Would it be possible to build double layer capacitors at all practically for one'self, as they can get quite expensive, especially if somebody were to be thinking about rigging up a few hundred to a thousand farads of storage.

I know capacitors can easily be made, but i do not think the ones that are easy to make are all that useful for what it is you are trying to do... However, I am sure if you look into how the bigger capacitors are made, it may not be at all that difficult to make those either - I believe I have some pretty large old capacitors - ones you couldn't wrap your hand around, and have a mass of several ounces, at least. I think they contain an oil or an acid of some sort with layers of metal - but I'm not sure.  I don't know how many volts they can hold or do hold -- but I am pretty sure whatever size you need is out there or can be made.

Yeah Sedit thats the buggers, some on ebay for car stereos have voltages up to maybe 36V but upto 100 farads capacitance, I was wondering about hooking a few up together, although granted my electronics skills are utterly shit, as a power pack, and using that to store energy to charge up a marx generator built from say, 600V, 450uF electrolytics which would be what actually creates the pulse used to discharge the railgun/coilgun.

Think I'll build a coilgun first though before railguns, as they are much easier and less demanding, although granted they won't shoot a copper-jacketed depleted uranium penetrator+incendiary filled round through steel plate surrounded by a cloud of superheated plasma that makes people who piss me off want to piss themselves and run like hell

Sedit, do you think it would be possible for an experimenter to fabricate nanotube based caps like that? granted they are in their infancy at the present time, and small ones that are relatively cheap have maybe 6V maximum values for a 5 farad cap, but if they could be DIYed, then why the hell not simply build them big enough to discharge at a few hundred volts?, as it would be much cheaper than buying them, and I cannot afford to pay hundreds of pounds.

Even at 100V it would be viable, as the voltage could be stepped up to multiple kilovolts easily enough, its simply for their damn fast charging and huge capacitance, no worries about shitloads of 9V PP3 cells if one were to rig up a fuel cell to a high end DIY ultracapacitor powerpack, all I would have to worry about then is replacing barrels every so often.

I would check out making a Cap Bank with lower strength Capacitors in parallel it can work just as well as a single capacitor.

People please look into the materials hes talking about. These are in no way normal capacitors. They are high density capacitors where the carbon nano tubes are used as the metallic plates with an organic(AFAIK) binding that make a normal capacitor look like a joke. They are so close to each other and the insulation is so thin they are able to store amazing amounts of voltages with little impeadence and discharge there currents in millaseconds with ease.

Just look into them its cutting edge technologies. If you wish for me to help you design a home made cap of normal standards Tsath PM me and I will get back to you in a couple days. I have made roll capacitors with so little impeadence they where able to run a TEA laser.

I can help but dont expect anything spectacular. Blowouts is a common issue due to unevenness of the capacitor plates but there is also the fact that my normal imput source is 10kv-15kv/ 25-35MA.

Did you consider to build a Leyden jar, and use it as capacitor?

Ultracapacitors

Due to the incredibly high surface area to mass ratio of graphene, one potential application is in the conductive plates of ultracapacitors. It is believed that graphene could be used to produce ultracapacitors with a greater energy storage density than is currently available.[114]

Very interesting, check http://en.wikipedia.org/wiki/Graphene

Ultra capacitors, or super capacitors as they're often called, have a huge capacitive density compared to other types of caps. They achieve this through the use of carbon nano-fibers which give the dielectric a huge surface area for it's size. They are typically limited to low voltages, and those on the higher end of the range are more expensive. They can be combined into banks to deal with higher voltages, but it can add up to a lot of capacitors.

Assuming you had a large bag of them, all the same type and rating, you could connect them in series (end-to-end) using enough of them to add up to the voltage you need. For example, say your bag contains super capacitors rated at 5 Farads and 4V. Now suppose you need to be able to handle 40V.  All you have to do is to use ten capacitors in seriers. The only trouble is that doing this reduces the overall capacitance.

The total capacitance of the string you just made is only 0.5 Farads, 1/20 of what you wanted. To get back to the original 5 Farads you would have to add identical strings in parallel with the one you first made. Since capacitors in parallel just add up, you would need a total of ten series strings, each of which contain ten capacitors.

That's an over simplification, of course, because you wouldn't want to run the capacitors right up to the rated voltage, so if you really needed 40V you'd probably want your strings to be at least 12 caps long, etc. There's also the matter of how fast you need them to discharge, because you will be limited by the maximum current rating. Naturally you can add more to increase the capability, but you see where this is going.

At present super capacitors can't stand up to batteries in terms of energy density, but there are advantages, too. One big on is that they can be charged almost instantly, and no battery can do that. Not yet, anyway.

PP