you’re showing a 40 minute programme of art films in a pitch black tunnel and you’re worried about electrocuting the audience.  Our solution was to buy this one from a well known high street electronics store, which offers a 14 day return trial period…  and make sure we didn’t scratch or damage it at all before we took it back.

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