Finsky, it's true. once an animal learns to go through a hot fence, it's no deterrent. most animals get zapped once and leave it alone, but every once in a while.....
i am with Allen for that very reason. the bigger the better. you can't go wrong with a 50 miler on 10 ft of fence!
and ground the charger, not the fence. the bigger the charger/drier the ground, the more ground rods. on my big charger in wet Oregon, i have two ground rods driven about 3 feet into the ground. make sure your clamps are good and contact is good. no ground on the charger, no zap on the fence.