Q: The battery is almost empty

What battery? An iMac does not a have a battery that shows full/empty. If you are referring to another Apple device, please be so kind as to include us in knowing what you have. No one here can see your equipment, and your surely don't want people guessing! You have to give enough details to get help.

Ah! I see. Thank you.

Please see this Apple article for instructions on changing batteries:

Apple Wireless Keyboard, Mouse, and Trackpad: How to install batteries - Apple Support

I understand now. You are seeing information coming from the biult-in Notifications utility. I looked at System Preferences > Notifications on my install of OS 10.10.4 and do not see a way to turn them off if they originate from the operating system.

You can turn all notifications off using information in this article:

http://osxdaily.com/2012/08/10/turn-off-notification-center-in-os-x-temporarily/

but that blocks all notifications. It only helps if you have no need to see if e-mail, Messages, reminders, or calendar alerts happen.