If your vehicle is showing headlights get noticeably brighter when accelerating, here are the most likely causes and the parts you'll need to fix each one.
The alternator's internal voltage regulator is failing, or its internal brushes are worn out. This causes it to produce insufficient electrical power at idle, but as engine RPM increases, it spins faster and generates enough voltage to brighten the lights.
A stretched, glazed, or loose serpentine or alternator belt can slip on the alternator pulley at low engine speeds. When you accelerate, the belt catches better or spins fast enough to temporarily restore proper electrical output.
The battery acts as an electrical buffer for the vehicle. If it has a weak charge or a dying cell, it cannot stabilize the system voltage at idle, causing the headlights to dim until the alternator spins faster to compensate.
Poor connections at the battery terminals or ground wires create high electrical resistance. This prevents the battery from properly buffering the electrical system at idle, leading to noticeable voltage swings. This often happens if terminals aren't tightened properly after recent battery work.
The spring inside the automatic belt tensioner wears out over time. It fails to keep the drive belt tight against the alternator pulley at low RPMs, causing the alternator to underperform until you accelerate.