There are a number of apps for monitoring battery health and current status. Battery Health (not yet upgraded) is a good one. It has more detail than anything else I have seen, including showing the actual charge and discharge rates so you can make your own estimates of time remaining. Its calculations of time remaining for different activities are a nice touch.
I suggest two improvements. The first is providing specific information on which apps are using the most power. The data is available from Activity Monitor, and Apple’s own battery monitor provides this “Power Usage Information,” although not very well.
The second is for electrical geeks - but this app is intended for geeks, and it claims to know about electricity. It measures the discharge rate in Watts and “mAh,” milli-Amp hours. mAh is wrong. Current is measured in milli Amps, with no hours. Just like you don’t measure power in “Watt hours,” but rather in Watts - which they got correct. One Watt for two hours is two “Watt-hours,” and 800 milliAmps for two hours is 1600 mAh. This error is common among people talking about solar energy. It’s like the difference between gallons and miles per gallon.