Eventually, your laptop's battery will die. It's inevitable. Just as humans are subject to death and taxes, batteries are subject to death. (Fortunately, the government hasn't figured out how to plunder tax money from a battery. Yet.)
You can tell when your battery is about to die by observing one unique trait: It suddenly becomes useless. It no longer holds a charge, and what charge it does hold is quick and unreliable.
Don't mourn a dead battery! Toss it out!
Keep in mind that batteries are considered to be toxic waste in most communities. You must properly dispose of or recycle dead computer batteries according to the rules of your community or jurisdiction. Never just chuck an old computer battery in the trash.
And don't get all Viking and try to burn your battery, either. That's just a bad thing.
You can buy replacement batteries from your laptop manufacturer or use a dealer- or manufacturer-recommended replacement. Be careful of replacement batteries that aren’t certified by their manufacturers! Those batteries can damage your laptop.