Do you still keep your old iPhone models? Like the 3G and the 3GS? Some of my friends and I still do. And we have a common problem, the battery can no longer hold a charge. The phone can be charged up to 100% but when you start using it the battery drains very quickly. The only sensible solution at this point is to replace it. And it can be done by sending to apple or replacing it yourself. The problem is, it can be expensive and complicated, but can be cheap and easy, as long as you know what to buy! Here's a useful guide, read through and see what's best suited for you.
Replace it yourself with internal battery
This is an option for the brave and with experience. You need to buy the battery which at ebay costs at least £4, plus the opening kit.
The iPhone's battery is located and kept very securely. It is not straightforward to open the case and change the battery like other mobiles. Opening it up is a problem, if you manage to open it, replacing the battery is an added problem, then putting it back together after replacing it is another problem! And your warranty becomes void when you open the case yourself.
See guide video on how to open and replace the battery of an iPhone 3G below.
Now you know it is not for the faint heart and needs a bit of an expertise and practicality to do it yourself.
Send it to apple
This is more reliable than doing it yourself. You will not lose your warranty and you can count on the service. The problem here is the costs and the waiting time. Service arranged through apple at the time of this post is £55, plus a shipping cost of £7.44. See link below for more information. While this service is reliable, it may not be sensible because of the cost. Especially when you just want to give away your old device. But if you still want to keep and use it as your main phone then this is the best solution.
Buy external battery case
If you want to revive the power instantly this is the most practical option. Buy a power sleeve, or external battery case for your iPhone. It costs at least £10 from ebay, see related products below, and other sources like at mymemory. You can buy it for £9, delivery included.
The power sleeve is doubled as a case for the iPhone, that's why it is also called a battery case. It is lightweight but a bit bulky. But the bottom line is, it works and it is cheaper and safer than the two other options above.
The unit is a polymer battery encased in a lightweight material. It has a mini USB port for charging. The battery indicator has 4 lights that lit up during charging to show how much power the unit been charged and a button to press to know how much power is still left. There's an on and off button as well to turn the sleeve to power up the iphone. The same button doubles up to charge the iphone as well while charging the power sleeve. The unit comes with a mini USB connector but without a power charger. This doesn't matter as you can use the iPhone's original wall charger to plug in the mini USB or any other 5 volts charger that has a mini USB socket.
Back view while being charged, note the lit up led light indicating the charge
The iPhone within the power sleeve
As simple as it is, it revives iPhone's battery instantly, cheaply and without sweat.