If the battery boils when charging at its last stage, then do not rush to panic. Abundant release of bubbles in this case indicates that the battery is charged. But when the battery boils when charging immediately or after just a couple of hours, this is a deviation from the norm. Moreover, it is very harmful to the battery life. Since there is a reason for everything, your task is to understand why the electrolyte boils quickly and, if possible, fix the problem.

Physics and chemistry of battery boiling

But before finding out the cause of any process, it is desirable to understand its nature. With the boiling of the battery, in fact, everything is simple. This phenomenon has two stages. One of them is practically harmless, as it is considered natural. The second one has a detrimental effect on the battery life. Also, a boiling electrolyte has absolutely nothing to do with boiling water in a kettle. Let's consider these theses in a little more detail.

The natural boiling of a battery is when gas bubbles moderately rise to the surface of the electrolyte at the end of the charging process. It is nothing but hydrogen. It is released as a result of an electrochemical reaction between acid and lead under the influence of an electric current. By and large, there is nothing wrong with such a boil. Although some experts argue that ideally, it is necessary to charge the battery so that there are not even bubbles.

The latter is easy to achieve if you use the so-called pulse charging mode. The current through the battery is passed in short pulses. Manages to effectively "digest" without causing gas. Unfortunately, the fact that you have a pulse charger does not mean that you charge, your battery that way. The pulse mode is implemented correctly only in some models, the cost of which starts from $ 200.

Electrolytic boiling of the battery is already the second stage. Harmful to the battery. It differs from the natural process in two ways. Firstly, there is not just the release of bubbles, but also abundant seething of the electrolyte. Secondly, electrolysis decomposes water into hydrogen and oxygen, which exit the battery in the form of an explosive mixture. Also, in the most advanced cases, significant overheating of the battery can be observed.

The electrolysis reaction in the battery occurs for several reasons, but always means the same thing. Namely, the battery either does not accept a charge at all, or does it extremely inefficiently. In addition, water is irretrievably lost. As a result, the density of the electrolyte increases, which is also bad. In addition to everything else, lead plates deteriorate. That's just the kind of boiling we don't need. We will look for its causes.

There remains the last fact about the boiling of the battery, which you definitely need to know. On the Internet, you can often find statements from couch "experts" that the electrolyte boils because the lead plates get very hot. That is, they work like a normal household boiler. This is utter nonsense. The boiling of an electrolyte has nothing to do with the boiling of water heated to +100°C. The processes are a bit similar visually, but their physics and chemistry are completely different.

You can boil the electrolyte in the battery like water in a kettle. But for this you need to withstand several conditions at once. That is, you have to try hard. Under normal adequate conditions, when the battery is charged under the hood from a generator or stationary from a working charger, there will be no natural boiling.

Since we have just looked at chemistry, formulas may be missing for some. No problems.

This is the formula for normal battery charging:

PbSO4 + 2H2O PbO2 + SO4 + 4H

This happens on the negative plates of the battery. For positives, the formula is different. But it is this one that is important for our topic. It clearly shows where those same hydrogen bubbles come from (H is hydrogen). Also note that there is no pure oxygen in the reaction results. This means that electrolysis harmful to the battery does not occur.

When electrolysis begins, the water contained in the electrolyte decomposes into its constituents - hydrogen and oxygen. The first is formed on the negative plates, and the second on the positive ones. As they accumulate, the same explosive mixture comes out of the battery. The main reason for which electrolysis begins in the battery is overcharging.

Signs of a boiling battery

If the battery is of a serviced type, then the signs of electrolyte boiling are obvious:

  • release of hydrogen bubbles (normal);
  • abundant seething (this is already electrolysis);
  • accumulation of electrolyte drops on the battery case (splashing).

All these signs are clearly visible when the battery is charged with open plugs. It remains only to learn how to determine boiling if the battery is maintenance-free. That is, there is no direct access to the electrolyte compartments. In such cases, only harmful boiling associated with the onset of water electrolysis can be detected. Here are indirect signs of this process:

  • hiss;
  • electrolyte leaks under the emergency pressure relief valve;
  • swelling of the battery case;
  • strong heating;
  • in the most advanced cases - the destruction of the battery.

Accordingly, if in your case there are any signs of electrolysis, the charging process must be stopped. Next, the cause is found out and, if possible, eliminated. The following describes in detail where to look for the problem, whether it can be solved, and if so, in what ways.

What threatens the boiling of the battery

But first, let's briefly summarize why when the battery boils when charging, it's bad. In general, if abundant seething occurs in a timely manner, then there is nothing wrong with that. Moreover, this is one of the signs indicating that the battery is charged. If the battery boils when charging immediately or in the first hours, then the following problems cannot be avoided:

  • systematic recharge;
  • electrolyte loss;
  • violation of its density (hydrometer vs refractometer);
  • shedding of lead plates;
  • short circuit in compartments;
  • capacity loss;
  • decrease in starting current;
  • possible destruction of the battery.

In the end, regular boiling of the battery ends the same way. At first, the starter turns badly. Later, the car won't start at all in the morning. Then comes the purchase of a new battery. If you do not find out the cause of the boil, then such a cycle can be repeated for you almost every couple of years.

Battery terminal corrosion
Battery terminal corrosion

Additionally, it does not hurt to note that the electrolyte outside the battery case is another serious problem. It leads to the fact that abundant salt deposits are constantly formed on the terminals. This means that electrical contact is deteriorating with all the ensuing consequences. That's not all. Acid rots power wires, the platform under the battery, and even parts of the car body. In the most advanced cases, it comes to weakening the structure.

Possible Causes of Electrolyte Boiling

More often than others, there are 5 reasons why the electrolyte in the battery boils. Two of them are hidden in the charger. The other three are problems with the battery itself. At best, only one of these five is an unremovable cause. If the battery is maintenance-free (no plugs), then two. If the electrolyte boils for more than a month, then the number of fatal causes increases by one more point. And that means buying a new battery. No options.

Let's get started.

Over charge voltage

Ideal voltage for battery charging
Ideal voltage for battery charging

The voltage at the battery terminals during charging is the most important parameter, which must not be exceeded. In concrete terms, the situation is as follows. A standard 12 V car battery has 6 compartments. They are connected in series. Each compartment must be charged with a voltage of no more than 2.4 V. This means that the entire battery cannot be charged with a voltage exceeding 14.4 V. If it is higher, electrolysis begins, which means boiling that is harmful to the resource.

Strictly speaking, less is also impossible. The battery will either not charge at all, or will not accumulate 100%. This is very often observed on cars if there is a malfunction of the generator. For example, in the current car of the author of this material, the voltage of the on-board network with the engine running is 14.1 V. It is not enough to fully charge the battery. You have to use the charger from time to time. Well, in the future - replace the relay-regulator.

Now about measuring the battery charge voltage. It is constantly necessary to talk about this, since many measure it incorrectly. The result is a delusion - the car owner thinks that the voltage is normal. And it is often elevated, which is why the battery boils. So. You should always measure the voltage directly at the battery terminals.

That is, navigate not by the voltmeter built into the charger. Not according to what is on the instrument panel (if any). And certainly not in Chinese, inserted into the cigarette lighter. All these methods of measuring voltage tend to give an error. It arises due to the fact that a few fractions of a volt are lost on the way from the terminals to the measuring device. This is called voltage drop or loss.

Therefore, if the battery boils while charging, first of all, measure the voltage directly at the terminals. To do this, you need the simplest multimeter. Practice shows that even inexpensive ones (which are two dollars) can show more or less accurately. Well, if there is a proven multimeter, then it’s generally good.

Important! At the very beginning of the charge, the voltage may be significantly lower than the specified reference of 14.4 V. This is normal. But do not forget that as the battery accumulates energy, the indicators will certainly grow. Therefore, the voltage should be monitored not only after connecting the charger, but also further in the process.

How to eliminate this cause of electrolyte boiling, if it takes place? Just. On adjustable chargers, you need to either limit this very voltage, or select other presets. If only current regulation is provided, then it can also help. By reducing the current, you decrease the voltage. If the device does not have any adjustments, and the voltage is high, you should not use it. Read - how to choose a charger - and buy a normal one.

Charging current is above normal

Voltage and current are two closely related characteristics. But in the case of the battery boiling during charging, something else is important. Namely, the compatibility of the charging current and the actual capacity of the battery. The word "real" is written for a reason. This should be emphasized, since in 90% of cases this nuance is not taken into account.

The point is this. Any non-new car battery has "two capacities". The first one is written on its body. This is the declared nominal capacity. For example, standard 65 Ah. The second is real. That is, the one that the battery has at the moment. Over time, the capacity inevitably decreases. For several reasons. Now it is important for us to remember that the real ampere-hours of a used battery are always less than what is written on its case.

You need to understand this because the recommended optimal charging current depends on the capacity of the battery. Or rather, its strength, expressed in amperes. It is calculated very simply, and many are familiar. 10% of the capacity is taken, and the battery is charged with this current. Example. For a 60 Ah battery, the optimal charge current is 6 A. And so on. This current cannot be exceeded, as this is another possible reason why the battery boils.

But the most important thing is not even that. There is also a real capacity, which motorists often forget about. As a result, during the entire service life of the battery, they charge it with a current that corresponds to 10 percent of the inscription on the case. But the inscription is not a real indicator, because from the first day the battery capacity decreases. Inevitably. All at different speeds. But it is decreasing.

If you are a careful car enthusiast and regularly monitor the battery charge level, then even with you it will lose 10-15% of its capacity in a couple of years after purchase. It was, for example, 60 A * h, and it became 45 A * h. It follows that it is no longer possible to charge it with a current of 6 A. It will accumulate charge inefficiently and, of course, quickly boil. For those who do not follow the battery and its condition, the numbers are much sadder. And boiling is more epic and with a bad ending.

Be mindful of the actual battery capacity
Be mindful of the actual battery capacity

How to solve a problem? There are two options. The first is to measure the actual capacity of the battery from time to time, and charge it with the appropriate current. But this requires special devices. Not cheap. The second is to reduce the maximum charge current approximately. As battery life increases. For example, one year left - we reduce it by 1 A. In two years, another ampere. If you are lucky, and the battery has served for five years, then charging it with a large current is generally a sin.

In addition to the natural "aging" of the battery, one should also not forget that many inexpensive models do not already have the declared capacity from the store. Unfortunately, in order to identify such a fake, an appropriate test is needed. Only a few make it, which, by the way, allows you to immediately return the goods according to the law of “two weeks”. If you bought a cheap battery, and there is nothing to measure the real capacity, charge it immediately not with the maximum allowable current, but with a slightly reduced one. For example, instead of 6-7 A, limit it to 5 A. This will not greatly affect the charging time, but it will have a positive effect on the resource.

Battery sulfation

Sulfation of a battery is the formation of an insoluble deposit of lead sulfate on its plates. This problem appears for several reasons. In short, this is a systematic undercharging and insufficient electrolyte level. In the first case, the sulfate formed during the discharge does not dissolve back in the acid. Over time, its crystals become larger and harder. If there is not enough electrolyte in the battery, then the upper part of the platinum generally works “dry”. In such areas, sulfates simply have nothing to dissolve in.

Battery sulfation
Battery sulfation

And what about the battery drain? And despite the fact that sulfation is the main cause of a sharp loss of capacity. It is the higher, the larger the area of lead in contact with the electrolyte. And when it is overgrown with sulfates, its participation in the reaction stops. This is where the capacitance comes from. And as mentioned above, in this situation it is easy to exceed the charging current, which is one of the main reasons for premature boiling of the electrolyte.

How to solve a problem? If sulfation is at an early stage, then it can be eliminated. This process is called battery desulfation. In short, this is such a special charging mode in which sulfates are partially or completely destroyed, freeing up additional lead area. There are these modes in some chargers, and they can also be simulated using conventional chargers and loads.

In advanced cases, sulfation is almost invincible. Neither electrolyte replacement, nor battery flushing, nor other folk methods help. Even super smart chargers mostly don't make things better. Accordingly, this problem is solved in two ways. Either by replacing the battery, or, if it is still "alive", by charging with small currents.

You need to understand that you can control (limit) the charging current only while charging from a stationary charger. Under the hood, the battery will take current to the maximum. And this means that the electrolyte is likely to boil regularly. Therefore, it is not worth operating a battery with obvious signs of serious sulfation. It is better to replace immediately. Away from sin.

Low electrolyte level

Electrolyte level is important
Electrolyte level is important

In the light of the foregoing, it should already be clear that due to the low electrolyte level, battery sulfation develops faster. But this is not the only problem. When the level is below normal, some of the plates are “bare” and do not participate in the process during charging. This can only mean one thing - the battery currently has a reduced capacity. This means that it is no longer possible to charge it with a standard current. It will certainly boil ahead of time.

The solution to the problem is obvious if the battery is of a serviceable type. You just need to regularly monitor the electrolyte level, and add it if necessary. As for maintenance-free batteries, of which there are a lot now, there's nothing you can do about it. The electrolyte level can neither be controlled nor topped up. If the battery has been boiling for a long time, then it only remains to throw it away.

The author is aware that craftsmen are getting smarter and adding water (or electrolyte) even to maintenance-free batteries. Bad business is easy. I drilled holes, took a syringe, and fill in as much as you like. Some pour water in this way into AGM batteries, and even into GEL. It is not worth repeating such experiments. Next time think twice before choosing a battery.

Short circuit in battery compartments

When a short circuit occurs in the battery, excessive boiling of the electrolyte occurs due to a combination of several factors. But let's start with what that reason is. A short circuit in a battery is when plates with different polarity begin to contact in one of the cells. As a result, the compartment from the battery turns into a blunt current conductor. By the way, the problem can be simultaneously in several cells.

What's next? When the plates are closed in at least one of the compartments, one element “drops out” of the battery. It has already been said above that there are six of them. Each should be charged with a voltage of no higher than 2.4 V. They are connected in series, from where those very ideal 14.4 V come from. With the “loss” of one such element, this voltage also decreases. Exactly 2.4 V. That is, if we charge a short-circuited battery with a voltage of 14.4 V, then it is already too large for it. Everything. The electrolyte boils almost constantly.

Short circuit in battery compartments
Short circuit in battery compartments

How to understand that there is a shorty in the battery? There are two ways. Both are not suitable when the battery is a maintenance free type. If there are plugs, then the first way to see a short circuit is to observe the electrolyte during charging. Where there are no hydrogen bubbles, while the rest of the cells are already boiling, there is a problem. The second way is to check under load. For example, you can rotate the starter several times while looking at the electrolyte. As a rule, if there is a shorty, under load, a shorted can immediately boils. So, look carefully!

What to do? Actually, a battery with a short circuit is already a dead man. Only replacement will help here.

Yes. There are folk ways to resurrect a shorted battery. But they are so barbaric that even in general terms they will not be discussed here. If you want to lose time and risk your own health, the Internet is full of "instructions for action." But it is better to immediately abandon such ideas. They are dangerous and hopeless.

Brief summary

If the battery boils when charging at the very end of it, this is normal. Light bubbles without bubbling in the process are not a problem at all. If it boils immediately after connecting the charger, in the first couple of hours, or hisses under the hood, this is a mess. There are only five main reasons: high voltage; current too high for a particular battery; sulfation; low electrolyte level; shorty.

VIDEO: about battery boiling