One of the annoyances of Mountain Lion has been the loss of the option to have the remaining battery time shown in the menu bar. Fortunately I can offer you not only a solution, but a better option, all for the low, low price of $0.
I have used SlimBatteryMonitor instead of Apple's menu bar battery monitor for several years, because it is much more flexible than what Apple has ever offered.
For example, I have never liked using "Time Remaining" while I'm on battery power, because I find that the estimate has always been unpredictable. So when I am using the battery, I prefer to have the "Percentage Remaining" displayed. However, when charging the battery, I like to have the "Time Remaining" shown, because I want to know about how long it will take before the battery is fully charged. If the battery is fully charged and I am using AC power, I don't really need to see the battery status in the menu bar at all.
Even before Mountain Lion, those various options would have been hard to maintain using Apple's battery menu bar item. Sure, I suppose I could have gone into the settings and changed the preferences each time I went from battery to AC to fully charged, but we all know that was never going to happen.
SlimBatteryMonitor has separate preferences for each of the three states I mentioned above: on battery, charging, and charged. Here's how I have mine configured:
You can have it show the time or the percentage (either with or without a battery icon), or just have it show the battery icon (which you can assign different colors for each state). When the battery is charged, SlimBatteryMonitor can even "hide" itself, meaning that it will not show any menu bar icon at all. (Because it is still running, it does stay in the menu bar, there's just nothing shown and the width is reduced to a few pixels. You probably won't even notice it unless you look for it.) The developer has a very detailed page explaining configuration options.
The app is free. Donations are accepted but there's no limitations or nag screen. The current version (1.5) hasn't been updated for a few years, but I've used it on Snow Leopard, Lion, and Mountain Lion with no problem. Those of you still hanging on to Mac OS X 10.3 or 10.4 can even use the 1.4 version of SlimBatteryMonitor.