The car battery, known as the accumulator, is responsible for the starting, lighting and ignition systems in a car. Typically, car batteries are lead-acid, consisting of galvanic cells that provide a 12-volt system. Each of the cells creates 2.1 volts when fully charged. Electrolyte density is a controlled property of an aqueous acid solution that ensures the normal operation of batteries.
Composition of a lead-acid battery
Lead-acid battery electrolyte is a solution of sulfuric acid and distilled water. The specific gravity of pure sulfuric acid is about 1.84 g/cm3, and this pure acid is diluted with distilled water until the specific gravity of the solution is 1.2-1.23 g/cm3.
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 industrial standard in Russia is 1.25-1.27 g / cm3 in summer and for severe winters - 1,27-1, 29g/cm3.
Specific gravity of electrolyte
One of the main parameters of the battery is the specific gravity of the electrolyte. It is the ratio of the weight of a solution (sulfuric acid) to the weight of an equal volume of water at a certain temperature. Usually measured with a hydrometer. The density of the electrolyte is used as an indicator of the state of charge of a cell or battery, but 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 allowable density. The electrolyte in the battery should not exceed 1.44g/cm3. The density can be from 1.07 to 1.3g/cm3. The temperature of the mixture will then be about +15 C.
Electrolyte of increased density in its pure form is characterized by a rather high value of this indicator. Its density is 1.6g/cm3.
Charge level
When fully charged steady state and under discharge, measuring the specific gravity of the electrolyte gives a rough indication of the state of charge of the cell. Specific Gravity=Open Circuit Voltage - 0.845.
Example: 2.13V - 0.845=1.285g/cm3.
Specific gravity decreases when the battery is discharged to a level close to that 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. Specificthe weight depends on the temperature and the amount of electrolyte in the cell. When the electrolyte is near the low mark, the specific gravity is higher than nominal, it drops and water is added to the cell to bring the electrolyte up to the required level.
The volume of the electrolyte expands as the temperature rises and contracts as the temperature falls, which affects the density or specific gravity. As the electrolyte volume expands, the reading decreases and, conversely, the specific gravity increases at lower temperatures.
Before you raise the density of the electrolyte in the battery, you must perform 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.
| % Sulfuric acid | % Water | Specific Gravity (20°C) |
| 37, 52 | 62, 48 | 1, 285 |
| 48 | 52 | 1, 380 |
| 50 | 50 | 1, 400 |
| 60 | 40 | +1, 500 |
| 68, 74 | 31, 26 | 1, 600 |
| 70 | 30 | 1, 616 |
| 77, 67 | 22, 33 | 1, 705 |
| 93 | 7 | 1, 835 |
Chemical reaction in batteries
As soon as a load is connected across the battery terminals, a discharge current begins to flow through the load and the battery begins to discharge. During the discharging process, the acidity of the electrolyte solution decreases and leads to the formation of sulfate deposits on both the 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 specified minimum voltage and specific gravity. A fully charged lead acid battery has a voltage and specific gravity of 2.2V and 1.250g/cm3 respectively, and this cell can normally be discharged until the corresponding values not reached 1.8V and 1.1g/cm3.
Electrolyte composition
The electrolyte contains a mixture of sulfuric acid and distilled water. The data will not be accurate when measured if the driver has just added water. You need to wait a while so that fresh water has time to mix with the existing solution. Before you raise the density of the electrolyte, you need to remember: the greater the concentration of sulfuric acid, the denser the electrolyte becomes. The higher the density, the higher the charge level.
For the electrolyte solution, distilled water is the best choice. This minimizes the possiblecontaminants in solution. Some contaminants may react with electrolyte ions. For example, if you mix a solution with NaCl s alts, a precipitate will form, which will change the quality of the solution.
Influence of temperature on capacitance
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 ranging from -17.8 to -54.4oC below 21oC, subtract 0.04 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. Pointing the infrared thermometer will also give the necessary information.
Density meter
The 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 1.255g/cm3 at 26oC. Specific gravity is a measurement of a fluid that is compared to a base. This is water that is assigned a base number of 1.000 g/cm3.
The concentration of sulfuric acid in water in a new battery is 1.280 g/cm3, which means that the electrolyte weighs 1.280 g/cm3times the weight of the same volume of water. A fully charged battery will be tested at up to1.280 g/cm3, while discharged will count from 1.100 g/cm3.
Hydrometer test procedure
The reading temperature of the hydrometer should be adjusted to a temperature of 27oC, especially regarding the electrolyte density in winter. High quality hydrometers have an internal thermometer that will measure the temperature of the electrolyte and include a conversion scale to correct float readings. It is important to recognize that the temperature is significantly different from the environment if the vehicle is being driven. Measurement order:
- Pour the electrolyte into the hydrometer with a rubber bulb several times so that the thermometer can adjust the temperature of the electrolyte and take readings.
- Study the color of the electrolyte. A brown or gray discoloration indicates a problem with the battery and is a sign that the battery is nearing the end of its life.
- Direct the minimum amount of electrolyte into the hydrometer so that the float floats freely without contact with the top or bottom of the measuring cylinder.
- Hold the hydrometer upright at eye level and pay attention to the reading where the electrolyte matches the scale on the float.
- Add or subtract 0.004 units for reading every 6oC, when the electrolyte temperature is above or below 27oC.
- Adjust the reading, for example, if the specific gravity is 1.250 g/cm3 and the electrolyte temperature is32oC, a value of 1.250 g/cm3 gives a corrected value of 1.254 g/cm3. Similarly, if the temperature was 21oC, subtract 1.246 g/cm3. Four points (0.004) of 1.250 g/cm3.
- Test each cell and note reading corrected to 27oC before checking electrolyte density.
Examples of charge measurement
Example 1:
- Hydrometer reads 1.333 g/cm3.
- Temperature is 17 degrees, 10 degrees below recommended.
- Subtract 0.007 from 1.333 g/cm3.
- The result is 1.263 g/cm3, so the state of charge is about 100 percent.
Example 2:
- Density data - 1.178g/cm3.
- The electrolyte temperature is 43 degrees C, which is 16 degrees above normal.
- Add 0.016 to 1.178g/cm3.
- Result is 1.194g/cm3, 50 percent charged.
| CHARGE STATUS | SPECIFIC WEIGHT g/cm3 |
| 100% | 1, 265 |
| 75% | 1, 225 |
| 50% | 1, 190 |
| 25% | 1, 155 |
| 0% | 1, 120 |
Electrolyte density table
The following temperature correction tableis one way to explain the abrupt changes in electrolyte density values at different temperatures.
To use this table, you need to know the temperature of the electrolyte. If the measurement is not possible for some reason, then it is better to use the ambient temperature.
The electrolyte density table is shown below. This data is based on temperature:
| % | 100 | 75 | 50 | 25 | 0 |
| -18 | 1, 297 | 1, 257 | 1, 222 | 1, 187 | 1, 152 |
| -12 | 1, 293 | 1, 253 | 1, 218 | 1, 183 | 1, 148 |
| -6 | 1, 289 | 1, 249 | 1, 214 | 1, 179 | 1, 144 |
| -1 | 1, 285 | 1, 245 | 1, 21 | 1, 175 | 1, 14 |
| 4 | 1, 281 | 1, 241 | 1, 206 | 1, 171 | 1, 136 |
| 10 | 1, 277 | 1, 237 | 1, 202 | 1, 167 | 1, 132 |
| 16 | 1, 273 | 1, 233 | 1, 198 | 1, 163 | 1, 128 |
| 22 | 1, 269 | 1, 229 | 1, 194 | 1, 159 | 1, 124 |
| 27 | 1, 265 | 1, 225 | 1, 19 | 1, 155 | 1, 12 |
| 32 | 1, 261 | 1, 221 | 1, 186 | 1, 151 | 1, 116 |
| 38 | 1, 257 | 1, 217 | 1, 182 | 1, 147 | 1, 112 |
| 43 | 1, 253 | 1, 213 | 1, 178 | 1, 143 | 1, 108 |
| 49 | 1, 249 | 1, 209 | 1, 174 | 1, 139 | 1, 104 |
| 54 | 1, 245 | 1, 205 | 1, 17 | 1, 135 | 1, 1 |
As you can see from this table, the electrolyte density in the battery in winter is much higher than in the warm season.
Battery Maintenance
These batteries contain sulfuric acid. Safety glasses and rubber gloves should always be used when handling them.
If the cells are overloaded, the physical properties of lead sulfate gradually change and they are destroyed, which disrupts the charging process. Therefore, the density of the electrolyte decreases due to the slow rate of the chemical reaction.
The quality of sulfuric acid must be high. Otherwise, the battery may quickly become inoperable. Low electrolyte levels help dry out the internal plates of the device, making it impossible to restore the battery.
Sulfated batteries can be easily recognized by looking at the changed color of the plates. The color of the sulfated plate becomes lighter and its surface becomes yellow. Such cells show a decrease in power. If sulfonation occurs for a long time, irreversibleprocesses.
To avoid this situation, it is recommended to charge lead-acid batteries for a long time at a low charging rate.
There is always a high possibility of damage to the terminal blocks of battery cells. Corrosion mainly affects bolted connections between cells. This can be easily avoided by ensuring that each bolt is sealed with a thin layer of special grease.
When the battery is charging, there is a high possibility of acid spray and gases. They can pollute the atmosphere around the battery. Therefore, good ventilation is required near the battery compartment.
These gases are explosive, therefore, open flames should not enter the space where lead batteries are charged.
To prevent the battery from exploding, which could cause serious injury or death, do not insert a metal thermometer into the battery. You need to use a hydrometer with a built-in thermometer, which is designed for testing batteries.
Power supply life
Battery performance degrades over time, whether used or not, it also degrades with frequent charge-discharge cycles. Life is the amount of time 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:
- Cyclic life. Timebattery life is determined mainly by battery usage cycles. Typically 300 to 700 cycles in normal use.
- Depth of Discharge (DOD) effect. Forgoing higher performance will result in a shorter life cycle.
- 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, the recommended temperature range is -17 to 35oC.
- Voltage and recharge speed. All lead-acid batteries release hydrogen from the negative plate and oxygen from the positive plate during charging. A battery can only store a certain amount of electricity. As a rule, the battery is charged to 90% in 60% of the time. And 10% of the remaining battery capacity is charged about 40% of the total time.
Good battery life is 500 to 1200 cycles. The actual aging process leads to a gradual decrease in capacitance. When a cell reaches a certain life, it does not suddenly stop working, this process is extended over time, it must be monitored in order to prepare in time for battery replacement.
