The influences of acid soaking time on the performances of VRLA power batteries

Abstract: In this paper, the performances of deep cycled power valve-regulated lead-acid batteries under different soaking time were investigated. The results showed that when the soaking time was 0.5 h at most, and the temperature was 35℃ ± 5℃, the batteries exhibited good cycle characteristic.

Keywords: deep cycle; valve-regulated lead acid battery; soaking time, formation temperature; cycle life; power battery

As key processes in the manufacturing process of lead-acid batteries, pasting, curing and battery/plate formation have received wide attention from researchers in the industry [1]. Jar formation battery after adding acid until the battery begins to charge this period is called the acid soaking time. During this period, the alkaline lead paste reacts violently with sulfuric acid and releases a large amount of heat, making the internal temperature of the battery rise sharply, and at the same time, the phase composition and pore structure of the solidified lead paste change . This process has a great influence on the setting of subsequent formation process parameters and battery performance .

1 The experiment

1.1 Battery assembly and formation

Positive and negative uncharged plates (plate size 138 mm×66 mm×2.5 mm) and AGM separator were assembled into 12 V20 Ah battery (assembly pressure of about 80 kPa) according to the plate group of 4 positive plates and 5 negative plates, and then proceed acid filling and formation. In the process of acid filling, the amount of sulfuric acid electrolyte (ρ = 1.252 g/cm3) was 11.2 mL /Ah. After finishing vacuum acid filling, the battery is quickly immersed in the cooling water (10 ~ 15 ℃), and the experimental battery is cooled according to the set acid soaking time. The battery formation time is 68h, and the net charging capacity is 8.5C 2. During the process, the battery formation temperature is controlled by a constant temperature tank.

1.2 Battery test

Adopt Battery performance tester , and in accordance with the national standard GB/T 22199-2017 requirements to test the initial performance C2 of the battery . The battery cycle performance was tested by charging the battery at 0.4C for 5 h to 14.8V, and then discharging it at 0.5C to 10.5V.The battery is charged and discharged continuously. When the battery was discharged for three consecutive times less than 96 minutes, the battery life was considered to be terminated.

2 Results and discussions

2.1 The plate test

Firstly, the phase composition of the plate was analyzed qualitatively and quantitatively by XRD. In order to better quantitative analysis of the plate, the samples were fully ground and the grinding time was controlled within 10min. The scanning Angle 2θ was 5° ~ 90°, the step size was 0.002°, and the scanning rate was 2 (°)/min. As can be seen from Table 1, 3BS and PbO are the main components of the substrate phase of uncharged plate.

Table 1 Results of XRD analysis of positive plates%

Then, the positive plate was soaked in 1.252g /cm3 sulfuric acid solution, and the corresponding plate was removed at a certain time interval, and the plate was washed quickly with distilled water, and then the plate was immediately transferred into a vacuum drying oven, and the phase composition of the plate was measured after drying. As can be seen from Figure 1, as sulfuric acid contacts the plate, PbO (α-PBO, β-PBO) and 3BS (3PbO•PbSO4•H2O) rapidly undergo acid-base neutralization reaction with sulfuric acid, generating a large amount of 1BS (PbO•PbSO4), and the contents of PbO and 3BS gradually decrease with the acid soaking time extended. When the soaking time reached 0.5 h, a large amount of PbSO4 was generated inside the plate and accumulated rapidly. The content of PbSO4 increased gradually and tended to be stable with the extension of soaking time to 6 h. When the soaking time exceeded 6 h, the content of PbSO4 began to increase rapidly. In the process of acid soaking , 1BS content showed a trend of decreasing first and then increasing and then decreasing gradually. When the soaking time was less than 2 h, the content of 1BS decreased gradually from 26.5 % (0.1h) to 17.5 % with the extension of the soaking time, and then increased to 34.7 % with the increase of the soaking time to 6 h, and then began to decrease gradually with the extension of the soaking time. This process may be related to the diffusion of sulfuric acid solution in the positive plate. When the acid soaking starts, the lead paste on the surface of the plate reacts with sulfuric acid to generate a large amount of 1BS, which also hinders the diffusion of sulfuric acid into the plate. At this time, a large number of PbO in the plate contacts with 1BS, and further forms 3BS (the soaking time is less than 0.5h). Then, with the continuous diffusion of sulfuric acid, a large amount of PbSO4 is formed in the plate. The content of α-PBO2 and the ratio of ω(α-PBO) /ω(β-PBO) in the plate decreased gradually with the extension of soaking time. This may be because with the extension of acid soaking time, sulfuric acid continues to spread into the plate, making the lead paste inside the plate participate in the reaction, and causing the change of the pH value and phase of the lead paste inside the plate. When the soaking time was 0.5 h, the ratio of ω(α-pbo) /ω(β-pbo) reached 0.327, and when the soaking time was increased to 2 h, the ratio decreased to 0.277. After soaking for 6 h, the ω (α-pBO) /ω (β-pBO) ratio in the positive plate decreased to 0.257, and when the leaching time was extended to 8 h, the ratio increased to 0.280. This change may be related to the increase of PbO phase content in the plate during acid soaking .

Figure 1 XRD analysis results of positive uncharged plates at different acid soaking time

2.2 Battery test

2.2.1 Capacity test

According to the requirements of GB/T 22199-2017, the capacity of the experimental battery at room temperature discharging and at low temperature of -18 ℃ discharging were tested. It can be seen from Figure 2 that the normal temperature capacity and low temperature capacity of the battery can both meet the requirements of the standard. However, with the acid soaking time extended to 8h, the capacity of 6-DZF-20 battery gradually decreased from 22.56Ah to 21.98Ah, resulting in a decrease rate of 2.6%. In addition, Figure 2 shows that too short soaking time (less than 0.5 h) is not conducive to the improvement of low-temperature discharge performance of the battery.

Figure 2 6-DZF-20 battery capacity and low-temperature performance

2.2.2 Cycle life test

FIG. 3 shows the cycle life curve of 6-DZF-20 battery at different formation temperatures. The figure 3 shows that as the battery water bath temperature increased from 20 ℃ to 30 ℃ battery cycle life increased by 278 times to 393 times, this may be because of low formation temperature, plate sulfuric acid salinization degree is low, is not conducive to form stable PbO2 in the late process of formation, make active material thermodynamics stability become low, spontaneous reactivity become high, Battery self-discharge increases [9]. When the temperature of the battery water bath is continuously increased to 45 °C ± 2 °C, the cycle life of the battery decreases to about 360 times. This may be because the excessive battery formation temperature causes the rise of the internal temperature of the battery, and then causes the deterioration of the performance of the battery plate, resulting in the decline of the battery life [10]. In addition, the capacity of the battery at the beginning of the cycle (< 10 cycles) increased with the increase of the cycle times, which may be related to the diffusion of sulfuric acid solution in the plate and the further reaction of lead sulfate, which did not participate in the reaction, to generate PbO2. When the number of cycles exceeds 100, the battery has good capacity retention characteristics.

Figure 3 Battery cycle life curve at different formation temperatures

FIG. 4 shows the cycle life curves of 6-DZF-20 battery under different soaking time when the formation temperature is 35℃±5℃. As can be seen from the figure, the battery cycle life curve also has decline period, plateau period and steep drop zone. The battery cycle life gradually decreased from 394 times to 261 times with the increase of battery soaking time from 0.5 h to 6 h. This may be because the corrosion layer formed by the solidification of lead-calcium-tin alloy grid battery is relatively thin, which is more likely to form lead sulfate during acid soaking, and then affects the corrosion layer structure between grid and active substances in subsequent formation, leading to the attenuation of battery life.

Figure 4 Battery cycle life curve at different acid soaking time

3 Conclusion

XRD analysis results showed that the content of α-PbO2 in the positive plate decreased gradually and the ratio of ω(α-PbO)/ω(β-PBO) decreased gradually with the extension of acid soaking time. In addition, the content of α-PBO2 has a certain relationship with the content of PbO phase in the plate during acid soaking. The results of battery cycle life test show that suitable water bath temperature (30 ~ 40℃) and short acid soaking time (0.5h) are more conducive to the prolongation of battery cycle life.