I have a MacBook Pro 5,1 running Yosemite 10.10.5.

It has had excellent battery life on a new battery, but 6 months after battery replacement suddenly the life had dropped by a factor of 2x-3x. The battery still has 95% capacity remaining as indicated by coconutBattery.

Nothing seems out of place in Activity Monitor, there are no apps using significant energy, yet the idle power consumption is never below 15W.

The battery life reduction did not coincide with any Apple updates.