If you notice that your iPhone began to work less and less after recharging, this does not mean at all that something has broken in it - just the capacity in its battery has decreased. In any battery, capacity decreases over time. In the iPhone, it is initially within 1600 mAh, and after a year it will decrease to 900. This greatly affects the duration of the device without recharging. Therefore, if you do not want it to work for two to three hours, you need to do such an operation as replacing the battery somewhere every two years.
The charge level does not always mean that the battery is working, because it only shows the voltage level on the battery. It sits especially fast if you frequently call or use wireless networks. Therefore, to extend the battery life, do not use it to the full maximum.
Also, its capacity drops rapidly if you have a habit of recharging the battery when it has not yet used its energy resources. So put the device on charge only when it is really needed.
Replacing the battery is a very useful process, because after it your device will again be able to work for a very long time. Therefore, do not neglect this from the principle of "better to save", because sooner or later the battery capacity will drop to 200-300 mAh, and then the iPhone will turn off right during calls, and this is not very nice.
If you want the battery replacement to cost you only the price of the battery itself, then you can try to replace it yourself. But this is a rather risky undertaking. First, to dismantle the battery, you need to disassemble almost the entire device. Secondly, if you do this incorrectly, your warranty will automatically end, and you will lose both it and the iPhone. Therefore, if you are unsure of your abilities, then it is better to entrust such a procedure to professionals - it will turn out much safer and not much more expensive.
So, how is the official replacement of the iPhone 3GS and 3G battery? If you bought a licensed device in an official store, then you can safely go to a service center - there they will do everything expertly and efficiently. If you have already lost the guarantee or lost it in any way (for example, replacing the battery manually did not go well and you accidentally ripped off the warranty sticker), then there's nothing to be done - you have to go to private offices. Here you should also be careful: be sure to ask the master for written confirmation that you left your device with him for repair - otherwise you may not get it back. It will not be superfluous to consult with friends who already know how to replace the iPhone battery.
Do not tighten the battery with a change of battery - this will negatively affect the device’s operating time. The most optimal interval for its replacement is one and a half to two years. During this time, the iPhone battery will work about half as weak. On the other hand, replacing the battery is a costly business, so you should not change it too often either. If you still want to save even more money, you can try to replace the battery manually.