If your headlights swing between dim and bright, check your alternator.
If you notice this happening, it’s more likely that your altenator is having a problem instead of your car battery. Dim headlights are a sign that they are drawing power from the battery because the alternator is producing little or no charge. If the lights get brighter when you rev the engine, it shows that the alternator is producing some current, but not enough to keep the battery properly charged when the engine is idle. If the lights have normal brightness and don’t change intensity as the engine is revved, your charging system is functioning normally. Other possibilities could be that your car battery is not able to hold a charge, or something else on the vehicle is constantly discharging it. You should have your car’s electrical systems checked because if it’s not a car battery problem, your car will keep draining every car battery you change and they won’t last very long.