I'm back on the road a the moment and I've noticed that my iPhone 5 battery is getting a lot more hammer than it's used to. It hasn't yet given up the ghost during the course of the day, but it isn't far off (perhaps I should cut back on the 12 hour days!).
I've not used a battery extender since my first iPhone, the iPhone 3G (I used a Mophie Juice Pack Air), but it did the trick and I decided to pursue a similar course with the iPhone 5. I didn't want to spend a huge amount of money but obviously wanted something reasonably reliable. In the end, I went for the Expower 2200Mah Rechargeable Extended Backup Battery Case for iPhone 5s/5 from Amazon.
This is a two piece case with an in-built battery and little pull-out stand. The case is a black rubberised material and weights in at just under 80g and adds about 1cm to the length of the phone and just a couple of millimetres to the width and depth. It does make the phone feel chunkier but adds a reassuring feeling of protection from the rubberised covering.
The Expower case charges with a normal lightening connector and charges the battery and phone simultaneously. Pressing the small silver button on the front of the case starts the battery, and a series of little blue LEDs gives you a visual indicator of what's going on. I haven't done any timings yet but it doesn't appear to take an unreasonable amount of time to fully recharge the system.
During the week, I'm up at 5:45am and on the way to work by 6:15am. I find that by 13:00 the phone has been down to about 65%. This is when I kick the extra battery in and in about 30 minutes I'm back to a full charge, which then lasts for most of the rest of the day, by which time I can get back to a mains charger.
I'm very impressed with the Expower. It's sturdy, reliable, and mine cost £9.99 (with an extra £4.41 P&P - it's not available through Amazon Prime). I even bought a second Smart Wallet to stick on the back!