I don’t really see the point in telling (in the sense of notifying) the customer that the battery needs to be replaced. One does recognize that a battery needs to be replaced, as it affects the usage of a device. When you notice that your battery doesn’t hold charge for long, you can assume that the battery needs to be replaced.

The implementation of telling that the battery needs to be replaced will probably be archieved with a(nother) notification message, which can be dismissed but will show up again after a certain time.

I see it as a scheme to steer the customer towards replacing the device altogether, especially when the battery is not designed as a removable part of the device.

It will be similar to a notification in an inkjet printer application that the ink cartridge will need to be replaced, (despite of the ink cartdrige is still half full) that you need to dismiss each time you want to print something.

Edit: typos