Why is voltage dropping slowly after installing a new alternator and a new battery on a 2002 Ford Focus SE?

Did you install a new voltage regulator? Is the belt tight? Maybe the "computer brain" is weakening.

Answer

If you have installed a new alternator and new battery, check the harness/wiring that plugs into the alternator, it may be corroded/bad connection causing your battery to not be recharged at idle speed; voltage should stay between 12-14.5v range and steady (some momentary drop may occur after turning hi beam lights, etc but it should return to proper voltage after a few seconds), as for the regulator, on most cases it is inside the new alternator so no further adjusting may be required; also, make sure you get the right alternator for your focus-there's like 3 types, the wrong one will cause your dashboard alternator lamp to be lit at all times-replace with right one or higher output if got power Windows,seats,etc.Good luck! Buena Suerte!