The batteries in your MacBook, whether you have a MacBook Pro, MacBook Air or MacBook all have limited battery life. They have designated charge cycles for which they last. After those charge cycles are completed, they will still last but the quality of the battery will begin to drop and drop quite significantly after some time.
Over time, you will notice that the battery will take much longer to charge and will also hold charge for very little time. Typically, a newer model MacBook or one bought after 2010 will last you only 1,000 charge cycles. A charge cycle is when your battery is charged from 0% to 100%. So, if you charge your battery from 50% to 100%, it will only count as half a cycle. As you can see, it can take a good couple of years before you go through 1,000 cycles. Older MacBook batteries are designed to only last about 300 to 500 charge cycles. So, if you have a MacBook that was bought in 2007 or so, there is a very good chance that you are well over the recommended charge cycle limit.
These are the kind of tips discussed at http://www.whyismymacsoslowallofasudden.com/, a site dedicate to helping you fix performance and speed issues on your Mac. You can visit and bookmark the site to be updated with critical daily tips on how to fix Mac OS problems, especially those associated with Mac OS Sierra, the latest operating system from Mac.
Coming back to batteries, some Mac users just pull out the batteries when they begin to malfunction. Though this can be a temporary solution, please understand that working without batteries can mean that your Mac can be a little slow, as it is designed to use assistive battery power in addition to wall unit provided power.