The article provides an overview of the lead-acid cell, focusing on their chemical reactions, charging processes, and safety considerations. It highlights their structure, functionality, and best practices for handling and maintenance.
The lead-acid cell is a very common secondary cell. It is the power source for the electric system of most cars, trucks, boats, and tractors. It can provide the large currents (hundreds of amperes) needed to crank internal-combustion engines.
A lead-acid cell produces about 2.1 V. Higher voltages are obtained by connecting cells together to form batteries. A 12-V automobile battery actually has a nominal voltage of 12.6 V because it contains six cells.
The structure of a lead-acid cell is shown in Figure 2. Notice that the cell terminals connect to a group of plates. The groups are tied together (both electrically and mechanically) at the top. The meshing together of the positive and negative groups effectively provides one large negative plate and one large positive plate.
Figure 2. Structure of a lead-acid cell.
Lead-Acid Cell Chemical Reaction
When a load is connected to the cell of Figure 3(a), current flows. Electrons leave the negative terminal (plate), flow through the load, and return to the positive terminal of the cell. At the surface of the positive plate shown in Figure 3(a), a molecule of lead peroxide PbO2 becomes three ions. Notice that each ion of oxygen (O) has two excess electrons, whereas the ion of lead (Pb) has a deficiency of only two electrons. Thus, the plate is left with a deficiency of two electrons; that is, it has a positive charge. In the electrolyte, two molecules of sulfuric acid H2SO4 provide four hydrogen (H) ions and two sulfate SO4 ions. The four hydrogen ions combine with the two oxygen ions to form two molecules of water H2O. One of the sulfate ions combines with the lead ion at the positive plate to form a molecule of lead sulfate PbSO4.
Figure 3. Lead-acid cell. (a) Chemical reaction. (b) Results.
On the surface of the negative plate in Figure 3(a), an atom of lead becomes a positive ion. In becoming a positive ion, the lead atom leaves two electrons on the plate. Thus, the plate has a negative charge. The lead ion combines with a sulfate ion from the electrolyte and forms a molecule of lead sulfate. The result of these reactions, shown in Figure 3(b), is that the plates change to lead sulfate and the electrolyte changes to water.
The above-described chemical reaction continues until one of two things happens:
- The load is removed; the chemical reaction stops. At this time, the charges on the plates and the charges of the electrolyte ions are in a state of equilibrium (balance).
- All the sulfuric acid solution has been converted to water and/or all the lead and lead peroxide have been converted to lead sulfate. In this case, the battery is fully discharged.
Lead-Acid Cell Recharging
A lead-acid cell is recharged by forcing a reverse current through the cell. That is, electrons enter the negative plate and leave the positive plate. When a cell is being charged, all the chemical reactions described above are reversed. The water and lead sulfate are converted back into sulfuric acid, lead, and lead peroxide. Cells are charged by connecting them to a voltage source that is greater than the cell’s voltage. In other words, the cell becomes the load rather than the energy source. As the load, the cell is converting electric energy (from the other voltage source) into chemical energy.
The six-cell (12-V) battery in Figure 4 is being recharged. Notice that the negative terminal of the charger is connected to the negative terminal of the battery. Since the charger’s voltage is greater than the battery’s voltage, current flows in the direction indicated in Figure 4. The exact voltage of the charger depends on the condition of the battery and on the rate of charge desired. The charger voltage is adjusted to provide the desired charge current. The manufacturer’s recommendation should be followed in determining the rate of charge for a specific type of battery or cell. Too fast a charge rate must be avoided because it will overheat the battery.
Figure 4. Charging a battery. The charger voltage must exceed the battery voltage.
A battery should not be overcharged. Overcharging can weaken the plate structure of the battery. When a battery is overcharged, water from the electrolyte is converted into hydrogen gas and oxygen gas. Having to add more than normal amounts of water to a battery is an indication that it is being overcharged.
Many of the newer lead-acid batteries are sealed. There is no provision for adding water. Such batteries require no maintenance other than keeping them clean—especially their terminals.
Specific Gravity
The charge of a battery can be determined by measuring the specific gravity of the electrolyte. The specific gravity of a substance is the ratio of its weight to the weight of water. If a substance has a specific gravity of 1.251, it is 1.251 times as heavy as water. Sulfuric acid is heavier than water. Therefore, the more sulfuric acid in the electrolyte, the higher the specific gravity of the electrolyte. Since the amount of sulfuric acid increases as the battery is charged, specific gravity indicates the state of charge of the battery.
The specific gravity of a fully charged cell is adjusted at the factory to suit the structure and intended purpose of the cell. For fully charged lead-acid cells, the specific gravity ranges from 1.21 to 1.28. The typical automotive battery is fully charged at a specific gravity of about 1.26.
A lead-acid cell is considered to be completely discharged when the specific gravity drops to 1.12. It should not be left in this discharged state for extended periods. To do so shortens the life of the battery. Discharged cells and batteries need to be protected from low temperature. A completely discharged cell will freeze at about −9°C16°F. At 50 percent discharge, it freezes at −24°C−11°F. When a cell freezes, the electrolyte expands and can break the jar (case) of the cell.
Hydrometer Working
Specific gravity is measured with a hydrometer like the one shown in Figure 5. The hydrometer is used as follows:
- Squeeze the rubber bulb. Insert the flexible tube into the electrolyte of the cell. Slowly release the rubber bulb, drawing the electrolyte into the hydrometer. When the float in the hydrometer lifts free, remove the flexible tube from the electrolyte and finish releasing the rubber bulb.
- Read the specific gravity from the float at the top surface of the electrolyte.
- Reinsert the flexible tube into the cell and slowly squeeze the bulb to return the electrolyte.
- When finished testing, flush the hydrometer (both inside and out) with clean water.
Figure 5. Hydrometer.
When the specific gravity of a cell cannot be raised by charging to within 0.05 of the manufacturer’s specification, it is a questionable cell. It will probably completely fail in the near future.
Obviously the condition of a sealed lead-acid cell or battery cannot be tested with a hydrometer. The condition of such a cell or battery can be determined by testing the terminal voltage under a specific load. The load and minimum terminal voltage are specified by the manufacturer. Some large sealed lead-acid cells have a built-in feature (colored balls with different specific gravities) to determine the specific gravity of the electrolyte. When the ball with the highest specific gravity (indicated by its color) floats on the surface of the electrolyte, the cell is charged. It is in a usable condition. When only the ball with the lowest specific gravity surfaces, the cell needs to be charged.
Lead-Acid Cell Safety
If not properly handled, lead-acid cells and batteries can be dangerous. The acid used in the electrolyte can cause skin burns and burn holes in clothing. It is extremely harmful to the eyes. Always wear safety glasses when working with lead-acid cells and batteries. If acid does come in contact with the skin or clothing, immediately flush with clean water. Then wash with soap and water, except for the eyes. If the acid is in the eyes, get medical attention immediately after repeated flushing with water. Wash hands in soap and water after handling batteries.
The gases released from charging batteries are explosive. Charge batteries in a ventilated area where there are no sparks or open flames. One final point: Although a 12.6-V lead-acid battery cannot deliver an electric shock, it can cause severe burns when shorted by jewellery such as rings, necklaces, and watches.
Lead-Acid Cell FAQs
- Describe the chemical reaction that occurs in a lead-acid cell as it is discharged.
- Describe how a lead-acid battery is recharged.
- List two precautions to follow to prevent battery damage when charging a battery.
- Define specific gravity.
- A(n) _____ is used to measure specific gravity.
- In cold weather a(n)_____lead-acid battery can freeze.
- The acid in a lead-acid cell is _____.
- List the safety rules that apply to handling and charging lead-acid batteries.
Answers
- Lead and lead peroxide are changed to lead sulfate while sulfuric acid is changed to water. This process leaves one plate deficient in electrons and the other plate with an excess of electrons.
- The battery is recharged by using the battery as a load rather than as a source. Electrons are forced into the negative terminal and out of the positive terminal. The chemical reaction is also reversed.
- Charge at, or below, the rate recommended by the manufacturer, and do not overcharge the battery.
- Specific gravity is the ratio of the weight of a substance to the weight of water.
- Hydrometer
- Discharged
- sulfuric acid
- Wear safety glasses, avoid spilling the electrolyte, and charge in a spark-free, ventilated area.
