I am looking for a battery extender case for an iPhone 4/4S for my partner. I bought a Logic3 PowerSleeve in early 2011 for my phone at the time, an iPhone 4. It was £25 and it has been fantastic, even on my iPhone 4S. It appears they are no longer for sale. Amazon has 1x used one for £21 and eBay has 3x from £28.15 (but that's an auction price, with 6 days still to go). What battery extender case(s) do you suggest for the iPhone 4/4S? I don't really want to pay over £25 considering my PowerSleeve was that price and has done the job perfectly since I bought it.