Battery Density

The car battery, known as the battery, is responsible for the start, lighting and ignition systems in the car. As a rule, car batteries are lead-acid, composed of galvanic cells that provide a 12-volt system. Each cell produces 2.1 V when fully charged. The density of the electrolyte is a controlled property of an aqueous acidic solution that ensures the normal operation of the batteries.

Lead Acid Battery Composition

The lead acid battery's electrolyte is a solution of sulfuric acid and distilled water. The specific gravity of pure sulfuric acid is about 1.84 g / cm ³ and this pure acid is diluted with distilled water until the specific gravity of the solution becomes 1.2-1.23 g / cm ³ .

Although in some cases, the density of the electrolyte in the battery is recommended depending on the type of battery, seasonal and climatic conditions. The specific gravity of a fully charged battery according to the industry standard in Russia is 1.25-1.27 g / cm ³ in summer and for severe winters 1.27-1.29 g / cm ³ .

Specific gravity of electrolyte

One of the main parameters of the battery is the specific gravity of the electrolyte. This is the ratio of the weight of the solution (sulfuric acid) to the weight of an equal volume of water at a certain temperature. Usually measured using a hydrometer. The density of the electrolyte is used as an indicator of the state of charge of the cell or battery, however, it cannot characterize the capacity of the battery. During unloading, the specific gravity decreases linearly.

Given this, it is necessary to clarify the size of the permissible density. The electrolyte in the battery should not exceed 1.44 g / cm ³ . The density may be from 1.07 to 1.3 g / cm ³ . The temperature of the mixture will be about +15 C.

The pure electrolyte in its pure form is characterized by a rather high value of this indicator. Its density is 1.6 g / cm ³ .

Degree of charge

In a fully charged stationary mode and during discharge, measuring the specific gravity of the electrolyte gives an approximate indication of the state of charge of the cell. Specific Gravity = Open Circuit Voltage - 0.845.

Example: 2.13 V - 0.845 = 1.285 g / cm ³ .

The specific gravity decreases when the battery is discharged to a level close to the value of pure water, and increases during recharging. The battery is considered fully charged when the density of the electrolyte in the battery reaches the maximum possible value. The specific gravity depends on the temperature and the amount of electrolyte in the cell. When the electrolyte is near the lower mark, the specific gravity is higher than the nominal, it falls, and water is added to the cell to bring the electrolyte to the desired level.

The volume of electrolyte expands when the temperature rises and contracts with decreasing temperature, which affects the density or specific gravity. As the volume of the electrolyte expands, the readings decrease and, conversely, the specific gravity increases at lower temperatures.

Before raising the density of the electrolyte in the battery, it is necessary to carry out measurements and calculations. The specific gravity for the battery is determined by the application in which it will be used, taking into account the operating temperature and battery life.

Chemical reaction in batteries

As soon as the load is connected through the battery terminals, the discharge current begins to flow through the load, and the battery starts to discharge. During the discharge process, the acidity of the electrolyte solution decreases and leads to the formation of sulfate deposits on both positive and negative plates. In this discharge process, the amount of water in the electrolyte solution increases, which reduces its specific gravity.

Battery cells can be discharged to a predetermined minimum voltage and specific gravity. A fully charged lead-acid battery has a voltage and specific gravity of 2.2 V and 1.250 g / cm ^3, respectively, and this cell can usually be discharged until the corresponding values ​​reach 1.8 V and 1.1 g / cm ³ .

Electrolyte composition

The electrolyte contains a mixture of sulfuric acid and distilled water. The data will not be accurate when measuring if the driver has just added water. You need to wait a while for the fresh water to mix with the existing solution. Before you increase the density of the electrolyte, you need to remember: the higher the concentration of sulfuric acid, the denser the electrolyte becomes. The higher the density, the higher the charge level.

For an electrolyte solution, distilled water is the best choice. This minimizes possible contamination in the solution. Some contaminants can cause reactions with electrolyte ions. For example, if you mix the solution with NaCl salts, you will get a precipitate, which will change the quality of the solution.

The effect of temperature on capacity

What is the density of the electrolyte - it will depend on the temperature inside the batteries. The user manual for specific batteries specifies which correction should be applied. For example, in the Surrette / Rolls manual, for temperatures in the range -17.8 to -54.4 ° C and temperatures below 21 ° C, 0.04 is removed for every 6 degrees.

Many inverters or charge controllers have a battery temperature sensor that attaches to the battery. They usually have an LCD display. Indication of an infrared thermometer will also give the necessary information.

Density meter

An electrolyte density hydrometer is used to measure the specific gravity of the electrolyte solution in each cell. The acid battery is fully charged with a specific gravity of 1.255 g / cm ³ at 26 ° C. The specific gravity is a measurement of a fluid that is compared to a base. This is water, which is assigned a base number of 1.000 g / cm ³ .

The concentration of sulfuric acid in water in the new battery is 1.280 g / cm ³ , which means that the electrolyte weighs 1.280 g / cm ³ times the weight of the same volume of water. A fully charged battery will be tested at levels up to 1.280 g / cm ³ , while a discharged one will be taken into account in the range from 1.100 g / cm ³ .

Hydrometer Test Procedure

The reading temperature of the hydrometer should be adjusted to a temperature of 27 ° C, especially in relation to the electrolyte density in winter. High-quality hydrometers are equipped with an internal thermometer that will measure the temperature of the electrolyte, and include a conversion scale to correct the readings of the float. It is important to recognize that the temperature is significantly different from the environment when the vehicle is in operation. Measurement order:

1. A few times, collect the electrolyte with a rubber bulb into the hydrometer so that the thermometer can adjust the electrolyte temperature and measure the readings.
2. Examine the color of the electrolyte. A brown or gray color indicates a problem with the battery and is a sign that it is nearing the end of its life.
3. Collect the minimum amount of electrolyte in the hydrometer so that the float floats freely without contact with the upper or lower part of the measuring cylinder.
4. Hold the hydrometer in a vertical position at eye level and pay attention to the readings where the electrolyte corresponds to the scale on the float.
5. Add or subtract 0.004 fractions of a unit for readings for every 6 ° C, at an electrolyte temperature above or below 27 ° C.
6. Adjust the readings, for example, if the specific gravity is 1.250 g / cm ³ and the electrolyte temperature is 32 ° C, a value of 1.250 g / cm ³ gives a corrected value of 1.254 g / cm ³ . Similarly, if the temperature was 21 ° C, subtract the value 1.246 g / cm ³ . Four points (0.004) from 1.250 g / cm ³ .
7. Test each cell and note readings adjusted to 27 ° C before checking electrolyte density.

Charge Measurement Examples

Example 1:

1. The hydrometer readings are 1.333 g / cm ³ .
2. The temperature is 17 degrees, which is 10 degrees lower than recommended.
3. Subtract 0.007 from 1.333 g / cm ³ .
4. The result is 1.263 g / cm ³ , so the state of charge is about 100 percent.

Example 2:

1. Density data - 1.178 g / cm ³ .
2. The electrolyte temperature is 43 degrees C, which is 16 degrees higher than normal.
3. Add 0.016 to 1.178 g / cm ³ .
4. The result is 1.194 g / cm ³ , charging 50 percent.

Electrolyte Density Table

The following temperature correction table is one way to explain the dramatic changes in electrolyte density at different temperatures.

To use this table, you need to know the temperature of the electrolyte. If measurement for some reason is not possible, it is better to use the ambient temperature.

An electrolyte density table is provided below. This is data depending on the temperature:

As can be seen from this table, the density of the electrolyte in the battery in winter is much higher than in the warm season.

Battery Maintenance

These batteries contain sulfuric acid. When working with them, you should always use safety glasses and rubber gloves.

If the cells are overloaded, the physical properties of lead sulfate gradually change, and they are destroyed, due to which the charging process is disrupted. Consequently, the density of the electrolyte decreases due to the low rate of the chemical reaction.

The quality of sulfuric acid should be high. Otherwise, the battery may quickly become inoperative. A low level of electrolyte helps to dry the internal plates of the device, after which it will be impossible to restore the battery.

Sulphonated batteries can be easily recognized by viewing the changed color of the plates. The color of the sulfated plate becomes lighter and its surface turns yellow. Such cells demonstrate a decrease in power. If sulfonation occurs for a long time, irreversible processes occur.

To avoid this situation, it is recommended that the lead-acid batteries be charged for a long time at a low charging current rate.

There is always a high probability of damage to the terminal blocks of the battery cells. Corrosion mainly affects bolted joints between cells. This can easily be avoided by ensuring the tightness of each coated bolt with a thin layer of special grease.

While charging the battery, there is a high probability of acid spray and gas. They can pollute the atmosphere around the battery. Therefore, good ventilation is needed near the battery compartment.

These gases are explosive, therefore, an open flame should not fall into the space where lead batteries are charged.

To prevent a battery explosion that could result in serious injury or death, do not insert a metal thermometer into the battery. You need to use a hydrometer with a built-in thermometer that is designed to test batteries.

Power Source Life

Battery performance deteriorates over time, regardless of whether it is used or not, it also deteriorates with frequent charge-discharge cycles. Battery life is the time when an inactive battery can be stored before it becomes unusable. It is generally believed that this is about 80% of its original capacity.

There are several factors that significantly affect battery life:

1. Cyclic life. Battery life is determined mainly by battery usage cycles. Typically, the service life is from 300 to 700 cycles under normal use.
2. Discharge Depth Effect (DOD). Abandoning higher productivity will shorten the life cycle.
3. Temperature effect. This is a major factor in battery performance, shelf life, charging and voltage control. At higher temperatures, more chemical activity occurs in the battery than at lower temperatures. For most batteries, a temperature range of -17 to 35 ° C is recommended.
4. Voltage and reload speed. All lead-acid batteries release hydrogen from the negative plate and oxygen from the positive during charging. A battery can only store a certain amount of electricity. Typically, the battery charges 90% in 60% of the time. And 10% of the remaining battery capacity is charged about 40% of the total time.

Good battery life - from 500 to 1200 cycles. The actual aging process leads to a gradual decrease in capacity. When a cell reaches a certain service life, it does not stop working suddenly, this process is stretched in time, it needs to be monitored in order to prepare in time for battery replacement.