Re: Is there a way to light a 25 watt light bulb using a battery?

Hi, Michelle – an interesting question as it raises more questions for me!

Let me start by making the assumption that you want to use a mains 
voltage (110v AC if you are talking USA mains, I guess) 25 watt bulb, but 
want to provide the power via a DC battery – presumably because you are 
working somewhere where mains supply is unavailable. OK? If that 
assumption is wrong, then much of what follows will not apply…

Very roughly speaking, your problem is that to achieve the light level 
you want from your bulb will require a voltage of 110v, which is not 
directly available from a battery. The filament resistance will be 
approximately 500 ohms, so you would only achieve a current of about .02 
amps with a 12 volt battery. The filament needs about .25 amps to get hot 
enough to provide the intended light, so you are short by over a factor 
of 10 (just the factor difference between the battery and mains 
voltages). So in a sense your question is how to boost the voltage of 
your battery. Unfortunately, there is no practical way of doing that, 
that would provide the 25 watts of power your bulb needs, and even if 
there were, the current drain on your battery would be quite heavy, and 
require you to have a high capacity battery for it to last for a while. 
(I assume lugging a car battery or similar is not an option.)

So, the questions I have relate to your purpose – which is to provide 
illumination for your photography – and why you would choose a mains 25 
watt bulb for the purpose. There are other options which will work well 
from a battery, and these days white LED (light-emitting diode) sources 
are quite inexpensive, very power efficient and small. Examples are to be 
found in bicycle lamps and here is the first example I found just by 
running a search on led bicycle lamp:
http://www.fcsurplus.ca/shopping/shopexd.asp?id=15772
(only $5)
There are loads of options, so you have a big choice and little expense – 
but you will probably want to find the whitest output you can. I think 
these devices will be similarly bright to a 25 watt bulb on mains power, 
and only need a set of AAA or AA batteries. If you find the source too 
specular,  a simple diffuser is easy to arrange with some cloudy plastic. 
(Lid of a Pringles tube for example.)

I hope I’ve been of help. Without knowing more about what you are aiming 
to do photographically, it is difficult to be more specific, but I doubt 
that trying to run a 25 watt bulb designed for mains AC supply from a 
battery is the best solution to your lighting problem.