Your laptop's battery doesn't last forever. You may think that, because it's rechargeable, you will never have to replace it. A few years down the road, when you get about 15 minutes from a full charge, you will see the truth. A standard laptop battery lasts about 300 to 500 charge cycles before sustaining significant loss of total charge capacity. When this occurs you will need to replace it.
Plug it In
When your battery is worn out, you won't really be able to rely on it for use of your laptop. This doesn't mean that you're out of luck. You will just have to sacrifice mobility. You will need to plug your computer into a wall outlet to continue using it. You will have full functionality; you will just be tethered to the wall. This will keep you working while you find and purchase a new battery for your new computer.
Purchase New Battery
When a laptop battery wears out to the point that it can't hold a charge, there is nothing you can do to fix it. The battery is garbage and you will need to replace it. Although you can purchase third-party batteries from various sites online, you will be better off buying it directly from your computer's manufacturer. Make sure to locate the exact battery that matches your laptop model to ensure it functions properly.
Install New Battery
When you have the new battery in hand, you will need to install it in your laptop. While swapping out components on a laptop may seem like a daunting task, replacing a battery is about as simple as it gets. You will need to shut down your computer, unplug it, close the top and turn it over. The battery will be installed in to the bottom of the computer. You will likely need to unlock it by turning or flipping a switch, and then pull it out or eject it with a separate button. Each model is different but the process should be obvious. If you can't figure it out, consult your computer's documentation for more details. Charge the new battery by plugging your laptop back into a wall outlet.
Swap Between Batteries
Now that your laptop has a functioning battery, you should be in business for another 300 to 500 charge cycles. If you would like to extend the time you have before having to worry about it, consider purchasing a second battery. Having two batteries allows you to swap back and fourth, extending the life of both batteries. Also, if you keep them both charged, you can run your laptop without external power for twice as long. You can use the first battery until its drained, then simply swap in the second. You will go from 15 minutes of battery life to multiple hours.