Why are ALL the iPhone 4S batteries gone bad?

Hi there,

I own a small phone and PC repair shop. Over the last few months the amount of people coming into me reporting iPhone 4S battery problems has more than quadrupled.

What's going on ? I can't keep the things in stock, and the ones I'm getting now seem to be of a lesser quality as many of them are coming back into me also.

Can anyone recommend a good supplier of batteries in Ireland or the UK ?