Lead-Acid Batteries: Function, Verification, and Failures
If a container has two electrodes immersed in distilled water and a battery connected, you will observe that distilled water is not conductive. However, if we sprinkle salt and dissolve it, it becomes conductive.
By passing an electric current for a few minutes, we see that the water picks up a greenish color. This effect is called electrolysis, and the liquid is an electrolyte.
Accumulators are used to provide electricity to the motor. They consist of two plates or bars of lead (Pb) immersed in dilute sulfuric acid (H2SO4). The plate serving as the cathode is coated with lead oxide, and the anode is lead.
- Discharge: Pb + PbO2 + 2 H2SO4 -> 2 PbSO4 + 2 H2O
- Charge: 2 PbSO4 + 2 H2O -> Pb + PbO2 + 2 H2SO4
In the process of discharging this type of battery, the active material reacts with sulfuric acid diluted with water. That is, the lead dioxide of the positive pole with the sponge lead of the negative pole becomes lead sulfate.
During the discharge process, lead sulfate (PbSO4) and water form, and sulfuric acid (H2SO4) is consumed. When connected to a DC generator (like an alternator in the case of cars) between the poles of the discharged battery and an electric current is passed, the reaction is reversed, so it is said that the battery is recharged.
The voltage obtained in an accumulator (a cell) is 2.1 V, so it must be connected from 3 to 6 battery cells in series (forming a battery), as a car requires 6 to 12 V. A lead battery stores electrical energy with an efficiency of 80%.
Volume changes of dilute sulfuric acid can cause the disintegration of the plates, hence the need for caution with this.
Battery Verifications
The battery voltage is usually measured at rest, but also during discharge. At rest, the tensions obtained are approximately 13.2 V when fully charged, 12 V at half charge, and 9 V when discharged.
To make the measurement, you can use a voltmeter and take a reading while the starter is activated.
Measurement of the electrolyte density is done with a hydrometer. The electrolyte is absorbed, and the measurement is taken in the piece that incorporates the densimeter.
A density of between 1.270 and 1.290 indicates that the battery is fully charged, a value between 1.200 and 1.240 indicates a half charge, and values less than 1.150 indicate that the battery is discharged.
Battery Failures
Three types of damage can be established:
External Origin: These come from the electrical system and are due to inappropriate and excessive work of the battery. This results in overload, which would require adding water frequently. The remedy would be compensation for the charging circuit malfunction that causes the overload. Deficient load is also external damage; the solution to this problem would be to check the connections. Finally, if a battery cannot move the starter, it should be replaced since it is not capable of maintaining a normal load.
Purely Mechanical: These are breaks in the box due to shock or excessive vibration, loss of output plugs allowing detachment of the electrolyte, and active material of the plates.
Internal Origin: The most common are defective separators, sediment deposited on the bottom of the box, impurities, and sulfation of the plates.