Battery Medic 4+

This little app does just what it says, viz., gives lots of helpful information about current battery health (current charge in mAh plus estimated amount of time before it'll need to be recharged. I especially appreciate the user-set 'low battery' notifications and 'full battery' notifications — very practical, so I'm never surprised when my MacBook just goes black while I'm so focused on a task I forgot to keep an eye on my battery's remaining charge. This is a convenient app that just works. Enough said.

This app is like having another widget to customize your desktop and have more oversight about what is going on under the hood. It does not do much, but if you need or want anything it is capable of but don't care about highly detailed specs/controls provided only by paid apps, then it is perfectly worth it (it is free and without in app purchases, so it is obviously worth it).
I rate it four stars because it does not provide everything you may want from an app such as this, so it obviously has room to improve (hence it not being a perfect score of 5/5), but the rating becomes arbitrary when you have nothing to lose from installing it, so just try it out.

The does a great job. My MacBook Pro would not report a low battery before it would power off. I reset the SMC but that did not solve my issue. This product allows you to set the alert to whatever baterry life percentage that you setup. One improvement would be to display the warning every minute with an audible alert until the power adapter is plugged in. The current alert can be overlooked if you are busy working. It displays but does not require any acknowledgement action to be taken.