No matter what Apple says about improved battery
life on the iPhone 4, there will always be those running out of power. Since there is no way to swap batteries on any iPhone, all these cases or sleeves have arisen for past models to allow for increased power, and here's one of the first for the iPhone 4: the MiLi Power Spring 4.
The MiLi Power Spring 4 claims to be both the thinnest and first external battery for the iPhone 4. Of course, first is debatable, because even at Amazon.com you can find other external battery cases. However, MiLi is well-known for prior products, so that's a plus, in its case.
The PowerSpring 4 has a 1600 mAh capacity (Amazon.com says 1200 in its product title, but if you look in the details, it says 1600). Pricing at Amazon.com
is currently $84.58, but the site says there's a 1 to 2 month delay (this will hopefully change soon). MiLi is selling it directly
for $79.95, however. We've always been fans of Amazon.com for newly released items due to their liberal return policy, though
These types of things have worked well in the past for other iPhone
models. Some of them don't do pass through charging, either well or at all, however (sometimes you'll get one of those famous "charging is not supported" errors no matter what the manufacturer says. Thus, it's always better to stick with a name brand for these things.
Of course, this could all be avoided with swappable batteries. These things do add to the bulk of the device, after all. Apple has always said they can get better battery life with non-swappable batteries, but it's also created the issue that eventually the batteries need to be replaced, that that involves cost, time and more money than a simple battery.