What is the danger of battery overcharging and how to protect against it

What is the danger of battery overcharge and how to prevent it

Motorists will rightly support the statement that if you use a quality battery and use it reasonably, the battery does not cause any special problems during its entire operating life. Some batteries last as long as the car, which means the need to replace them does not arise as often.

The process of recharging the battery

But sometimes the battery is under the influence of increased loads, operated in extreme conditions. One of the most dangerous phenomena is overcharge.

Not all motorists know what it is and what the real threat is. This gap in knowledge must necessarily be filled by further understanding how to act correctly in the event of such a situation.

The concept of overcharging

Here it’s simple. It is the charging of the battery above the norm. But it is not quite clear what to consider as a norm.

In the case of passenger cars, the normal voltage reading on the battery terminals, which is fully charged, is about 12.7 V. As soon as the engine is started, the reading drops. This is due to the fact that the battery gives up some of the charge for starting.

Later, when the engine is already running, the alternator comes into play. It feeds the onboard part of the car, supplying a voltage of about 14.5 V. Current goes to the battery electrode plates, and thus the restoration processes are started. That is, the battery replenishes the lost charge.

But if the charging process takes place in conditions of an increased voltage, exceeding the mark of 20 V, and it happens long enough, the lattices of electrodes in the battery begin to heat up strongly. The water inside the battery gradually evaporates and boils. At the same time, due to the evaporation of water, the concentration of sulfuric acid increases. Its density increases. The boiling water decomposes into two components. Hydrogen and oxygen. The excess gas does not have time to leave the battery. At this point, overcharging carries the greatest danger, as the battery case can deform and even rupture.

Basically, this problem is typical for old foreign cars and used domestic cars. Overcharge on more modern and relatively new cars is unlikely. But it may not be excluded.

Not the least important is the quality of the battery. Therefore, it is necessary to choose the battery carefully and according to the recommendations of the vehicle manufacturer.

Since overcharge carries a potential threat, you should learn its characteristics and ways to avoid such situations.

Car battery overcharge

Signs of overcharge

It is logical to start with the question of what is the reason for which there is an active battery overcharge, and how you can determine the malfunction. After all, until recently it was functioning normally and did not cause the car owner any problems.

Since the charging current to the battery comes from the generator, in many ways it is the latter that often provokes overcharging.

An accurate diagnosis of overcharging, when the battery current is higher than required, can be made with a conventional multimeter.

  • The multimeter is switched to constant current measurement mode. The limit will be in the region of 20 V.
  • The probes of the instrument are connected to the battery terminals. Observe the correct polarity. The minus wire is black and the plus wire is red.
  • If the battery is fully charged, the voltage should be 12.7 V when the motor is off.
  • Now the engine starts.
  • If the alternator, the relay and the onboard network are working, the multimeter will show about 14.5 V. An error of 0.5 V in either direction is allowed.
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It is possible to determine the fact of overcharge by external symptoms. These are traces of leaking electrolyte on the battery body, oxidized terminals. Also information from the on-board computer can help.

One of the characteristic signs of overcharge is the excessive brightness of the dashboard and cabin backlight.

Noticing such signs, it is worth taking appropriate measures to eliminate them.

High brightness of dashboard backlight

Consequences of frequent overcharging

The motorist should think about the possible consequences of constantly overcharging the battery, because here we are talking not only about the expense of replacing the battery, but also about the risk of encountering malfunctions of the electrical system of the car.

Therefore, be sure to examine the list of what is potentially dangerous to frequent or constant overcharging of the car battery.

  • When you overcharge, the electrolyte boils. This is an acid that partially leaks out gradually from the battery. With prolonged boiling, the sulfuric acid gets on the body, radiator, wiring, hoses and other underhood components. It eventually corrodes the elements.
  • If the overcharge is low, the electrolyte will accumulate on the battery case and settle on the terminals as condensate. This entails oxidation processes, deterioration of contacts. This is manifested in the form of a green deposit.
  • Evaporation leads to a decrease in the level of electrolyte inside the battery. Some of the electrodes are not covered with a special solution, the plates are excessively heated, they crumble. The cans can short-circuit, and the battery fails.
  • Strong and constant overcharging leads to severe overheating, which causes the decomposition of the electrolyte. The gas mixture of hydrogen and oxygen may well explode, and all the acid pours out under the hood.
  • Fuses, dashboard lights burn out, and electronics fail.
  • In the most complicated situations the onboard network wiring melts and the fire of the car is not excluded.

Yes, the danger is not so serious at initial stages. But ignoring the symptoms of overcharging can provoke dangerous and sometimes catastrophic consequences.

AGM batteries are considered to be the most sensitive with respect to overcharging.

This is due to the fact that the electrodes here are separated by separators made of glass-matte material. When heated, the electrolyte evaporates and the plates dry out. The active mixture may crumble and the battery should be replaced.

But the situation with gel batteries is not better. With such a problem, gel settles, exposes the top of the plates, and they short out. Only replacement of the battery will help in such a situation.

AGM battery device

What are the causes and how to fix them

Now it remains only to deal with the causes of battery overcharge and their elimination. Everyone decides for himself whether to do it with his own hands or ask for help from specialists.

  1. Faulty alternator relay. Almost the most common cause of active and constant overcharging of the battery. Diagnosing the fault and trying to find out what the cause is, it often turns out that the problem is on the side of the relay regulator. The relay itself works, but does not function properly. Due to the relay, the onboard voltage does not go over the 14.5 V mark. If it turns out to be faulty or with a factory defect, then the relay will pass more current. As a result, overcharge, though not strong, but stable and constant. The solution is quite logical. It is to replace the problem relay.
  2. Failure of the generator relay. In this case at engine speed a strong overcharge of the automobile battery will be observed, since the relay has fallen out of the circuit, and now the voltage goes from the generator directly to the battery. The limiter in the form of a relay is no longer active. Sometimes the voltage applied to the battery can exceed 20-24V. This is already a serious overcharge. Again the relay needs to be replaced.
  3. Problems on the side of the alternator. Most often damage of diode bridge, armature or winding is diagnosed. As a result, some of the voltage passes the relay and ruins the battery. The problem can be solved by repairing the alternator. If this is not possible, then a complete replacement.
  4. A faulty contact in the electrical circuit. If the contact is bad, then the relay may perceive this as exceeding the normal on-board load. To compensate for the problem, the regulator starts to pass voltage with higher parameters than necessary. Hence the overcharge. The contacts should be checked visually, and tested with a multimeter. Detected problems can be eliminated by cleaning the contacts or replacing the wiring.
  5. Improper operation of the charger. Sometimes overcharging is not caused by the generator, but by a separate charger. Older models have a manual setting. By selecting the wrong mode, the battery can be put out of action.
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Alternator relay for replacement

Some motorists, in order to resuscitate an old battery, try to purposely increase the charge by deliberately increasing the density of the electrolyte used. But this can have a devastating effect on the battery.

Measures to Protect Against Overcharging

There is no need to reinvent the wheel to prevent overcharging. The car manufacturers themselves do everything possible to protect against such phenomena.

A good example is the functionality of modern on-board computers. They can display the battery’s operating parameters in real time, and the electronics will monitor the condition and adjust the correctness of its functioning.

Since some cars stand idle for a long time, it’s not always possible to replenish the charge by the generator. You have to remove the battery and use a battery charger. It is recommended to get an inexpensive but good charger with automatic voltage regulation.

Charging a Full Battery

One last question may be worth answering. It is related to whether it is possible to charge a car battery, if there is an overcharge in it.

If overcharging is related to the alternator, you should not allow the alternator to interact with the battery until it is repaired or replaced.

If the battery is fully charged, it may only be recharged using a working battery charger with automatic voltage regulation.

At the same time the device should display the current parameters. If the battery charger does not regulate the voltage correctly in the process of replenishing the battery, active boiling of the electrolyte will occur if a charged battery is connected to the device. At the same time the battery capacity will decrease.

If such manipulations should be carried out, then only when the battery is discharged to at least 60-70%.

In each case, there is a certain reason that provokes such a change in the normal state of the battery. It must be found, eliminated and only after that continue to operate your car.

What are the dangers of car battery overcharge?

Attention! The increased overcharging from the generator, can lead to battery failure, and the battery in turn can also explode, such cases have already happened in my practice, just saw with my own eyes, as after the popping of the battery, a man ran and climbed into the bath, pre-filled it with water, in all working clothes. If he didn’t, in the morning there would be nothing left of his clothes, the acid in the battery would have burned them all!

What is overcharging a battery?

In essence, it is overcharging, and to understand more about the subject, you should first understand what the norm is. The normal voltage across the terminals of a fully charged battery is, on average, 12.7V. When you start the engine, this figure drops as a significant amount of electricity is given off.

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After the start, the alternator supplies power to the machine, providing a voltage of about 14.5V. The current flows to the electrode plates of the battery, triggering the reduction processes in the electrolyte – thus the battery is charged.

If this process takes place at a higher voltage (more than 20V) for a long time, the electrode lattices of the battery begin to heat up and the water in the jars evaporates and boils, which creates a boil. The concentration of sulfuric acid in the electrolyte increases, i.e. its density increases.

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During boiling, water breaks down into oxygen and hydrogen, the resulting excess gas does not have time to be vented. At this stage, overcharging is especially dangerous – the battery case can be destroyed and even explode.

How to detect the presence of overcharge

You can accurately determine that there is an overcharge with the help of a multimeter. To do this:

  • Set the multimeter to DC mode at about 20V;
  • Connect the contacts to the terminals of the battery (black wire to minus, red to plus);
  • When the engine is not running, fully charged battery gives 12,7 Volt;
  • Start the engine;
  • If the alternator is good, the snout is working and there are no problems in the onboard network, the multimeter should show 14,5V (+ – 0,5V).

Battery overcharge is detected by external signs (oxidation of terminals, leakage of electrolyte on the case) or by the on-board computer. If there is no on-board ECU in your car, you can buy an external one.

You should pay attention to the interior lighting and dashboard light. One of the first signs of overcharge is the excessive brightness of glow of onboard equipment lamps.

What constant overcharging can lead to

  • During overcharge, the electrolyte boils, with it partially leaking out through the gas holes and plugs of the cans. If the process continues long enough, the leaked sulfuric acid solution will end up on the radiator, wires, cooling hoses and bodywork. The concentration of acid is not great, but over time it will corrode some of the above.
  • If the battery is slightly overcharged, the electrolyte precipitates on the battery in the form of condensation on the terminals, which oxidizes and causes a green stain on the terminals.
  • Evaporation of the electrolyte leads to lowering of its level in the banks, exposes part of the electrodes, the plates overheat and the active mass crumbles from them. This threatens to short-circuit the jars and the final failure of the battery.
  • Overcharging in the final stage is accompanied by significant overheating, during which the electrolyte water disintegrates into oxygen and hydrogen. The resulting mixture of gases can explode, with the acid ending up on the underhood components.
  • Fuses, lights on the dashboard and the passenger compartment burn out, other onboard electronics stop working (voltage limit they can withstand is not over 17V). In some cases wires of onboard network may melt and catch fire.

Results of car battery overcharge

By the way, there are such miracle drivers who think that it is possible to drive with overcharge without any problems and nothing will happen. This is not true, and here’s why:

Overcharge of the battery provokes boiling of electrolyte, it splashes out on the surface of the power source itself, and then flows down on many parts under the hood, for example to say: – terminals, sockets, body metal, radiator, wires and more. And I think everybody knows that there is acid in the electrolyte, it is not concentrated but it is still there and it easily corrodes all of the parts that I have listed, yes it does not happen immediately, but it will happen and you can be sure about that. Then, it will oxidize the battery terminals. In particular, the acid gets on the terminals, they will oxidize extremely quickly, then you will see an unpleasant green stain on them.

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Causes of occurrence and their solutions

Excessive battery charge occurs due to the following reasons:

  • Faulty alternator snout . This is a situation when the regulator is working, but its functions do not perform correctly. Alternator snout does not allow the onboard voltage to exceed 14.5V. A defective or faulty device gives slightly higher currents to the network of the car, which leads to a stable, but not critical overcharge, because the voltage is usually slightly higher than normal.
  • A malfunctioning alternator snout . In such cases, electricity is fed to the battery directly from the alternator without any restriction, and the voltage can exceed 24V, which as a result causes significant overcharging of the battery.
  • Faulty alternator . If the diode bridge is damaged, the winding or armature is broken, some of the current will flow past the snout, resulting in overcharging.
  • Poor contact in the circuit. A good relay will perceive the resistance created by a bad contact as an increased load on the vehicle’s mains. In an attempt to compensate, the regulator will allow more voltage into the circuit, and the battery will overcharge as a result.
  • Incorrect charging from the charger . Some older chargers are manually adjusted. The wrong mode leads to overcharging.

Important! Do not ignore the partial failure of the alternator snout, because the defective part sooner or later may be completely inoperable.

The battery is sometimes intentionally overcharged in an attempt to restore performance. This is done to increase the density of the electrolyte. Under high voltage, the temperature rises, boiling begins, water evaporates, and the concentration of sulfuric acid increases.

The reasons for this phenomenon

Guys, a word or two about the “special” overcharging from the charger – many do it on purpose! REMEMBER! Thus – increase density of an electrolyte, up to necessary level – in our strip it is approximately 1,27 g/cm3, if density below (already at the charged AB), then the battery can freeze at subzero values. It has to be increased! And how to do it? Very simple – you need to evaporate a small amount of water from the electrolyte, so the acid concentration will grow and the density will increase.


So many motorists – “boil” the battery on a low current, from the charger, but only up to a certain density value. After that, turn off the charger. Otherwise you will simply “ruin” the battery. It is especially important not to let the plates “bare”.

Now the “unspecial” overcharge, which is said under the hood of the car, its main causes:

  • The alternator charge regulator relay has failed . This relay “sees” the charge, and when it reaches 12.7 volts, shuts off power from the generator. If this relay fails, then the generator will constantly charge the battery, and he has a lot of currents, it will boil very quickly! This is the most common cause. Fortunately, this relay costs pennies. Here is a little video, take a look.
  • Failed the alternator itself, this also happens. For example, changed the relay, and nothing helps, constantly goes charging! It is necessary to repair or change the generator, here the repair is already more complicated and expensive.
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Alternator is out of order

  • On some cars, such as trucks, also on some UAZ, is a voltmeter, it shows the voltage from the alternator to the battery, which is how he charges it. Usually should not exceed 14 volts, but often readings are 15 – 17 volts, it is very much. I had a case in my practice, and replaced the relay and alternator, all new, and the voltmeter shows 17 volts, have already puzzled over what to do! It turned out out of the sensor failed, changed the display and everything is normal, the voltage leveled at 14 volts. So the moral is this – sometimes the sensor itself fails – no overcharge, just shows “false” readings.

VAZ voltmeter

These are the most common reasons for overcharging, in fact, there is nothing else to break, if you have not some Lexus in which the sensors are just a lot, there may be something else, although it seems to me and there, unlikely.

Fortunately, in new cars, you light up two indicators on the panel, CHECK ENGINE, as well as a battery icon.

Many will say – so what, it overcharges, and “hey” with it, what will be something? But guys do not say, read about the consequences.

How to protect the battery from overcharging

To charge the battery outside the car it is necessary to use modern chargers with automatic voltage regulation. The functionality of budget devices is quite modest, but it is enough to charge the battery without any negative consequences.

The most simple and effective protection of the car battery from overcharging by an alternator is use of an on-board computer. In some cars, the ECU is installed from the factory, but if there is no built-in, you can always buy an external one. In addition to the state of the battery, on-board computer displays a lot of other useful information.

Naturally, no equipment will save the battery from the negligence of its owner. The car requires attention, this is especially true for domestic models with mileage. At the first signs of overcharging, (increased brightness of lights, oxidation of terminals, EC readings), you should take action immediately, otherwise you can not avoid serious repairs in the aftermath.


Often the causes of overcharging are not in the battery itself, but in the alternator. Here is a list of the most common overcharging: 1. Problems with the relay. It is this part that is responsible for disconnecting the battery from charging, after reaching a certain voltage level. Accordingly, if there are any problems with it, then the current continues to flow to a charged battery. Replacing the relay is usually no problem, and the repair is cheap.

2. The alternator itself is out of order. The scheme is the same, however, in the case of replacement, be prepared for much higher costs to buy a new part. 3. Unsuitable alternator. If you have purchased a new alternator, and the problem remains, it is worth checking whether the part is suitable for your car. 4. A problem with the voltage sensor. As a rule, such failures are quite rare and occur in expensive cars, which are full of electronics and often something can “short out”. It is solved by replacing the sensor, the cost differs from the class of the machine. In fact, the listed problems are the most common, otherwise there is nothing else to break in this system.

In new cars, electronics will warn the owner that there are problems. The “check” and the battery icon will light up on the “dashboard”.

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