Why batteries 75% power only once fully charged on MagicMouse?
I am using Apple Battery Charger (including the batteries)..
MacBook Pro (Retina, 13-inch, Mid 2014), OS X Yosemite (10.10)
There are a combination of factors. The batteries reach a state of charge, then relax, so to speak, and can then accept more charge. The first figure recorded on your machine may be 100% but within minutes can drop to 80% in this 'relaxed' state. If you take the batteries from the charger immediately the green light shows, they are rarely fully charged. You can test this by unplugging and plugging the charger back in a few minutes later and the light will turn to amber .. and stay that way for some time. This second charge when the light turns green is more likely to register as 100 %.
Additionally, after several recharges, the batteries do not charge up quite so efficiently and last for a shorter time. Then, if batteries are stored fully charged for a few days, they tend to run down. And to finally confuse things, some machines do not show the correct amount of charge.
Best to accept this relatively minor inconvenience in exchange for having charged batterioes in reserve.
Ususally, I find the batteries are good for between14 to 17 days on an iMac.