There are a few reasons why you may need to replace your motorcycle battery, says Motor Gear Expert:

Your motorcycle's generator or alternator should charge the battery while you are running the bike, but if the battery isn't holding a charge properly, your bike may not start the next time you try it.
The engine cranks over slowly or does not crank at all when you try to start it.
Your battery is between two and five years old, and you want to replace it as a preventive measure so you don't run the risk of being stranded somewhere.

When you first notice that your motorcycle battery isn't holding a charge, you may wonder if it is flat and can be recharged, or if it needs to be completely replaced, says MotoFour.com. They explain that this is an important distinction because of the high cost of new motorcycle batteries. You can check the battery yourself if you have a voltmeter, or you can bring it the nearest motorcycle shop and have them charge the battery, then run a load test. If your battery fails the load test, you'll need to replace it.