Could very well be time to change the voltage regulator brushes in the alternator. Easy, two screws, and cheap, $35 or so. Depending on how long it's been since you checked or changed them, you may just want to order a new one and change it. Keep the old one if it's good as an emergency spare.

But as the others have suggested, make sure you don't have a constant drain from a malfunctioning circuit.

Len