Ignition & Electrical / Q&A

Mailbag: Why Does My New Vehicle’s Alternator Output Voltage Keep Fluctuating?

Jeff-Smith-Ask-Away-Alternator
(Image/Jeff Smith)

Q. I recently installed an aftermarket USB port with a digital voltage display in my newer Ford truck. I noticed my battery voltage varies significantly, between 12.9 and 14.1 volts, with no alternator warning liught on my dash.

Is this a sign that my alternator is failing?

A. Not necessarily. On modern vehicles, charging voltage can be as high as 16 volts, and as low as 13 volts. And there are situations where your alternator might not be doing any work at all if your battery is fully charged.

In many of today’s cars and trucks, the function of the voltage regulator is managed by the vehicle’s ECM. Alternator output is very precisely controlled—sometimes down to the milliamp.

Why a New Vehicle’s Alternator Output Voltage May Change

There are a couple of reasons why. First, the accessories and sophisticated electronics in late model vehicles place tremendous demands on the electrical system. Increased alternator control helps meet those demands.

Another less obvious reason is fuel economy. To reduce the parasitic drag of the alternator on your engine’s accessory drive system, the ECM may increase charging output during coasting or braking. This reduces the need to power a heavily-loaded alternator while accelerating or cruising. It sounds insignificant, but every little bit helps.

Finding & Preventing a Problem

But if you do suspect a problem, ensure contacts are clean at the battery, connectors, and grounds. Use a corrosion preventing agent when necessary. Connect a scan tool to monitor alternator output and look for battery and charging system fault codes.

Regularly test the battery’s health with a battery tester. If it fails, use the proper charger for your battery type, being careful to charge at a rate that will not overheat the battery. Retest the battery with a full charge.

Understanding New Vehicle Battery Management Systems

Also, many new vehicles now have a Battery Monitoring/Management System (BMS). If the battery is replaced in a vehicle with a BMS, a scan tool must be used to reset the system and enter the new battery information. The BMS manages batteries differently based on parameters like battery age, size, and type. Properly resetting the BMS will lead to longer battery life and better charging system performance.

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2 Comments

  1. Tim Henderson says:

    I just add my own ground metal to metal.

  2. brian Tousignant says:

    Why do the all the lights flicker/fluctuate on my 2011 Challenger when driving. There is no change in flickering with a rpm change? I was told am need a new battery cause my present battery is 5years old.

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