Signs and Causes of a Low Car Battery Voltage – and How to Fix it

Is your car displaying an arrange of symptoms indicative of a weak low battery? Are you wondering if indeed it is a case of low battery voltage but are not sure?

Then this post is for you, you’ll find signs indicative of a low battery voltage, reasons why your car battery voltage may be low, and what you can do to fix a low battery voltage.

Signs of a Low Car Battery Voltage

The best way to tell if your car battery voltage is low is to measure it with a voltmeter or multimeter set to read DC voltage.

That being said, signs that your car’s battery voltage may be low (not confirmatory) are:

#1. Slow cranking of the car at the start. The car may start eventually or fail all together. This can be caused by a low battery voltage or can also be a result of say loose battery connections.

These are just two of the possible reasons why the car engine may be slow to crank.

#2. Car engine makes a “machine gun sound” when starting. This is also sometimes a sign that the battery voltage is low.

Car not starting
Car not starting

#3. Battery light indicator on the dashboard comes on and stays lit. Other possible causes of the light coming on include loose wire connections, a faulty battery alternator or bad ground connections.

What Voltage Is Too Low for a 12V Car Battery?

If a battery voltage is as low as 10.5V then that battery is as good as completely drained. That being said, it is advisable to always keep the car battery topped up (above 12.4V) and not in a partially charged state for it to last as long as possible.

Other voltages that you should be aware of that can point to a problem with the battery or the car’s electrical system are:

#1. Resting battery voltage after the battery has fully charged, with the ignition off, should be about 12.5V, ideally 12.7 V indicative of a fully charged battery.

#2. It is normal for a car’s battery to momentarily drop to as low as 11.5-12V when it is starting. The starter draws the high initial current necessary to turn the car engine over.

#3. The battery voltage immediately after starting the car should be in the range 13.5-14.5V DC, if the alternator is supplying enough voltage to charge the battery.

5 Possible Reasons Why the Car Battery Voltage maybe Low

There are several possible reasons why the voltage of the car battery may be low:

#1. The battery terminals may be corroded and limiting the battery’s ability to charge properly

#2. There might be a loosely fitting or damaged positive, negative or ground battery or alternator wire that is stopping the battery from charging properly.

#3. The alternator is faulty and is not enable to generate enough voltage to fully charge the battery

#4. The car battery is faulty

#5. The car battery is being drained by the car accessories. The battery’s charge is constantly drained by the onboard electronics – car alarm, sensors even when the car’s ignition is turned off.

Have you installed aftermarket accessories such as an audio amplifier? If the alternator and battery are not adequately sized the car battery can be drained flat in a short period of time.

Expect the battery voltage to drop over a few days if a car is parked and is not driven regularly unless you connect it to a battery maintainer.

How to Check a Car Battery’s Health – Testing on Load (Complements Voltage Measurements)

Whereas the battery voltage can give you an indication of the battery health, it has its limitations.

A battery can have a voltage of say 12.4V but is not be able to deliver the required amps to turn the car over.

So, consider the battery voltage reading as a preliminary test. A load test on the battery may also be necessary.

#1. Charge the battery fully and then measure how long the headlamps can light before dimming. Depending on the reserve capacity of the battery, the runtime can be anywhere from 60 – 90 minutes.

If it is significantly lower say 30-45 minutes then there is a high chance that the battery has lost some of its capacity.

You can also ask some one to start the car when the battery is fully charged.

Monitor the car battery voltage. It should momentarily drop to 11.5-12.0V. If it drops to below 11.5V, there is a good chance it is about to fail.

#2. Connect the battery to a good quality battery tester. They usually have a display that can indicate whether a battery is faulty or aging and needs to be replaced.

#3. Have your battery tested at an autozone near you.

What to Do – if the Car Battery Voltage is Low

#1. Check Clean the battery cable connections and posts. Inspect the battery cable connections for any corrosion and remove it.

Corrosion on the battery terminals can stop it from charging properly or even causing sluggish starts that give the mistaken impression of a weak or low battery voltage.

#2.Have you added any other accessories to the car’s electrical system? If you have a low battery voltage after adding aftermarket accessories to the car, consider upgrading the alternator, wiring and or battery.

#3. Always connect the car battery to a good quality maintainer if you plan to park it for several days or longer to replenish the charge drained by the onboard systems

#4. You may also need to replace the car battery

Closing Thoughts

If you’re wondering if the car battery voltage is low, best that you measure it with a good quality multimeter.

Even then, beware that using voltage measurement as an indication of battery health has its limitations. You may need to have the battery load tested at an autoparts store in addition to using the voltage measurements.

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