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I want to make a UV light that runs off batteries (maybe a car battery) but it needs to last quite a long time and be powerful enough to help a small plant to grow in a dark area (for a school biology project)
You could probably buy a few dozen UV LEDs, those won't use much power at all.
Beyond that, you could look into a blacklight (I'm pretty sure those are UV) or a UV cold cathode the inverter for which often runs straight off 12v, as from a car battery. Check computer modding shops for the latter.
Is the no way of wiring this area to mains with an extension cord or something, or at least have say a 12v power supply plugged into mains outside the cubpoard this plant is in, because that would certainly simplify your project.
A plant is probably not going to do well with just UV light. If you can choose the parameters for your experiment use red light on one plant and blue light on another. Record the differences after a few weeks. Red light stimulates stem growth and you'll end up with a tall spindly plant. Blue light promotes leaf growth and you'll have a shorter bushier plant.
If the intention is to grow plants like normal, you need a traditional "grow light" that is similar to the daylight spectrum. If this is an experiment to see how well plants fare with UV light as their source, that's another matter altogether. However, you must realize that the "black light" UV used to illuminate Day-Glo posters is long-wave black light while the UV used for EPROM erasers and germicidal systems is short-wave black light and dangerous to the eyes (although I've never heard exactly what it is that short-wave does to damage eyes and how).
I used to use a regular florescent light to help grow plants indoors and it seemed to work well. I never got the Gro-Luxe type but I did read that although regular florescent tubes gave off UV, the amount decreased with age. The reason that I mention this is that there are 6 volt lanterns sold in the flashlight department that have 1 or 2 small florescent tubes (Everready #F4T5) I do not have the circuit that makes these little florescent tubes work or any information as to how much (if any) UV they produce but perhaps some research at the Everready web site would help. It might be a battery operated solution.