Headlights brighten and dim when accelerating

I don't think this is a major problem, but it could indicate an underlying battery or alternator issue.

A typical 12v battery in good condition with no output load has a voltage of 12-12.7v.

A typical alternator produces 13.8 to 14.2v, somewhat depending on rpm.

A typical alternator voltage regulator will control the voltage of the alternator over a wide rpm range.

Therefore, if you turn your lights on without the motor running the light produced by he headlights is based on the battery voltage of less than 13 volts.

When you start the motor the alternator voltage should be at least 13.2 volts, which should make the lights a bit brighter.

When you increase the motor rpm the alternator voltage will increase to approximately 14.2v, before the alternator voltage regulator limits the output voltage. Headlights would/should get brighter.

The result is that your headlights could increase in brightness because of the increased voltage from the alternator at higher rpm, to the limit setting of the alternator voltage regulator.

You probably wouldn't notice this unless you see the reflection of the headlights on something in front of you, like you have observed. You can make this reflection check intentionally on your garage wall or some store windows at night. Most of the time you wouldn't notice this, unless you are in traffic and see the reflection of you headlights on cars in front of you.

You can also check this more accurately with a simple voltmeter.

Check you battery voltage with the car key off and no lights on. It should be 12v +/-.5v.

Start the motor and check the battery voltage, again. The alternator output voltmeter reading at the battery should increase to =/>13.2v.

Increasing the motor rpm to 1500 rpm should produce a voltage reading at the battery of 14.2v, but certainly not more than 15v.

Now do the same checks with the headlights on and compare the voltage readings.

Without the motor running and the headlights turned on the voltage may drop to 11.5v or less.

If the voltage readings are in this range under each condition, especially with the motor running, the alternator is working properly.

The headlights getting brighter with increased rpm could indicate that your battery is starting to get one or more weak cells. Each cell of a lead acid battery in new/good should produce 2.1v. Therefore, 6 (cells)X2.1=12.6v, for a new/good 12v battery.

Most auto parts stores or repair shops have a battery tester and can check your battery condition very easily. Some will do it for free.

Older cars, especially 6v battery cars, with generators and mechanical voltage regulators were notorious for headlight fluctuation. Alternators and solid state voltage regulators eliminated some of the issues.

I hope this makes sense. There should be a lot of internet articles on automotive alternators, batteries, and how they work.

Best regards,
Jerry

 

Hi Spazz, just a thought: have you also checked this while not driving, i.e. do the lights brighten based on the revs? Otherwise it could also have to do with the change in beam angle because of accelerating (nose goes up slightly) and braking )nose dives slightly). As said, just a thought.....

 

My 575 will do it every so often but it is only for an instant and than goes back to normal illumination.