If my cellphone's battery drains slowly from 100% to about 40% then drains very quickly, does that mean the battery is bad?

If you've ever noticed that your phone's battery drops from 60% to 50% in a matter of minutes, only to stay at 50% for what seems like an eternity, it's likely time to calibrate the battery.
Batteries degrade naturally over time, and their capacity gradually diminishes. However, your phone isn't always accurate in this regard—if your battery has declined to 95% of its original capacity, your phone may still show it as 95% full rather than 100% full (the 'new normal').
Allowing the battery to go from entirely full to completely empty on a regular basis might harm it and reduce its capacity over time. On the other hand, leaving your phone plugged in when it's totally charged can cause the battery to degrade.
Select Settings > Device > Battery or Settings > Power > Battery in most Android versions. To get a list of all apps and how much battery power they're using, go to Settings > Battery. Consider deleting an app you don't use very often if it appears to be using a lot of power.
By tapping it on Android 9, you can observe how power-hungry apps are wasting battery. You'll be able to enable 'Background limitation' for some apps. Keep 'Battery optimization' turned on for all apps.

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