A forum for people who enjoy electronics as a hobby.
Smart battery chargers monitor battery voltage and temperature and there is no concern for shortening battery life. The type of battery makes a difference too.
Most modern cell phones with lithium batteries are smart and safe. If the battery is flat, square and has three connections it is probably smart. The third terminal is a temperature sensor.
Home phones with NiCad batteries or NiMH batteries are not smart but charge at a low current and are safe enough.
When in doubt read the book that came with your phone.
Toys or other cheap devices are not smart or safe. The less you paid for it the dumber it is. Battery chargers? Read the book that came with it. The less you paid for it the dumber it is.