Which battery extender case(s) do you suggest for the iPhone 4/4S?

[King Mustard]

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.

 

[Trip]

I don't use a case but use the Spyder i4X from Digital Research. PowerShadow i4X Battery Extender for iPhone/iPod - Spyder Digital Research

Shop around. I caught one one sale fro $21.95US. It holds a full charge for an iPhone 4 or 4s. Charges/Discharges at 500mAh. Very small and lightweight. It will plug into my 4s with a Otter Defender case on it. A Lifeproof will not allow it to plug in.