KitKat (ver. 4.4.2) was released in 2013 so it's a very dated version of Android. A typical battery that's also from around that time will be most likely close to the end of its use-life so it's not surprising if the battery is starting to fail. They don't last forever, most are rated to last at least a couple of years before their performance starts to degrade. It's an unavoidable occurrence.
Note that even if you buy a new battery to replace the old one, there's very little chance the new battery will have been manufactured recently. Any battery manufacturer won't bother to continue making batteries that match up with old, unsupported devices, there's no financial incentive to stock-pile old batteries that may or may not ever be sold. Another unavoidable issue is a lithium based battery will age no matter if it's constantly in use or just sitting idle on a store shelf. (It will degrade faster being used, but it still ages at a much lessor degree being completely idle). So whatever battery you buy now for your old device will probably hold a charge better than your current battery but will still be at a diminished capacity.
A workaround would be to just always keep your device plugged into its power adapter, or if portability is a factor buy a new, high-capacity, external battery and keep it plugged in constantly.
