Regular undercharging of the battery is a problem that first steals the battery resource, and then the time and nerves of the car owner. Contrary to the seeming complexity, it is quite easily eliminated. To do this, it is enough to understand the following: how does undercharging differ from similar problems; how to define it; under what conditions the battery is fully charged on board the car; why these conditions are not met in your case and how to ensure them. In this material on the ABS website, you will find answers to all questions, and after studying them, you can easily eliminate the undercharging of the battery in your car.

Undercharged starter battery - definition

Battery undercharging — is when the battery is being charged by the car's alternator but never (or almost never) fully charged. In practice, this problem is usually noticed too late. To detect undercharging in a timely manner, you need to regularly monitor the level of battery charge, which few people do. Actual undercharging can also include cases when the battery is fully charged on board the car, but after parking it turns out to be in a semi-discharged state.

It is too early to talk about the reasons. But, if in general terms, then there are only three of them. Firstly, undercharging occurs due to malfunctions in the car's charging system or defects in the battery itself. Secondly, the ambient temperature may not contribute to a full charge. Thirdly, inaction or certain actions of the car owner also lead sooner or later to this problem.

Methods for determining the undercharge of the battery

To determine if there is an undercharge of the battery in your case, you can resort to two simple and affordable methods. The essence of any of them is to measure the degree of charge of the battery after it has been charged from the generator. Both methods are working, accurate enough, and do not require expensive professional equipment. Both have disadvantages too. But they are not critical. That is, you will establish the fact of undercharging in any case.

Method 1. Electrolyte density

The density of the electrolyte in a lead-acid battery directly indicates how much it is charged. At the same time, it is so accurate that the level of charge in this way can be determined with an accuracy of 5%. And this is more than enough to establish the presence of a chronic undercharge of the battery on your car. To measure density, you only need a hydrometer. It already has a color scale, according to which you can roughly understand how much the battery is charged. To find out more accurate numbers, you can use publicly available tables, of which there are a million on the Internet. Here is one of them.

Density (g/cm3)

Charge (%)

< 1.12






















> 1.26

Density above normal


Advantages of the method. First, high accuracy. Secondly, the density can be measured whenever it pleases, regardless of whether the battery was inactive or charged the day before. The second advantage is especially valuable, since this is a weak point for the second method of determining the battery charge level.

A hydrometer is a device for measuring the density of an electrolyte
A hydrometer is a device for measuring the density of an electrolyte

Disadvantages. The most significant is that not every modern battery has access to electrolyte. Technologies are constantly improving, and at some stage, manufacturers have come to the conclusion that the batteries have become quite capable of working out the prescribed period without user intervention in the density and electrolyte level. In the people, such batteries are called maintenance-free. They don't have plugs. Accordingly, if you have such a battery, this method will not work for you. About charging maintenance-free batteries.

With the measurement of electrolyte density in some modern batteries, there is another problem. The electrolyte in the upper layers, from which the hydrometer sample is taken, can differ markedly from that which is in the battery below, in direct contact with the lead plates. This problem appeared when calcium batteries were invented. The electrolyte in them is “protected from boiling”, and therefore from normal mixing. As a result, when the battery is charging, its density near the lead increases, and the indicators lag behind from above.

If you have a calcium battery, or any other, which, according to the manufacturer's idea, should not "boil" at a normal charging voltage (up to 14.8 V) - consider this factor. It may turn out that there is actually no undercharging, and measurements of the electrolyte density of the upper layers will indicate the opposite. How to achieve a uniform density and get adequate performance - see the materials on calcium batteries on the ABS website.

Method 2. Battery terminal voltage

The voltage at the terminals of the battery under certain conditions depends on the degree of its charge in the same way as the density of the electrolyte. Unlike the first method, this one is more versatile. You can resort to it regardless of whether your model has traffic jams or not. Absolutely any, even the cheapest, multimeter is suitable for measuring voltage. To convert the measured volts into a percentage of the battery charge, a simple plate is enough. On the ABS website, this version of it appears everywhere.

Voltage (V)

Charge (%)

< 11.90























Incorrect measurement


Advantages of the method. Measuring voltage is much easier than fiddling with a hydrometer. This method is universal, and is suitable not only for car batteries, but also for all other power sources. Only the tables are different there, since everything depends on the rated voltage. But the principle is the same everywhere - the more fully charged the battery, the higher the voltage at its terminals.

Voltage measurement at battery terminals
Voltage measurement at battery terminals

Disadvantages (rather, features). Voltage cannot be measured when it pleases. In order to obtain adequate percentages of charge by voltage, measurements must be carried out on a “rested” battery. The fact is that during the first 8...12 hours after charging from the generator (from the charger too), natural processes continue in the battery, and the voltage at the terminals at this time is always slightly overestimated. Something around 13 volts. Therefore, in order to take adequate indicators, you must wait for the specified time, allowing the battery to “calm down and stabilize”.

The consequences of undercharging the battery

The systematic undercharging of the battery only at first does not significantly affect anything, and therefore is rarely noticed in a timely manner. However, after a few months of operation, the battery begins to degrade. In particular, its real capacity decreases, the maximum starting current weakens and, in the end, the battery starts to fail regularly in the morning when trying to start the engine in cold weather.

If we consider the problem more specifically, we can distinguish three sad consequences of chronic undercharging:

  2. Reducing battery life.
  3. Unreliable

Let's dwell on each point for a moment.


Sulfation, in the case of lead-acid batteries, is a natural chemical process that occurs every time the battery is discharged. When a battery supplies power to a load, lead sulfate forms on its lead plates. It is a gray-white plaque, which under a microscope looks like this.

Sulfation under the microscope
Sulfation under the microscope

If the battery after the next discharge is soon charged to 100% or so, sulfates disappear. Due to this, the density of the electrolyte increases. If the battery is often and for a long time in a semi-discharged state, which happens when undercharged, the sulfate coating becomes thicker, stronger and more difficult to remove. As a result, part of the lead area ceases to interact with the electrolyte, that is, it does not participate in the operation of the battery. The capacity and starting current of the battery are reduced.

Of course, many have heard of such a thing as desulfation. If not, then the ABS website has material about it. In some cases, desulfation makes it possible to eliminate this consequence of systematic undercharging. But only in the early stages. That is, when the problem is identified in a timely manner, and eliminated promptly. In advanced cases, desulfation, even with the help of advanced and expensive chargers, is powerless. And sulfation flows into the next problem.

Service life reduction

Any battery that does not regularly receive a full charge will serve much less than it should. In the case of car batteries, a normal resource is 5 years. Whereas in the case of a systematic undercharging, a new battery can go to waste for the second winter. The service life largely depends on how significant the undercharge is. That is, if the battery is constantly missing some 10%, it will last almost as long as its sister in ideal conditions. If the battery most often stood idle only half charged, its resource will be less than declared by the same amount. Or even worse.

The reason for accelerated wear is simple and understandable. Sulfation tends to accumulate if there are "favorable" conditions for this. First of all, the undercharging of the battery. Due to sulfates, it becomes less and less capacious, which sooner or later leads to failure to turn the starter. Again, desulfation sometimes helps to regain some of the lost capacity and, thereby, increase battery life. But, unfortunately, only sometimes.

The battery often fails

Most often, an undercharge of the battery is detected by car owners already at this stage. Until the battery is enough to successfully start the engine, few people bother with control measurements of density and voltage. As they say: it goes, and it’s good, I won’t get into something that works anyway. However, if there is an undercharge, then even if you are not aware of it, battery failure is an inevitable consequence of the problem. How annoying it is when the car suddenly does not want to start, many people know. Therefore, there is no point in dwelling on this for a long time.

Normal conditions for a full charge of the battery on the car

What really makes sense is knowing and understanding the conditions under which it is impossible to undercharge a battery on board a car. There are three fundamental ones:

  1. The battery is in good condition and free of defects that cause inefficient charging or severe self-discharge.
  2. The voltage of the vehicle's on-board network is within 14.2...14.8 V.
  3. The operating time of the battery charging system (generator) is sufficient to restore the energy spent on starting the engine.

Battery defects. Perhaps we will consider them later, as they relate to the causes of undercharging the battery.

On-board network voltage. It is important to understand here that only the voltage that is actually present directly at the battery terminals with the engine running is taken into account. The voltage on a regular or home-made voltmeter, in the cigarette lighter and other places of the car's wiring - in this case, we are not interested. It depends only on what comes to the conclusions of the battery whether it will be fully charged or not. In theory, of course, a battery with a nominal value of 12 V can be charged to 100% or so, even if charged with a voltage of around 13 V. However, it will take a very long time at this voltage.

In order for the battery to have time to recover from the generator in real operating conditions of the car, the latter must produce at least 14.2 V, and preferably more. But not higher than 14.8 V, since this is already overcharging, and its unpleasant consequences.

Time. No battery is able to charge instantly, as capacitors can. To restore even a fully charged battery, which turned the starter for a couple of seconds, it takes at least 20...30 minutes. If, at the time of launch, the battery was not fully charged, the engine was frozen and did not start on the first try, and there was also a negative temperature outside, much more time is needed. How much exactly is impossible to say. Too many variables.

The battery is constantly undercharging - causes

Actually, knowing the conditions necessary for full charging, it is not difficult to find and eliminate the cause of undercharging the battery. Below are five common problems that cause the battery to undercharge from the alternator. The essence of the problem is described, how to identify and eliminate it in garage conditions, that is, without service stations.

Reason 1. The battery is not enough voltage

Recall that for normal and relatively fast charging of a car battery, there must be a voltage of 14.2...14.8 V on board. It is checked elementarily. A multimeter set to measure DC voltage within 20 V and connected directly to the battery terminals with the engine running steadily.

If at idle with the consumers turned off, the device shows you something from the range indicated above, the battery voltage is enough. The undercharging is not due to this. If the norm is reached only when the speed is increased to 2-3 thousand, this indicates a defective operation of the generator. A drawdown in the region of 0.2...0.6 V is allowed when powerful consumers are turned on. That is, with headlights, the voltage can decrease from 14.4 V to 13.8 V. This is normal.

If the multimeter shows significantly less, this is the reason for undercharging the battery. You will have to look for its source in the generator, but most likely it will be found in the voltage regulator relay. You can find out how to check all this without specialists from the materials on the ABS website (see the "Site Map" section). It is impossible to describe it in a few words.

Replacing generator slip rings.

Reason 2. Battery is running out of time

Despite the fact that this reason comes second here, it must be determined last. When everyone else is excluded. The main problem is that there is no exact time that the battery needs to fully charge. It depends at least on:

  • initial charge level;
  • technical condition of the battery;
  • on-board network voltage;
  • air temperature.

Nevertheless, approximate figures, nevertheless, can be designated. If the battery is in good condition, it was at least 70% charged before starting the engine, the voltage on board is within normal limits, and there is no severe frost outside, 40...90 minutes will be enough. In winter, as a rule, this is not enough. Therefore, it is recommended by many during this period of the year to more often monitor the state of the battery and, if necessary, recharge it from the charger in the garage.

Reason 3. Battery "freezes"

Everything is quite simple here. If the car has been standing in the cold for some time, then after starting the engine for the first 30...40 minutes, the battery takes a charge from the generator extremely poorly. This problem is often tried to be solved with the help of thermal covers for batteries. However, as practice shows, in most cases they do not help much. Alternatively, you can bring the battery overnight in a warm room, which many do. This is effective, since immediately after starting the engine, the “warm” battery immediately begins to fully charge from the generator.

If neither the thermal case, nor the daily removal of the battery and carrying it back and forth is not for you, then there is only one way out - to regularly recharge the battery from the charger. However, if you drive a car often and far, then this reason is simply irrelevant for you. Undercharging the battery, if any, occurs for other reasons.

Reason 4. Batteries with defects

Batteries with the following defects may suffer from undercharging:

Sulfation, which is mentioned above, can also be attributed to battery defects. Please note that some of the listed jambs can be easily fixed. For example, wash the case with water and soda, clean the leads from plaque, add water to the electrolyte. Other defects from the list, unfortunately, can only be fixed by purchasing a new battery.

Reason 5. The battery sits down during parking

The key factor here is the leakage current. About this on the website of the ABS there is a full-fledged material. In addition to the leakage current, it is also worth considering the duration of stops. It will also be useful to find out when exactly the battery is not fully charged - immediately after the next trip, or, nevertheless, after parking. Unfortunately, only the density measurement method (hydrometer vs refractometer) is suitable for this, since it is pointless to measure the voltage immediately after the engine stops.


Chronic undercharging of the battery occurs both due to malfunctions and due to factors beyond the control of the machine. It is diagnosed and eliminated relatively simply - by checking and providing optimal conditions under which the battery can quickly and fully be charged from the generator. Even if the problem cannot be eliminated promptly, but it is detected in a timely manner, it is strongly recommended that the battery be “rejuvenated” regularly from a stationary charger. This will help save its resource and not face the consequences described here for a longer time.

VIDEO: about undercharging a car battery