## 3 Results and Discussion
### Performance Tests of VRLA Batteries  
#### 3.2.1 Discharge Capacity Tests  
As shown in Figure 4, VRLA batteries with different carbon black materials at 2-h rate, low-temperature performance and 1.8 C\({}_{2}\) discharge performance were compared to those of the control batteries with acetylene black as a negative electrode additive.  
Figure 4a shows the 2-h rate capacity test of VRLA batteries. As can be seen in the figure, the discharge times of CB1, CB2, CB3 and the control batteries at room temperature are 126 min, 127 min, 128 min and 125 min, respectively. When CB3 is added as a negative electrode additive, the discharge capacity of the batteries is highest and the discharge time is 128 min, which is 2.4% higher than that of the control batteries. Experimental results show that with the increase of SSA of carbon black, the discharge capacity of the batteries is also increased.  
A battery's low-temperature capacity is related to temperature. If the battery's discharge capacity is lower than the standard value (85 min) at low temperatures, the battery efficiency will decrease under cold conditions, especially in northern China. In this paper, there are two research methods for low-temperature performance: one is charging at 25 \({}^{\circ}\)C and discharging at \(-\)15 \({}^{\circ}\)C, and the other is charging and discharging at \(-\)10 \({}^{\circ}\)C. Figure 4b shows a significant improvement of the discharge time of the batteries containing carbon black at a low temperature of \(-\)15 \({}^{\circ}\)C. The discharge times of CB1, CB2, CB3 and the control batteries are 103 min, 105 min, 110 min and 100 min, respectively. It can be found that the discharge time of the batteries with carbon black has a marginal improvement of 3-10%  
Figure 3: (**a–c**) Scanning electron microscopy (SEM) images, (**e–g**) transmission electron microscopy (TEM) images, (**h–j**) high-resolution TEM (HRTEM) images of CB1, CB2 and CB3, respectively.  
compared to the control batteries [29]. The low temperature \(-10\)\({}^{\circ}\)C performance test is shown in Figure 4c. These data indicate that batteries with carbon black additives have a significant improvement of 2-5% when compared to the control batteries. The results of the low-temperature discharge test at \(-15\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C indicate that carbon black as negative additives have better low-temperature performance and have a longer lifetime in low-temperature environments. In addition, CB3 has better low-temperature performance, which is due to the larger SSA of the CB3 additive, which can better improve the migration rate of ions [30].  
E-bikes have a large current discharge when they start, which has a great impact on the life of the batteries. It is observed from Figure 4d that the discharge time of the batteries with carbon black is slightly better than the control batteries at 1.8 C\({}_{2}\) rate discharge. Compared with the C\({}_{2}\) rate discharge at 25 \({}^{\circ}\)C, the discharge capacity of the large current discharge is significantly decreased. This is probably because the reaction occurs only on the surface of the active material on the negative plate when a large current is discharged. PbSO\({}_{4}\) crystals form an irreversible PbSO\({}_{4}\) layer on the surface of the negative plate, which further impedes the HSO\({}_{4}\)\({}^{-}\) ions into the interior of the plate and reduces the reversible electrochemical reaction, which leads to the premature reduction of the battery's capacity [31].