I think most people are missing Mr Singh's point. In basic terms the cap as he says is the storage tank, when the demand is on the storage tank helps to keep the flow, because the original source (battery/alternator) is stressed out. Once the source stays under stress too long the tank will empty and you are back to square 1 with original dimming prob. So as the man says and everyone seems to agree...beef up the source then you wont need additional storage.
Good to see we all paid attention in physics class :D