What is it you want to repel?
If it's cat's or foxes, perhaps to allow birds in your garden, then you would need to have a mains one for strength as the majority of battery ones only reach the part of the garden they face. Cats also get used to it as you've found out.
I'm only guessing here, but the best and kindest thing you can do if you want to encourage birds into a cat garden is to give them collars with bells attached. This at least gives the birds half a sporting.chance - and the cats won't suffer from the sonic vibes.