Lead-acid batteries, used as starting batteries in automotive systems, play a crucial role in providing power for the ignition of the vehicle’s internal combustion engine, as well as for other auxiliary functions during high-speed driving. During the startup process, automotive systems rely on starting lead-acid batteries to supply electricity; during normal operation, the vehicle’s power generation system performs a floating charge on these batteries to replenish their energy. Therefore, the charging acceptance capacity of starting lead-acid batteries is of vital importance for their sustainability, stability, and even their service life. This capacity is determined by factors such as the additives used in the negative electrode plates, the product design, and the process control during battery production. In this article, the author investigated the influence of the mass ratio of lignin to humic acid in the negative electrode additives, the amount of electrolyte used, as well as the production process during the battery formation stage on the charging acceptance performance of these batteries.
1 Experiment
1.1 Preparation of the test batteries
The batteries used in this experiment were 55D23 type SLI lead-acid batteries. Both the positive and negative electrodes were made using thin, mesh-like grids made of aluminum-calcium-tin alloy. The positive electrode was coated with 435 lead paste, whose main components were 4PhO–PbSO₄; the negative electrode was coated with 3BS lead paste, whose main components were 3PhO–PbSO₄. These pastes were produced using high-temperature curing processes. The additional materials used for the positive and negative electrodes included Type A wood fibers, ultra-fine barium sulfate, humic acid, fulvic acid, and high-strength short fibers. The electrode arrangement consisted of 6 positive electrodes paired with 5 negative electrodes; the separators were made of single-sided PE material. The height of the electrodes relative to the distance between them was 955. The batteries were manufactured using the internalization process, and the density of the electrolyte used in this process was 1.285 g/cm³. There were three types of negative electrode plates used in the experiment; the mass ratios of humic acid to lignin in these plates were 1:3 and 1:1.3:1, respectively.
1.2 Test Methods
The VICTOR-9803 digital voltage meter, the MDX-600 internal meter, as well as the BNT 100-20-5ME Dicarlon battery discharge and charge comprehensive tester were used in the tests.
A constant-temperature water bath and a high/low-temperature alternating temperature test chamber were also employed. The charging acceptance capacity of the battery was tested in accordance with Article 5.6 of the “Part 1: Technical Requirements and Test Methods for Starting Batteries for Ships” (CMT 5008.1-2013), while the water loss of the battery was measured according to Article 500 of the same standard.
2 Test Results and Discussions
2.1 Tests on the Addition Amounts of Lignin and Humic Acid as Auxiliary Materials in the Negative Electrode Lead Paste
Both lignin and humic acid are organic expanders. When added to the negative electrode lead paste, they form a polymer electrolyte layer on the surface of the negative electrode, which effectively prevents the sulfation of the negative electrode plate. During charging, the protons (P+) in the electrolyte need to diffuse to the surface of the electrode through both external and internal diffusion processes in order to gain electrons and complete the reduction reaction. This polymer electrolyte layer reduces the diffusion rate of lead ions (Pb2+) from the electrolyte to the electrode surface, thereby decreasing the battery’s charge acceptance capacity.
Lignin and humic acid contain various functional groups such as phenolic and quinone groups. Lignin can improve the battery’s low-temperature capacity and low-temperature discharge performance. Humic acid contains aromatic rings and other functional groups, which can effectively reduce the overpotential for hydrogen evolution at the electrode, thereby minimizing water loss during battery operation. Although both lignin and humic acid can decrease the battery’s charge acceptance capacity, humic acid’s aromatic rings and quinone groups possess a stronger complexing and adsorbing ability for protons, resulting in a more significant negative impact on the battery’s charge acceptance capacity. Therefore, when designing the process for adding organic expanders to the negative electrode, it is necessary to comprehensively consider factors such as the battery’s cold-start performance, power consumption, and charge acceptance capacity.
Based on the assumption that the addition amount of organic expanders was 0.2% (expressed as a percentage of the total mass of the materials), the author adjusted the mass ratio of lignin to humic acid in the negative electrode lead paste in order to minimize the negative impact of these organic expanders on the battery’s charge acceptance capacity. As shown in Table 1, the total amount of organic matter and humic acid in the negative electrode of Battery No. 1 was relatively low. During charging, the battery’s charge acceptance capacity was high, but the water loss during discharge increased. However, increasing the mass ratio of humic acid to lignin in the negative electrode lead paste improved the battery’s charge acceptance capacity, while also reducing water loss. Nevertheless, the battery’s charge acceptance capacity still decreased slightly. Overall, the comprehensive performance of the battery was improved.
Table 1: Battery performance in terms of the mass ratio of the three types of humic acids to lignin
| Battery Number | w1/%① | w2/%② | Charging acceptance | Water loss (g/Ah) |
| 1 | 0.15 | 0.05 | 3.6 | 0.82 |
| 2 | 0.10 | 0.10 | 3.2 | 0.66 |
| 3 | 0.05 | 0.15 | 2.1 | 0.62 |
;Note: ①The mass fraction of lignin in the total material; ②The mass fraction of humic acid in the total material.
2.2 Orthogonal Experiment of Formation Process
During charging of lead-acid batteries, PbSO₄ within the plates is reduced, causing SO₂ to detach from the active material surface. It then diffuses through internal plate pores into the electrolyte to complete the reaction. If pore size distribution within the plates is uneven or porosity is too low, SO₂ cannot be promptly transferred to the electrolyte and instead accumulates on the active material surface. According to reaction kinetics, high concentrations of SO₂⁻ inhibit further SO₂ dissociation, thereby reducing the electrochemical reaction rate. SO₂⁻ diffusion becomes the rate-limiting step of the electrochemical reaction, manifesting as a decrease in the battery's charge acceptance capacity.
In the production process, aside from plate manufacturing, the battery formation process has the greatest impact on the internal structure of the plates. Three primary factors dominate: formation electrolyte density, acid immersion time, and formation temperature.During acid immersion, the porosity of positive plate active material initially increases then decreases (4), while negative plate active material porosity undergoes a gradual decline followed by a rapid decrease.Formation electrolyte density and immersion time are the key determinants of active material porosity. Electrochemical reactions occurring during formation generate Pb and PbO₂. As this is an endothermic reaction, adjusting formation temperature alters Pb and PbO₂ deposition rates, thereby modifying internal plate structure,electrochemical reactions occur to form Pb and PbO₂. Since this is an endothermic reaction, adjusting the temperature during formation can alter the precipitation rates of Pb and PbO₂, thereby modifying the internal pore structure of the plates. The pore structure within the battery plates is established during formation, and subsequent charging and discharging processes find it difficult to alter this structure. Therefore, a three-factor, three-level orthogonal experiment was designed, with the factor levels shown in Table2, with results presented in Table 3. Orthogonal calculations indicate that formation temperature is the most significant influencing factor. Increasing the formation temperature can markedly enhance the battery's charge acceptance capability. However, further elevating the formation temperature intensifies gas evolution reactions during formation and causes corrosion to the grid. The overall optimal process is A2 B3 C3, though this specific combination does not appear in the orthogonal experiment table. Experimental battery No. 10 employed the optimal process: formation electrolyte density of 1.20 g/cm³, acid immersion for 1 hour, and controlled formation temperature at 60°C. Post-preparation testing revealed a charge acceptance capacity of 4.5, with stable performance maintained during mass production.
2.3 Single-Factor Test on Electrolyte Acid Content
Starting lead-acid batteries are predominantly maintenance-free units,typically employing a flooded design with a substantial volume of electrolyte. During battery operation,water content within the electrolyte diminishes ,yet the total molar quantity of sulphuric acid remains constant. Consequently, the author designed the molar ratio of sulphuric acid to active material (referred to as the acid-to-active-material ratio) as a single variable to validate the impact of acid content on the battery's charge acceptance capability, thereby establishing a basis for battery product design. When the acid-to-active-material ratio is maintained between 2.0 and 3.0, the charging acceptance capacity declines gradually. However,when this ratio exceeds 3.0,the charging acceptance capacity decreases rapidly.For maintenance-free flooded batteries,the acid-to-active-material ratio should not exceed 3.0.
Table: Factor levels
Level | Acid ImmersionTime/h-A | Electrolyte density(g/cm3)-B | Converted temperaturetime/°C-C |
| 1 | 0.5 | 1.05 | 40 |
| 2 | 1.0 | 1.10 | 50 |
| 3 | 2.0 | 1.20 | 60 |
Table : Effect of Acid Content on Charging Acceptance
| Battery Number | Acid Content Ratio | Charging Acceptance Capacity |
| 1 | 2.00 | 4.90 |
| 2 | 2.25 | 4.82 |
| 3 | 2.75 | 4.80 |
| 4 | 3.00 | 4.69 |
| 5 | 3.25 | 4.52 |
| 6 | 3.5 | 4.10 |
| 7 | 3.75 | 3.62 |
| 8 | 4.00 | 3.12 |
| 9 | 4.25 | 2.43 |
| 10 | 4.50 | 1.78 |
3. Conclusions
When the total amount of organic expanders added is 0.2% and the water loss properties of the battery are maintained at an acceptable level, setting the mass ratio of lignin to humic acid at 1:1 can enhance the battery’s ability to absorb charge from the organic expanders.The influence of forces is reduced to a minimum level. The optimal process for producing batteries using the positive vacuum snow test method was determined through relevant experiments. The current density during the charging process was 1.20g/em²,and the charging time was 1 hour. It was also observed that when the ratio of the amount of sulfuric acid in the battery electrolyte to the amount of active material ranged between 20 and 4.5, the charging capacity of the battery decreased as the amount of sulfuric acid increased.