Phones aren't like cars, where changing the battery is as easy as disconnecting the old and connecting a new one. They used to be, but thinner phones and higher battery capacity demands meant that they are now small foil bags sealed up inside your phone. They are designed to last a specific number of charge cycles, and a charge cycle is taking the battery from its lowest point to its highest. It's worth noting that this doesn't mean zero to 100, even though the phone will report it that way. Batteries are designed to be used at 80% of the listed capacity (but still safe up to 125%) and never brought down to a zero-charge state. The circuitry inside your phone takes care of that and makes it simple for us by reporting the scale to 0-100.

The number of charging cycles that a battery is rated for is an approximation; the reported number is based on an average. There are ways to "game" the system when it comes to charging cycles, and you've probably seen advice that says to charge your phone a certain way to extend the usable life of the cell. These do work, but you won't be able to get too much extra life from your phone's original battery, so it may not be worth it. You have to decide if micromanaging the way you charge your phone is worth another two or three months of life.

I just charge my phone whenever it needs it because life is too short. #YOLO.

Signs your battery is ready to move on

A battery doesn't just suddenly die under normal circumstances; it gives some warning signs that it's getting close. Sometimes batteries can just short or have another type of fault that kills them, but those instances aren't very common. A lot of time and energy went into designing a safe and long-lasting battery for a phone.

You might notice when your battery is going downhill because it rapidly discharges while you're using it. If you used to go half the day and your battery was still at 60%, for example, and it now hits 30% during the same time, it's a sign that the battery is starting to sour. Other things like bad apps or a bad update can do this, too, so it can be tricky to diagnose.

One of the first warning signs is your phone losing charge right away. If you take your phone off the charger when it says the battery is at 100% and it immediately drops to 90% or 80%, the battery is getting a bit old. It's normal for some phones to lose a few percentage points of battery charge right away (at least according to the battery gauge), but a drop of 10% or more is a sign of a problem.

Normally the life span of a normal persons mobile phone how uses it regularly and charges there device each night is approx 2 Years. After this time the battery degrades at a very fast pace. 

If your phone is bulging or swelled up, stop using it right now.

If you notice your phone bulging in the middle or getting very hot on or off the charger, that's a sign of a bad battery too, but you also need to stop using it right away and take it somewhere that a tech can look at it. We've all heard stories of phones bursting from a bad battery, and the whole Note 7 thing is still fresh in our minds. Phone batteries are safe when used as intended. Because of things like Samsung's excellent response to Note 7s catching fire and bursting, they are safer now than ever before. But accidents and malfunctions happen. Don't ignore any signs of excessive heat or swelling because nobody likes it when their pants (or anything else) catch fire.

You probably won't see any of these signs for at least 18 months after you bought your phone if you bought it new. Battery life may be listed in charge cycles, but the goal is to make them last two years since that's the standard length of a carrier contract. But if you really like your phone or really like not spending money on a new one, you will see your battery die eventually.

What can you do about it?

You used to be able to go online or take a trip to the carrier store and buy a new battery for your phone. Those days are gone, and I doubt they will ever come back. But the battery in your phone probably can be changed by someone who knows what they are doing like us at Repair My Device

Changing the battery in a sealed phone isn't hard for people who know what they are doing. Most of the time.