 # Accelerated test on cycle life of lead-acid batteries

The cycle life of lead-acid batteries is usually tested by the method of charge and discharge cycles. The test takes a long time, often several months. Mr. Ma Shaohua proposed the following technical test methods, which can greatly shorten the test time.

1. The principle of accelerated life test

The life of the product is directly related to the amount of stress applied. The greater the stress, the shorter the life of the product. For a product with a long life, it takes a long time to measure its life by normal methods, which is time-consuming and labor-intensive. For such products, the accelerated life method should be used to measure its life. The principle of accelerated life is to accelerate the failure of the product by increasing the stress and calculate the life of the product under normal use conditions without changing the failure mechanism of the product. The types of applied stress are divided into constant stress, step stress and sequential stress.

Among them, the constant stress accelerated life test theory is the most mature and widely used. In accelerated life testing, current, voltage, power, temperature, etc. can be used as acceleration variables.
If the current is used as the acceleration volume, the life and the current passing through satisfy the inverse power law relationship, namely (1):

In the formula, I is the charging current; t is the life of the lead-acid battery; K1 and C are constants.
Taking the logarithm of both sides of the above formula (1), we can get (2):

It can be seen from the above formula that the logarithm of the product life t has a linear relationship with the logarithm of the current I passed through. As shown in Figure 3, if four stress levels of I1, I2, I3, and I4 are selected for the test, the life spans of the battery are measured as t1, t2, t3, t4, (I1, t), (I2, t2), The coordinates of (I3, t3) and (I4, t4) are A, B, C, and D in sequence. In the bilateral logarithmic coordinate system, if the four points A, B, C, and D can be fitted into a straight line. Then, the current value I0 of the product under normal use is substituted into the linear relationship determined above, and the lifespan to of the product under normal use can be calculated, as shown in Figure 3.

1. Experimental method

This test uses a 6-DZM-12 battery produced by a certain manufacturer, that is, the 2h rate current I2=C2/2=6A.

(1) Selection of acceleration variables Because most electric bicycle lead-acid batteries fail during the charging process, the specific reasons are mainly due to excessive water loss and grid corrosion. When the battery terminal voltage reaches a certain value, the water inside the battery will decompose, the positive electrode will release oxygen, and the negative electrode will release hydrogen. Although the positive electrode of the battery is coated with PbO2, the electrolyte will still pass through PbO2 and react with the Pb of the lower panel grid, and oxidize the metal Pb to PbO2 during charging. The increase of the charging current leads to a rapid increase in the terminal voltage of the battery, which increases the rate of water decomposition, and in conjunction with the oxygen cycle process, the corrosion of the positive plate is accelerated, thereby accelerating the failure rate of the battery. Therefore, in this test, the charging current is selected as the acceleration variable, and the battery life is tested by using the constant stress as the accelerated life test. (Want to learn more about the inverse of battery life with us? go now.)

(2) Determination of stress level Under the premise of not changing the failure mechanism, the range of charging current is 2.25I(A)~5.3I2(A), and the level of charging current is determined according to the following relationship (4):

In the formula, k is the number of stress levels of the acceleration variable. Generally, k is not less than 3, preferably K≥4. In this test, k=4 is selected. According to the above formula, the stress level of charging current is determined as shown in Table 5.

(3) Determination of other parameters This test is carried out in the environment of (25±2)℃, hereinafter referred to as room temperature. Take the same number of test samples under each stress level, that is, n1=n2=n3=n4=8, divide the 8 batteries into two groups, and each group has 4 connected in series. The discharge current is determined according to the actual 48V electric bicycle riding speed, the corresponding discharge current and the battery surface temperature rise. Under the condition of a normal load of 80kg and a smooth road, when the speed is 20km/h, the discharge current is 8.9A, and the temperature rise of the battery surface is 3.5℃. This speed is the riding speed of most users and the temperature of the battery surface is not high. For convenience, choose 9A as the discharge current, which is expressed as 1.512 (A) with the 2h rate. Because the undervoltage value of this type of lead-acid battery is 10.5V. Therefore, within the specified discharge time, when the voltage value of any two of a group of batteries drops to 10.5V for 3 consecutive times and the total voltage value drops to 42V, the test is terminated.

(4) Test process The 80% DOD accelerated life test process of the 6-DZM-12 lead-acid battery is as follows.
① Preparation stage. After connecting the battery with the matching charger, charge the battery. The charging method is operated according to the instruction manual of the charger. After charging, discharge the battery with a constant current of 1.512 (A), and stop when the discharge reaches the undervoltage value of 10.5V, as one cycle. After discharging, let it stand still until the surface temperature of the battery is close to room temperature, and then recharge it. Repeat this 3 times to test whether the consistency of this group of batteries meets the requirements. If it meets the requirements, the 3 cycles are included in the calculation of the total number of cycles of the battery. If it does not meet the requirements, replace the test sample.

② charging stage. Connect the test circuit on the test bench as shown in Figure 6, record the surface temperature of the battery with a data recorder, push the test bench into the simulated experimental box, adjust the temperature of the experimental box to (25±2) °C, and the relative humidity to 50%, charge the battery with a current of 2.2512 (A), the charging time is 53.5min, and the fluctuation of the charging current cannot exceed ±1% of the specified value. After charging, the battery is cooled until its surface temperature is close to room temperature and then discharged. Figure 6 – Connection diagram for charging test, Figure 7 – Connection diagram for discharge test

③ discharge stage. Connect the test circuit according to the discharge connection diagram, as shown in Figure 7. Use a data logger to record the surface temperature of the battery, discharge the battery with a constant 1.5I2 (A) current for 64 minutes, and the fluctuation of the discharge current should not exceed ±1% of the specified value. After discharge, the battery is cooled to a surface temperature close to room temperature and then charged for the next cycle. During the discharge process, observe the voltage indications in real time. Within 64 minutes of discharge, when any two voltage indications are lower than 10.5V for 3 consecutive times and the total voltage value of a group of batteries drops to 42V, the cycle life of this group of batteries is considered to be terminated. The 3 charging times are not included in the total charging time.

According to the above process, alternate with the first group to do the second group of tests, record the total number of cycles, and then take the average value of the total charging time of the two groups of failed batteries for the total charging time under constant current charging. Then adjust the current of the constant current source to 3A, 4A, and 5.3A, respectively, and the charging time is 40min, 30min, and 22.5min, respectively. The data obtained are compared with the data of 2A charging, and other conditions remain unchanged, and the above test process is completed.

1. Test results and analysis

According to the above test plan, the test results are shown in Table 8. According to the experimental results in the table, the charging current and life can be obtained! The relationship of , using the least squares method to fit a curve, the fitted curve relationship is:
Igt=-1.50321gl +5.0909