I'm dead broke. So, I have a cheap battery for my dead-broke-esque 2008 MacBook. It only lasts about 2 hours on a full charge. However, it's a little goofy at times. Not in a dangerous way, just that it seems to miscalculate how much time it has left sometimes, and suddenly dies at 25%. I know, terrible choice to get an old battery. But I'm poor and was sick of outlet-surfing, so I got a cheap battery. Anyway, since my upgrade to OS X Lion (in 2018, mind you), I haven't had as much of a problem, but still might have it sometimes. Plus, I'm going to be letting a friend/colleague of mine use it for a while, and would rather tell him to "plug it in when it gets red" than sit and explain the goofy battery more than I already have.
So what I am wondering is how I can, if possible, change the percentage of battery remaining that will cause the battery icon to turn red in the Menu Bar. I also would think it nice to be able to spring the "Reserve power" notification at a certain percentage as well. I know I can also just AppleScript Finder to pop up and say "ey, you're low on power" but I think it would be better to change the actual alerts.
Best Answer
You can't.
It's not something that can be modified.
The battery icon changes to red when you have less than 15% charge remaining (macOS 10.8 and earlier). It changes when there is less than 10 minutes remaining in Mavericks (10.9 and later). See About Mac notebook batteries
You'd need a 3rd party app like Coconut Battery (Paid edition) so you can get notifications when your battery reaches a certain level.
(IMO) If you're going to pay for an app to tell you this, just buy a new battery and be done with it.