(Pocket-lint) - While the new iOS 4 update goes some way to fixing the battery on the iPhone 3GS you have to admit it's still not great.
There are a number of solutions to solving this problem: one is to use your iPhone less than you currently do, it's not a practical suggestion of course but one that we thought we should mention.
The second is to carry around an iPhone cable wherever you go and jack into your laptop or a power socket whenever possible. Grab a quick bit of juice and be on your merry way.
The third is to opt for an external battery cover that not only protects your iPhone, but also gives you an extra charge when you need it to keep you going beyond 4pm; after a heavy session of internet surfing, twitter checking and general email pushing.
In steps the Mili Power Spring, the latest battery case come cover for the iPhone.
Improving on previous models, this one promises to be slimmer, offer just as much "extra" juice on the go and allow you to get that extra phone call, at least, before the phone becomes a rather expensive paper weight.
New here is a top element that now stays connected to the rest of the shell, and the ability to easily turn the charging capabilities on or off depending on your need, rather than just plugging it in to charge.
In extended tests we found that the Mili Power Spring doubled the life of our iPhone 3GS, giving it another complete charge when we were out and about - handy if you can't find a power socket to plug into.
There is a pay off of course for all this extra power. Like its predecessor it almost doubles the size of your iPhone making it rather large, while at the same time kinda defeating the point of having such a slim phone in the first place, that said it does work and goes about the task in hand.
In terms of aesthetics, the case is fairly non-descript. You get to choose a range of colours inside and out (although inside you'll never see), and detailing is kept to a minimum.
Rather than featuring a 30-pin dock connector, the battery is charged via a USB socket instead. It means you'll have to carry around another cable in your bag and it would have been nice to have seen a 30-pin connector socket so you could just use either the iPhone 3G or 3GS cable or other iPhone accessories to charge it.
It's only ever so slightly slimmer than the previous Mili Power Spring battery case that we previously tested here at Pocket-lint, and the capacity is around the same.
That said it will give you the use of your favourite mobile phone for considerably longer, so if you're heading out for the weekend or planning a long day out of the office this is certainly one to consider.
However, because it is just a case, the good news is that you can simply whip it off when it runs out of juice.