My battery life has reduced significantly.
My battery life percentage runs out more quickly than it used to. Why is that?
The battery after many charge and discharge cycles will lose it's ability to hold a charge. Extreme changes in temperature can also reduce battery life, as well as keeping it plugged in all the time. You should let the battery completely discharge once and a while to help it have a longer life. If the battery is unusable for your needs, you should replace it.