If at the final stage of charging the battery one cell does not boil, then it either has not been fully charged, or is not charging at all. There is a separate article on the boiling of electrolyte in car batteries on the site. It would be nice to have a look at it as well. As for the non-boiling cell, only two problems are possible here. One of them is removable. The second is a guaranteed purchase of a new battery.

The theory of electrolyte boiling in batteries

A standard 12V car battery is a six-cell battery. They are connected in series - plus to minus. Each such cell consists of a dozen lead plates, moreover, half of them are positive, and the rest are negative. One cell is designed for a nominal voltage of 2 V. Therefore, six cells give a total of 12 V.

In order for the battery to work, the lead plates are immersed in an electrolyte. It is a solution of sulfuric acid in water. When the battery is completely discharged, the composition of the electrolyte is more like water, since the acid in the form of lead sulfate is on the plates. This can be verified by measuring the density with a hydrometer.

When the battery is charging, the lead sulfate dissolves in the electrolyte, which is indicated by an increase in its density. The battery stores energy. But, since it consists of six separate cells, each of them is charged, so to speak, independently of each other. If the conditions are the same in all cells, upon completion of the charge, energy is spent on the course of the electrolysis reaction. Water from the electrolyte is split into hydrogen and oxygen, which in the gaseous state we can observe in the form of bubbles.

This process is called battery boiling. Although it has nothing to do with boiling water in a kettle or saucepan. One way or another, if bubbles are abundantly released in all six cells, then the battery is charged.

A boiling battery is not always fully charged. If, for example, the battery is charged with too much current or voltage, then electrolysis will begin much earlier than the cells have time to accumulate the maximum possible charge for them.

Two reasons why one cell does not boil

One cell may not boil if:

  1. It has a short circuit.
  2. The battery is unbalanced.

Let's consider these reasons in more detail.

Short circuit in the cell

A short circuit in the battery is the end
A short circuit in the battery is the end

In each battery cell, positive and negative plates are located through one. For the battery to work, they must not be in contact with each other. To do this, each plate is placed in a so-called envelope made of a porous (electrolyte-permeable) dielectric (current-impermeable) material.

Over time, these envelopes break down, while more and more lead sulfate forms on the plates along the way. From vibration, its lumps fall off, and get stuck between adjacent platinum plates so that a local short circuit is obtained. As a result, such a cell ceases to function normally. It instantly loses capacity and potential. When you try to charge the battery in such a bank, nothing happens. Accordingly, the electrolyte in it does not boil.

Battery imbalance

Battery with the strongest plate sulfation
Battery with the strongest plate sulfation

In order for electrolysis to begin evenly in all compartments, they must have the same parameters. The main one is the area of the plates in contact with the electrolyte. When insoluble sulfate is formed in certain areas of lead, the charge reaction does not occur here. As a result, a cell with such a defect is not charged equally efficiently compared to the other five. As a result, it “lags behind” its brothers, and does not boil with them at the final stage of charging.

Most often, in an unbalanced battery, one cell boils much weaker than the rest. That is, gas bubbles are just appearing in it, while in the rest the electrolyte is already on the verge of bubbling.

Now consider the question of what to do if one battery cell does not boil when charging.

Diagnosis of a battery with a shorted cell

There are several ways to find shorty
There are several ways to find shorty

Calculating shorty in a car battery is quite simple. There are even several ways to do this.

The simplest is to load the battery with a current close to the starting current of the starter. Depending on the battery model, this is about 150-400 A. In the process of such a load, you must simultaneously look at the electrolyte in all cells. If there is a short circuit in one of them, it will instantly manifest itself as active seething. Normal cells should not boil under short-term load.

To load the battery with such a current, you can use a car starter, a load plug, or a piece of thick wire.

You will need an assistant to check the starter. Remove the high-voltage wires from the spark plugs so that the engine does not start, but the starter has a hard time. With the candles unscrewed, it will be easy for him, and he may not take the required current. Then one looks at the electrolyte, and the second turns the starter. If within 3-5 seconds the electrolyte does not boil in any bank, then there is no short circuit. If in some cell the activity is higher than in the others, the shorty is in it.

Similarly, you can try to load the battery with a load plug or a regular piece of wire. With the latter one must be especially careful - check quickly, and do not keep the terminals short-circuited for more than a second. This time is quite enough for the shorted cell to show itself.

If a shorted cell is detected, the diagnosis can be confirmed by measuring the density of the electrolyte in a charged battery. If there is such a defect, then the density in the damaged cell will differ sharply from the rest. In advanced cases, when the battery is operated for a long time with a short stack, the density of the electrolyte in the defective bank will be close to unity or equal to it. That is, there will be pure water, not electrolyte.

So, you have found out - due to a short circuit, one cell does not boil. What to do? Unfortunately, this is exactly the second option, when you will inevitably have to buy a new battery. Read this article on how to choose the right one. Trying to somehow remove the shorty is useless. Although there are “specialists” on the Internet who drain the electrolyte, wash the cells with anything, fill in fresh electrolyte and say that the short circuit is gone. You can, of course, try. Waste your time, or worse, burn yourself with the electrolyte.

Simple battery balancing

First assistant in balancing
First assistant in balancing

First you need to make sure that one cell does not boil when charging the battery due to imbalance, and not because of a short stack. This can be done by measuring the density. If the suspicious cell is not dead, but only lagging behind, then the density in it will differ from the rest of the cells by 0.01-0.15 units. For example, in all compartments on the hydrometer you see 1.26, and in one 1.16-1.25. This density indicates imbalance.

The main cause of imbalance between battery cells is plate sulfation. As a rule, it occurs approximately evenly throughout the battery. But there are also cases when there are more sulfates in any one jar. It is when charging and does not boil with the rest.

The second way to diagnose an unbalanced battery is to measure the voltage on each individual cell. To do this, you will need a digital multimeter and extended probes. The latter can be made from wire, or bought ready-made from the Chinese. When the battery is charged, the voltage on each cell should be at least 2.1 V. If any of the indicators lag slightly behind, then the battery is not balanced.

The third way to determine the imbalance is to look into each cell with a good flashlight, and compare the color of the plates. If you find lighter plates in one of the compartments than in the others, then they are sulphated and are the cause of an unbalanced battery.

To balance the battery, it needs to be trained. The workout is covered in more detail on ABS in the battery desulfation article.

Either do this:

  1. Disconnect the battery from the car's electrical system.
  2. Discharge the battery to a voltage of 10.5 V by loading it with a headlight bulb.
  3. In no case do not allow a deeper discharge, especially if you have a calcium battery.
  4. Measure the density of the electrolyte in all cells and record the readings (hydrometer vs refractometer).
  5. If the density is the same, then it is enough to simply charge the battery in the usual way.
  6. If the density is different, charge the battery with a current of 5% of the capacity (for example, for a 60 Ah battery it is 3 A).
  7. Do not allow the charge voltage to exceed 14.4V.
  8. When, at the specified voltage, the charge current drops to 0.1 A, stop charging.
  9. Measure the electrolyte density in all cells.
  10. If the density does not even out, repeat all steps, starting from the second.

To increase the efficiency of balancing according to this algorithm, periodically load the battery with the same light bulb during charging. For example, charge for half an hour, then discharge for 5 minutes.

Battery training, control-training cycle, desulfation, balancing are, in fact, the same thing. There are, of course, exceptional cases. But if the problem is only that one cell does not boil when the battery is charging, then it does not apply to these cases.


When one cell in a lead-acid car battery does not boil, this indicates that one of the cells is either not charging or is lagging behind the rest. There are only two reasons. The first is a short man, which cannot be corrected by adequate methods. The second reason is imbalance due to selective sulfation. It is treated by training or battery desulfation.

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