Lithium batteries wear with every charge cycle, and after two or three years most phones show it. The first sign is the obvious one: a full charge doesn't get you through the day anymore, even though your usage hasn't changed.
The second sign is sudden shutdowns. The phone reads 30 or 40 percent and then turns off, sometimes restarting to show a much lower number. That's a battery that can no longer deliver enough current under load, and it gets worse in cold weather or when the camera is running.
Third, heat. A worn battery runs warmer while charging and while in use. Fourth, slow charging or a phone that only charges to a certain point. Fifth, and most serious, a screen or back panel that's lifting at the edge. That's a swollen battery, and the phone should stop being charged and come in right away.
On iPhone, Samsung, Motorola and Pixel phones, a battery replacement is one of the quickest repairs we do, usually finished the same day, and it costs a fraction of a new phone. The diagnostic, including a battery health check, is free.