My iPhone 4 is now approx. 18 months old, and the battery has finally started to deteriorate. It'll barely make 12 hours from full to empty. I've seen a few guides and it looks really easy to replace, but I'm struggling to decide which battery to buy. They range in price from about £4 to £10 on Ebay. Some claim to be genuine, many don't, and all photos look exactly the same! So my question is how do you know if one is genuine, and does it really matter?