The alternator is your car’s primary source of electricity when running. It generates power to recharge the battery, and powers most of the auxiliary electrical systems when you are driving. Sometimes the alternator will generate too much electricity and this can lead to it overcharging the battery.
In general, the symptoms of an overcharging alternator include a red battery light on the dashboard, a check engine light, blown fuses and bulbs, and damage to the car battery.
All modern vehicles have safety features that will protect your vehicle’s electrical system from an overcharging alternator. However, an overcharging alternator can still cause damage if it is not repaired or replaced.
What are the symptoms of an alternator that is overcharging? (5 common symptoms)
The most common symptom of an overcharging alternator is a high voltage reading when the alternator is running. The first signs of overcharging will often appear as damage to the charging or electrical system.
Most standard car alternators will generate a voltage of between 13.5 and 14.5 volts when the engine is running. However, it’s not uncommon for an alternator to supply up to 16 volts on some vehicles, often for short periods of time. The specific voltage that points to an overcharging alternator will vary from vehicle to vehicle.
It all depends on the vehicle and the amount of power required by the vehicle to run all of the electrical systems. Many modern vehicles also generate electrical power through brake regeneration and use special batteries that are adapted for use with stop-start systems and hybrid drivetrains.
Symptom 1: High voltage reading from the alternator when the engine is running
One of the most common symptoms of an overcharging alternator is a continuous, higher than normal voltage output from the alternator when the engine is running. It is normal for the alternator voltage to be slightly higher on engine startup, but it should settle into a normal voltage range when the ECU has taken stock of the current engine conditions and power requirements.
If it’s a newer vehicle, then the alternator output is partially controlled by the ECU. The power from the alternator is adjusted in response to the power needs of the vehicle. In this scenario, a diagnostic tool is needed to test the state of the alternator under different conditions to see if it is overcharging when under extra loads.
On older vehicles, it’s usually enough to measure the alternator output using a multimeter. The power output should be fairly constant, although it may go up by around 2 volts when more power is needed (when headlights are on with air conditioning for example). Older vehicles sometimes have a voltage dial on the dashboard that can give a warning when the alternator is overcharging the battery.
Symptom 2: Physical Damage to the battery
Another way to spot an overcharging alternator is by checking the condition of the battery. Newer vehicles will usually have a battery monitoring system that will regulate the health of the battery and won’t allow any extra voltage to damage the battery. On these vehicles, you will usually see a red battery light appearing on the dashboard if there is a problem such as overcharging. If the ECU detects a problem it may restrict the amount of power getting to the battery preventing it from charging fully.
Older vehicles and machinery that don’t have an ECU monitoring the alternator will usually allow the alternator to overcharge the battery until it fails. Signs of damage to an overcharged battery include swelling and cracks to the battery case caused by a buildup of gas and a loss of electrolyte fluid inside the battery. A battery will usually feel hot to touch after it has been overcharged. Some batteries may even start to leak if they have been overcharged for a long time.
Symptom 3: Check engine light on dashboard
On newer vehicles, an overcharging alternator will trigger one or more error codes that will usually cause a check engine light on the dashboard. As outlined above, modern vehicle charging systems are controlled and monitored by the ECU. In some cases, the ECU can adjust the amount of power that is being generated by the ECU. If the ECU detects a problem with the alternator such as overcharging it will record this in memory and take the appropriate action necessary to protect the rest of the charging system and battery.
A common error code in this instance is P2504 – Charging System Voltage High. This error is triggered when the ECU detects a high voltage somewhere in the charging system. It often points to an overcharging alternator, but can also be caused by problems elsewhere such as a bad voltage regulator, bad battery wiring, or corrosion at the battery terminals.
Symptom 4: Frequently Blown Fuses and Bulbs
Another symptom of an overcharging alternator is damage to the electrical system in the vehicle. This commonly starts with fuses and bulbs that repeatedly blow due to voltage surges. It’s not the most common symptom these days due to the extra monitoring of the charging system by the ECU, but on older vehicles, it can become a real problem if it is not addressed promptly.
Over time, an overcharging alternator can damage the wiring looms in the charging system, causing them to become burnt or brittle. This can sometimes lead to vehicle fires if the alternator is not repaired or replaced. It’s not as common in newer vehicles but it can still happen, so it’s not a good idea to ignore the red battery light or check engine light on the dashboard for too long after it appears.
Symptom 5: Battery Fails Quickly After Replacement
If the battery is constantly being overcharged it will drastically shorten its lifespan. Traditional, sealed car batteries contain a sulphuric acid and distilled water electrolyte that facilitates repeated charging and discharging of the battery. Continuous overcharging when the engine is running causes this liquid inside the battery to heat up to the point where a chemical reaction occurs that releases hydrogen and oxygen.
Vehicles with sealed batteries usually have a venting system to release these gases so it’s rare that an overcharging battery will explode. However, loss of the electrolyte reduces the ability of the battery to discharge and recharge properly.
Newer vehicles often use AGM battery technology where the acid electrolyte is bound to a fleece textured mat that helps to protect the electrolyte from damage caused by overcharging. This also allows for more frequent charging and discharging.
Next Steps – How to test if an alternator is overcharging
A basic alternator test can be performed using a multimeter. If it’s a newer vehicle, then a diagnostic reader that you can hook up to the ECU is a better option. Using a diagnostic tool such as an OBD II reader will give you access to stored error codes and some can allow you to record the voltage output of the alternator in real-time under varying conditions.
If you want to carry out a quick test using a multimeter, then before starting, it’s a good idea to look up what the default alternator voltage should be. This is usually somewhere between 13 and 15 volts when the engine is running, but it can be higher or lower on some vehicles.
When checking the voltage output of the alternator you should try to get a reading at the alternator terminals instead of at the battery terminals. This will give you a more accurate reading without wiring or auxiliary device voltage loss.
Testing the alternator is as simple as starting the engine and measuring the voltage by placing the tip of the multimeter leads on the corresponding output terminal on the alternator. There are lots of different alternator designs so it’s a good idea to look up a service manual for your exact vehicle to see where to put the multimeter to get the best reading.
If the basic multimeter reading is way above the expected voltage range, then there is a good chance that the alternator is overcharging. If it doesn’t appear to be overcharging, then ask a friend to rev the engine and hold it around 2,500 RPM. This should increase the voltage output. You can further test it by turning on some auxiliary systems such as the headlights and air conditioning or heating. This is not the most accurate way to test the alternator but it will give you an idea of how well it responds to increased engine speed and increased load.
1. Can I drive with an overcharging alternator?
No, as a general rule, you should not drive your car if the alternator is continuously overcharging the battery. An overcharging alternator will cause damage to the battery and wiring of the charging system. If it’s essential, in most cases, you can drive the vehicle a short distance if you need to get home or to a mechanic.
Most newer vehicles have a battery management sensor that monitors the current state of the battery and the electric charge that is coming from the alternator. If the voltage is too high, the alternator may be disengaged so it no longer produces a charge, or the battery circuit may be isolated to protect the battery from damage. In this case, you may find that the vehicle won’t start or it may loose power when driving. For this reason, it’s safer to not drive the vehicle long distances if you are aware that the alternator is overcharging.
2. What stops the alternator from overcharging the battery?
In general, a voltage regulator stops the alternator from overcharging the battery. The voltage regulator can be part of the alternator or on some older vehicles it is fitted near the battery as a separate component.
The voltage regulator can be a mechanical device that physically breaks the circuit when the voltage exceeds a preset value. In modern vehicles the voltage regulator is electronic and is controlled by the ECU. Batteries on newer vehicles are much more sensitive to over and under charging. Many vehicles also have start/stop and brake regeneration systems fitted, that affect how the battery is charged and used.
3. Can overcharging alternator kill a battery?
Yes, an overcharging alternator will kill a battery if it is allowed to continuously overcharge the battery. Overcharging a car lead acid battery will cause it to heat up and possibly swell. This excess heat causes the electrolyte inside of the battery to boil, creating a chemical reaction that release hydrogen and oxygen and a loss of electrolyte. This affects the ability of the battery to charge efficiently and can eventually kill the battery completely.