The mission. Fully deplete the battery in an iPhone 3G a long way from home to see if a Kensington Mini Battery Pack/Charger could keep it alive for the return journey.
The purpose. To simulate that "Damn, I forgot to charge my iPhone last night" moment.
The method. Time the charge depletion so that the iPhone would power itself down at Sydney airport terminal. (For those who don't already know, I'm from Adelaide, so Sydney is a long way from home.) Then connect the Kensington charger and monitor the results.
How it played out. Attaching the charger to the iPhone revived it, but it took about seven minutes before it was operational, as it won't fully boot without a minimum charge level. It took a further 70 minutes for the charge to syphon from the Kensington charger to the iPhone. Once the Kensington was depleted, the iPhone registered a 70 per cent charge.
However, because it wasn't a deep charge, it took only a few minutes of use (checking SMS and emails) before the charge level dropped to about 50 per cent. This charge level was recorded at about 11am. The phone was still working in the taxi on the way home from Adelaide airport at about 5pm, though I expect another hour's use would have been available had I not played SimCity on the plane. During the day, only 3G was enabled. No Wi-Fi or Bluetooth.
Conclusion? Kensington's Mini Battery Pack/Charger will get your iPhone back on its feet again, probably long enough to get you through the day and back to home base where you foolishly left your sync cable that otherwise could have juiced your iPhone from your laptop. If you had a laptop.
It bears mentioning there are a lot of variables at play with iPhones. My 3G iPhone is more than a year old (yes, that's right, I was in line on 11/7/08) and its battery is not as efficient as it used to be. Combine this with differences in power consumption profiles between one iPhone and the next (Bluetooth on/off, fetch enabled/disabled, screen brightness setting, etc) and it's safe to say that your mileage will vary.
Kensington would like you to think you could plug their charger into a fully-charged iPhone and leave it attached for play time of "up to 30 hours of music and 6 hours of video, 3 hours of talk"?. Yeah, well you could, and you might get those benefits, but I found having the charger hanging off the bottom of my iPhone pretty awkward. It was easy, with the wrong grip applied, to disconnect the charger from the iPhone's socket, though this had a little to do with my Capdase hard case, which prevented a really snug coupling.
The battery can be attached to the iPhone while it charges from the same source but having the charger attached prevents iTunes syncing. A little inconvenient. LEDs on the front face of the charger indicate partial and depleted charge states. The charger comes with a retractable USB cable. With a $59.99 RRP, it rates as an affordable solution to being caught short of juice, and it gets the job done.
Just one more thing. If you're going to use it as I did, as a recharging device as opposed to a supplementary power source, you'll have to be sure to have the Kensington battery charged and packed for the ride. Which kind of brings us full circle to that "Damn, I forgot to charge my iPhone" moment.