Low on-board voltage in the car network is a common malfunction, which causes problems with the battery, engine and consumers. However, this breakdown is one of many, the cause of which can really be found and eliminated without the help of a professional auto electrician. All you need is the ability to use a multimeter and the information provided below.

Consequences with low on-board voltage

With low voltage in the on-board network, the car continues to work somehow. For this reason, many car owners drive like this, at best, postponing the solution of the problem indefinitely. Others never even think about what kind of voltage there is in the on-board network of their own car. Goes - and well. Attention is drawn to this issue already when the consequences of low voltage begin to appear.

These include:

  1. Constant undercharging of the battery. If the voltage on board the vehicle is generally below 14.4V, the battery will never "see" a 100 percent charge from the alternator in its lifetime. In the best case, the charge level will float around 40-60%. And it will be fully charged only from a stationary charger.
  2. Lack of time for battery recovery. If you use the car for short and infrequent trips, the battery simply will not have time to return the energy spent on launches. This problem tends to accumulate. That is, after each trip, the shortage will be greater and greater. As a result, the battery will eventually discharge to zero.
  3. Accelerated battery degradation. Car batteries last longer the more time they spend in a well-charged state. If deep discharges occur, the battery is mostly charged by less than 60% - its resource is reduced, if not by several times, then much. Sometimes he does not go for more than a year. It quickly loses capacity and inrush current.
  4. Lack of power for powerful consumers. The first thing they usually pay attention to is the head light. At low voltage, the headlights shine weaker than they should. The radio tape recorder (with a subwoofer is generally tin), inverter, air conditioner suffers from this problem.
  5. Unstable engine operation. Namely, floating idle, misfiring, sudden stops. Actually, there may also be problems with starting the engine, since a weakly charged battery does not turn the starter well.
  6. Failure of electronic elements. It happens extremely rarely. But it happens. Especially if the voltage drops of the on-board network reach critical values.

In most cases, the cause of low on-board voltage not found in a timely manner tends to progress. That is, at the beginning of the problems, it seems like, and there are none. But over time, they accumulate and become more noticeable. In the end, if nothing is done, one day the car will become a stake. And it’s also good if it’s in a garage, and not somewhere on a deserted highway.

Normal voltage of the on-board network of the machine

Ideal voltage for battery charging
Ideal voltage for battery charging

A few words about what is the normal voltage of the car's on-board network, as opinions often differ. Someone and 14.0 V for the eyes. Others have 14.8 volts on board, and that's not enough for them. The reason for such a significant spread most often lies in the incorrect interpretation of the instructions for charging the battery. There are especially many disagreements in the case of calcium batteries. Even in this matter, it is necessary to take into account the natural voltage drops that occur as the load on the on-board network increases.

Everything said below is based on the following “postulates”:

  1. The reference (normal) voltage of the on-board network of a car is 4 V.
  2. With the load on, a drawdown is allowed, but not lower than 14.1 V.
  3. When using calcium batteries, a voltage of 14.7 V is allowed.
  4. The reference voltage without the included load should be observed at any engine speed, including idle (but not lower than the minimum).

That is, if you start the engine and turn off all consumers, the ideal voltage of the on-board network should be considered 14.4 V. If in your case it is noticeably lower, the following are the most common 10 reasons to start looking for. If this does not help, then it is most likely better to contact a professional auto electrician. The dog is buried too deep.

Reason 1. Erroneous measurements

Cheap and incorrectly connected voltmeter - "lying"
Cheap and incorrectly connected voltmeter - "lying"

First of all, you need to understand that the normal voltage of the on-board network must be "search" directly at the battery terminals. Not in the salon. Not in the cigarette lighter socket. Not anywhere else. Only on battery terminals. Therefore, if you are guided by the indicators of a standard or self-installed voltmeter, check its indicators with the voltage at the terminals.

The second important point is the multimeter. It must be calibrated, and show real voltage. As a rule, many modern multimeters are checked and adjusted in production. But not all. But almost any device can be calibrated independently. As a last resort, check what it shows with some standard. As an example, you can go to an electronics repair shop. They always have a normal laboratory power supply and calibrated meters.

The last important point is the current load on the on-board network. This has already been said above. The reference voltage of the vehicle's on-board network should be measured with the consumers switched off. The issue of voltage drop under load is another topic. Here we consider exceptionally low voltage under all the above conditions.

Reason 2. Battery heavily discharged

Discharged battery = voltage drop
Discharged battery = voltage drop

A fairly common cause of low on-board network voltage is the current battery charge level. If it is low, then the voltage of the on-board network may also be slightly underestimated. This happens because a discharged battery, after starting the engine, begins to charge intensively, consuming a large current. That is, it is a serious load on the generator.

To broaden your horizons on the topic of batteries, read the articles “Checking the battery with a load plug” and “Voltage of the car battery” on the ABS website.

History from life. One good friend of the author decided one day to defeat the low voltage of the on-board network in a well-known artisanal way. We are talking about a diode that is soldered into the gap between the battery and the relay-regulator. This solution allows you to "deceive" the relay-regulator. About 0.5-0.7 V drops on the diode, the regulator “detects” this, and “requires” more from the generator.

Before installing the diode in the on-board network of the car, there was a voltage of 13.7 V. After - 14.3 V. Almost reference. But the author's friend did not take into account one point, namely the current battery level. Since the voltage of the on-board network was too low, the battery was constantly undercharged. The situation was aggravated by the fact that the car was used for short trips.

The installation of the diode just coincided with the case when it was necessary to drive a car over a fairly long distance. During this journey, the battery was charged normally, and the voltage of the on-board network crept up. It crawled right up to 14.8 V. And this is the voltage at which the battery boils (not everyone). As a result, as this experience showed, the diode as a solution to the problem of low voltage in this particular case did not fit.

But the replacement of the relay-regulator helped.

Reason 3. Malfunction of the voltage regulator relay

Relay-voltage regulator
Relay-voltage regulator

The task of the relay-regulator is to monitor the voltage in the on-board network, and as it changes, adjust the degree of excitation of the generator. That is, the voltage in the network drops - the relay-regulator "forces" the generator to produce more volts, and vice versa. If this part is buggy, then there is (more often) low voltage in the on-board network. This cause can be established by a simple check of the relay-regulator. Solve - its replacement.

The relay-regulator is buggy for various reasons. Most often due to overheating. Less often, electronic components that are responsible for the correct operation of the circuit burn out in it. There are also low-quality models that work fine at first, and then the voltage starts to float again. Original (but expensive), as a rule, immediately and for a long time help to solve the problem of low voltage.

Reason 4. Insufficient generator power

The regular generator is designed for regular energy consumers
The regular generator is designed for regular energy consumers

The first point is the natural wear of the generator. If it has reached a critical level, then the power it produces is simply not enough to power all consumers.

The second point is the increased load on the generator. If you have installed a powerful subwoofer, inverter, xenon light, refrigerator in your car, there is every chance that this “burden” is too tough for the generator. The problem is solved very simply. Replacing the generator with a more powerful one. And do not worry about the myths that because of this the battery will deteriorate, the engine will somehow suffer, and so on.

Reason 5. Generator belt slip

Generator drive
Generator drive

It's simple - the generator generates electricity in the required volume as long as it is spinning. If, however, it is powered by a belt that is worn out, torn, stretched, slips, the low voltage of the on-board network will not take long. Often the belt slips for no apparent reason. That is, it is whole or even new, and properly tensioned. In such cases, water from puddles that has fallen on the generator pulley, or engine oil flowing from the engines of old cars from all the cracks, may be the cause.

Reason 6. Burnt diode bridge

Power diode bridge
Power diode bridge

The task of the diode bridge is to rectify the alternating voltage that the car's generator generates. It consists of at least six diodes connected according to the bridge scheme. If at least one of the diodes of the bridge burns out (breaks through) - the voltage of the on-board network drops. A very simple check of the diode bridge allows you to establish the problem. Solve - replace burnt out (broken) diodes or the entire "horseshoe" at once.

Reason 7. Car wiring problems

Oxidation + overheating of wires
Oxidation + overheating of wires

There are 3 problems with wiring, due to which a low voltage of the on-board network can be observed:

  1. Short circuit.
  2. Break.
  3. Bad contact.

It is not so easy to search for all this, and it is impossible to describe the process in a nutshell. Either we study the topic more deeply, or we go to an auto electrician. Perhaps you are "lucky" and one of the above problems will be somewhere on the surface, where you yourself can identify and eliminate it on your own.

Reason 8. Unreliable contact in the battery charging circuit

This is what reliable contacts look like
This is what reliable contacts look like

The first point to check is the battery terminals. If there is a bad contact here, the voltage of the on-board network will definitely be floating. The problem is solved in an elementary way. The battery terminals are cleaned of oxides and salts, and the clamps are checked for integrity and securely tightened. For prevention, it is desirable to protect these points with some kind of composition that prevents the deposition of oxides and salts on the terminals and clamps.

Even if outwardly the battery terminals look clean, do not be too lazy to remove the clamps and check the contact points in more detail. Over time, deposits form between the clamps and terminals, due to which the voltage of the on-board network may decrease. It is recommended to visit here at least twice a year.

The second point is the mass of the engine. As a rule, this is a thick wire that goes from the battery minus somewhere to the bottom of the motor. Conditions there, to put it mildly, are not the best. The contact point quickly oxidizes, rusts, becomes dirty. Checking the mass of the engine is very simple (you can do it with the engine turned off). To do this, first measure the voltage at the battery terminals. Then leave the positive probe on the battery, and touch the negative probe to the motor near the generator. If there is a difference between the indicators, the ground wire needs to be revised.

Reason 9. Insufficient cross-section of wires in the battery charging circuit

Thick power wire
Thick power wire

Occurs after a serious and inept repair of the car's wiring. Insufficient cross-section of wires in the generator-battery circuit is a guarantee of low voltage on-board network. In addition, even "native" power wires can have a small cross section. Over time, at the bends and at the attachment points, the veins gradually break, as a result of which the resistance of the chain section increases. It is repaired that the first, that the second is elementary.

Reason 10. Generator malfunctions


A faulty or worn-out alternator can quite naturally be the cause of low on-board voltage. Unfortunately, the diagnosis and repair of this node requires serious experience. Start simple. Read about typical generator failures.

Replacing generator slip rings.

Brief summary

Low on-board voltage is a common car breakdown. In most cases, an auto electrician is not needed to fix it. If desired, even the most complex of the causes of the malfunction described above can be found and eliminated on your own. But be aware. The car's on-board network does not forgive many mistakes. Therefore, if there is no clear understanding and confidence in what you are doing, study the issue better, and only then open the hood.

VIDEO: testing a car generator