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The future is clearly in maintenance-free batteries — no access to the electrolyte, no problems. Conditionally. Nevertheless, these days there are still quite a lot of batteries, as they say, with traffic jams. There are several reasons for this, ranging from the manufacturer's capabilities to the fact that not all technologies allow making maintenance-free batteries. Accordingly, the issue of electrolyte density remains relevant. It should be at the desired level, moreover, the same in all cells of the battery. How to control and correct it - you will learn from this article.

What is the density of an electrolyte?

The density of an electrolyte is the ratio of its mass to the volume it occupies. It is measured in grams per cubic centimeter (g/cm3). The battery electrolyte is a liquid composed of water (H2O) and sulfuric acid (H2SO4). The density of water is 1 g/cm3, and that of concentrated sulfuric acid is 1.83 g/cm3. When mixed, a certain average density is obtained between these two values.

The normal density of the battery electrolyte is the concentration of acid in water at which the functionality of the battery is ensured. If the indicator is above the norm, wear and sulfation of lead plates occurs at an accelerated pace. At low density, the risk of freezing of the electrolyte in the cold season increases. For regions with a temperate climate, the norm is in the range of 1.27...1.28 g/cm3. In the northern regions with severe frosts - 1.28...1.30 g/cm3. Closer to the equator, the normal density of the electrolyte can be about 1.23 g/cm3.

Explanation. The lower the density of the electrolyte, the less frost it freezes. This is due to the fact that when acid is added to water, the freezing point decreases. Pure water turns to ice at 0°C. The electrolyte with a density of 1.28 g/cm3 remains liquid up to -74°C. While at a density of 1.12 g/cm3 freezing begins at a frost of only -10°C.

Different cell densities are almost always the result of battery misuse. May refer to:

In a healthy lead-acid battery, the density of the electrolyte depends on the state of charge. Its measurement allows you to fairly accurately assess the battery charge level and its technical condition. The change in density is due to the principle of operation of a lead-acid battery.

As the battery discharges, the acid reacts with the lead to form sulfate (PbSO4) on the electrodes. The density of the electrolyte tends to the density of pure water - 1.00 g/cm3. When the battery is charged, lead sulfate is destroyed and the acid returns to the electrolyte, increasing its density. In general, this process can be called desulfation.

Battery electrolyte density measurement

The density of the electrolyte is measured with a hydrometer. It consists of a flask and a float with a load, on which there is a scale with indicators. The higher the density, the better the float will float, showing a higher density. And vice versa. In maintenance-free batteries, the indicator does the same, about which there is a separate article on the Auto without Service Station website.

In order to obtain real electrolyte density values, measurements must be performed correctly. Mistakes are often made in this simple procedure. Receiving incorrect values, the user draws erroneous conclusions and, even worse, takes actions that are dangerous for the battery life.

In order not to mislead yourself, before measuring, carefully read the following rules:

  1. It is advisable to disconnect the battery from the on-board network and charger, and also remove it from the engine compartment (so as not to splatter the surroundings with acid).
  2. The electrolyte temperature must be +25°C. For other values it is necessary to use special tables. If the temperature is within + 15...+30°C, then you should not focus on this. The readings will be distorted by an imperceptible amount.
  3. The electrolyte level should be normal.
  4. Density cannot be measured after correcting the electrolyte level with distilled water.
  5. In calcium batteries, after classical charging, the electrolyte needs to be mixed. How it works is described and shown in a master class article on recharging a Ca/Ca battery.

Also from personal experience, I recommend taking measurements for each can at least three times in a row. Firstly, this will once again make sure that you did not make a mistake when reading the readings on the scale. Secondly, in this way you will additionally mix the electrolyte and get more real data. Be sure to keep the hydrometer at eye level when reading. If the battery is more than 5 years old, return the electrolyte to the jars very carefully, avoiding vigorous mixing.

Attention! The sulfuric acid contained in the battery electrolyte is hazardous to your health. When manipulating the hydrometer, be extremely careful and careful. Avoid contact of the electrolyte with the skin, eyes, respiratory organs. If this happens, quickly flush the affected area with plenty of running water. If necessary, call an ambulance.

The density of the electrolyte is always measured in all cells in turn. This is necessary for comparison of indicators. For the functionality and resource of the battery, it is very important that all banks have the same density. A variation of ±0.01 g/cm3 is allowed .

Advice! Do not be lazy to write down the results of measurements. This will avoid confusion and draw correct conclusions about the density of the electrolyte.

Methods for adjusting the density of the battery electrolyte

The need to adjust the density of the electrolyte arises only if the car battery is used improperly. If from the first day the battery was monitored and looked after in a timely manner, this procedure may not be necessary until its natural "death" and replacement.

There are only three adequate methods for adjusting the density of the electrolyte:

  1. Equalizing charge.
  2. "Boiling" the battery.
  3. Corrective electrolyte.

Before attempting a forced increase in electrolyte density, make sure you charge your battery properly. Pay special attention to calcium batteries. Follow the link for a detailed article about the features of their operation.

Attention! I am categorically against any manipulation of the electrolyte if the battery is maintenance-free. No need to try to disassemble them, pick, drill, look for plugs hidden by a cunning manufacturer. It usually doesn't end well. Everything that can be done with such batteries, you will find in this article.

These three alignment methods should be applied in the order in which they are described below. You should not rush to the most radical actions. There is a high probability of making it even worse than it was. Up to the "kill" of a fully functional battery.

Equalizing charge

This method is based on the principle of battery operation. When it is charged, the density of the electrolyte increases. This property can be used quite successfully if in some cells the indicators lag behind by 0.01...0.1 g/cm3. The essence of the method is to charge the battery for a long time with low currents. Usually it is 0.5...1 A. The voltage, in this case, in no case should exceed 14.4 V. Otherwise, electrolysis may begin in the cells, during which energy accumulation does not occur, but water is lost from the electrolyte .

If there is free time, then it is advisable to apply an equalizing charge after the forced discharge of the battery. Charging from zero to 100% with low current will take several days. However, this technique is very effective. And for batteries in a running state, it is simply necessary.

Attention! Modern batteries quickly lose capacity when they are left in a deeply discharged state for a long time. When applying an equalizing charge to correct the density of the electrolyte, start charging immediately after discharging. Also, the voltage at the terminals should not fall below 10.5 V. If you leave a discharged battery for a week or more, sulfation will begin. It will be more difficult to fix it, the longer the battery will forgive in deep discharge. After a couple of weeks, lead sulfate becomes so strong that no amount of desulfation helps.

"Boiling" the battery

Boiling is conditionally called the process of electrolysis of water, of which the electrolyte consists to a greater extent. The name is given because of the visual similarity with the boiling of water when heated to a temperature of +100°C. Electrolysis occurs in the battery, which is similar to boiling water only visually. The nature of this phenomenon is quite different. During electrolysis, water decomposes into hydrogen and oxygen, which leave the battery in a gaseous state.

In old batteries, to start electrolysis, it was enough to raise the charge voltage above 14.5...14.8 V. Now this problem has been partially solved by alloying lead plates with calcium. To make a modern battery boil, a voltage of 15...16 V must be applied to its terminals.

This feature can be successfully used to equalize the density of the electrolyte by physically mixing it. It is not worth boiling the battery for a long time and often. Each long electrolysis not only evaporates water, but also reduces battery life. Long - how long is it? If the electrolyte "boils" for more than 20 minutes in a row, then this is already considered long enough.

Do not boil the battery for more than 40 minutes. Maximum 1 hour. Also, do not apply this procedure more than a couple of times a year. If this does not help to equalize the density of the electrolyte, then you need to resort to more radical methods.

Correction electrolyte

If none of the methods described above helps to equalize the density of the electrolyte, this is a bad signal. It indicates serious problems in the battery. Since there is not much to lose, all that remains is the use of a corrective electrolyte. I must say right away that you should not rely too much on this method. A positive result is extremely rare.

Advice! Don't waste time resuscitating a battery with an internal short circuit. Carefully read the material on this topic on this site (the link is above), put up with it and just buy a new battery.

Density equalization algorithm using corrective electrolyte:

  1. Fully charge the battery.
  2. Correctly measure the density in all banks.
  3. Mark the cells where the density needs to be adjusted.
  4. Using a hydrometer, select a little electrolyte from the desired cells.
  5. Instead, pour the same amount of correction electrolyte into the jars.
  6. Conduct an equalizing charge or "boil" as described above.
  7. Repeat measurements.

The operation can be considered successful if the density of the electrolyte in the cells is the same or with a spread of up to ±0.01 g/cm3.

How not to adjust the density of the electrolyte?

In conclusion, I will give several inadequate methods for adjusting the density of the battery electrolyte. They are offered by bloggers who are poorly versed in batteries and make content for the sake of views. Don't do it. Waste your time and with a high degree of probability you will lose a completely serviceable battery.

Inappropriate methods:

  1. Replacing the entire volume of electrolyte.
  2. Mixing electrolytes in neighboring cells with a syringe or hydrometer.
  3. Aggressive mixing of the electrolyte using a syringe or hydrometer.
  4. Battery reversal.
  5. Flushing the insides of the battery with various miracle liquids.

To avoid problems with the density of the electrolyte, regularly monitor the condition of the battery. Recharge it in time, avoid deep discharges, freezing, mechanical damage. Periodically measure the density of the electrolyte, if the design of the battery allows it. This will allow you to identify "not comfortable" operating conditions for your battery in time.

Recommendation! I strongly advise you to familiarize yourself with all the materials that are referenced in this article. This is guaranteed to help expand your knowledge on this topic and increase the life of your batteries to a normal 5-7 years. Believe my experience - even the cheapest batteries can last much longer than 2-3 years.

Friends, the author vitally needs your help.


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