One of the reasons why I choose a certain model of a mobile phone is the battery. The long life battery is essential for me, a person who sometime forgets to charge battery for long time and yet need to have important phone call all the time. Some mobile phones have a lot of compatible battery available while some only the original.
Some models like samsung mobile batteries will be hard to get, and the only available battery is the original. Compare with Nokia which you can find the compatible Chinese brands of battery.