Goldilocks! 

If you use NiMH batteries (such as Sanyo eneloops), believe it or not, Goldilocks describes a problem with knowing an accurate battery status.  NiMH and alkaline batteries have different discharge curves as illustrated in the figure below.  It used to be that Apple’s reported battery status for bluetooth devices was based solely on the alkaline curve.  Because NiMH batteries are 1.2 volts instead of 1.5 volts, they appear less full when they are fully topped up.  This means that a fully charged NiMH which tops out just under 1.5 volts, would display as around 90% full, and would very quickly taper off to only about 50% full.  Then when the Magic Trackpad came out, Apple added an innovation in which they automatically detect the battery type, and use an alkaline discharge curve to calculate battery status for alkaline batteries, and an NiMH curve for NiMH batteries.  This would be a huge improvement except that the NiMH curve they use is woefully inaccurate.  I measured this curve which is represented by the dotted line in the graph below. 

Unfortunately, Apple’s efforts leave a reading of 100% charged for an NiMH battery all the way down to the point where the battery is only about 60% full.  This brings us back to Goldilocks.  Depending on which device you are using, NiMH readings will either be too hot, or too cold, but never just right.  This is illustrated in the second figure.  In that figure, I have highlighted a single voltage at which Apple will either display 100% charged, or around 60% charged, when the real status is around 90%.  So who is Goldilocks in this story?  MightyMonitor is “Just Right!”   MightyMonitor 5 adds new optimizations for NiMH batteries in Apple keyboards and mice!

Make your Mac “Just Right!”  Download MightyMonitor 5 and NiMH optimize percent remaining for your mouse or keyboard by selecting the appropriate setting from the preference dropdowns.  At some point Apple will likely fix this, but until then, there’s MightyMonitor to the rescue.  One caveat.  Since the Trackpad reports out status of NiMH batteries on the dotted curve, there isn’t any way for MightyMonitor to optimize its status.  Bummer.

A Plea!!!

If anyone at Apple (or anyone else) knows how to get the actual battery voltage for the mouse, keyboard, and trackpad, I would LOVE to be able to get that.  Based on my own fairly extensive testing, the battery % values displayed in the bluetooth device menu are incorrect for NiMH batteries, as are the NiMH data for the Magic Trackpad.  I am able to calculate much more reasonable values with the ioreg data, but would be MUCH more accurate if I had the actual voltages from the devices.  A little help would be GREAT (a little objective-C perhaps?)  :)  If you can help, please shoot me an email.