[Android Central Question]

It was charging normally this morning then all of a sudden it slowed down. According to the Ampere app, it's charging at 450mA.

The USB cable is new and I'm using the original wall charger that came with the phone. If it's the battery that needs replacing, how would I know?

[Rukbat]

The battery wouldn't be flat.

Lay it on a flat table and tap a corner to spin it a little. If it turns about half to 3/4 of a turn, but seems to be dragging, it's probably okay. If it spins, it's bulging, and time to properly dispose of it.

There are battery apps that will give you the condition of the battery. (Stay away from apps that claim to "save" battery [they're usually the ones that also claim to "clean" your RAM], but the ones that just check the battery [like the health screen in Accu​Battery] will give you a good indication.)