For most of us, Macs have two types of batteries. The battery that runs all Mac notebooks. And the batteries that drive our connected Bluetooth devices; mouse, keyboard, trackpad.
Check the battery level for the Bluetooth connected devices is not exactly complicated, but it takes a few clicks for each one. Open System Preferences, click Bluetooth, see the Bluetooth devices connected to your Mac.
Unfortunately, Apple does not make it easy to see exactly how much battery is left on each device. Hey apple. The percentages would be fine. Minutes as well. You can also click View Bluetooth in the menu bar and a few clicks will tell you the percentage of batteries left on each device.
Or just add the free BatteryAlert utility to your Mac.
BatteryAlert is a surprise, a Dock-based app, so it doesn't care about the Mac's already crowded menu bar. In the Dock, BatteryAlert displays the remaining battery charge for the device with minimum power.
Click BatteryAlert and you will be met with battery status for each connected device mouse, trackpad, keyboard, and alert options. You can receive notifications when the battery level of any device reaches a certain threshold email, message, alert or alert.
Nicely done, easy to set up and use, and it's free. Apple should do this in OS X (the only warning is you need OS X Yosemite).