I'd advise using a large leisure battery. We have three 120Ah, which are pretty heavy so do need to be moved with a trolley. You need two because it takes ours two days to charge. Not sure about the cows, but if they are like sheep they need to be trained at a young age about electric lines, otherwise they just charge through them. I'm not a fan of electric nets as such. I'd be worried about the chickens killing themselves on it and they are very difficult to stop shorting to earth and shorting to earth will quickly discharge the battery. Our arrangement is to use the net as the earth circuit which avoids the previous issues and then run live lines around the outside. You are after all trying to keep predators out, as a simple non-electrified net will usually keep chickens in, although youngsters will occasionally fly out.