Decrystallization with high current pulses technique for capacity restoration of industrial nickel-cadmium battery

Received Feb 19, 2020 Revised Apr 5, 2020 Accepted Apr 20, 2020 The process of crystallization occurred due to the process of charging and discharging during the usage of the Nickel-Cadmium (Ni-Cd) battery where crystalline formed on the surface of the battery plate. This situation causes the impedance of the Ni-Cd battery increased and contribute to the increment of the battery temperature and battery impedance. High battery temperature will cause the performance of Ni-Cd battery deteriorates. Therefore, this study is investigated on the performance of industrial Ni-Cd battery during the process of crystallization and de-crystallization with high current pulses. By this technique, it is capable to break the formed crystalline to recover back the capacity loss and enhanced the performance of Ni-Cd battery. Therefore, the study results shown that the life cycles and capacity of the NiCd battery increased up to 41% of its capacity after the de-crystallization take place by injecting high current pulses. Consequently, the life span of the Nickel-Cadmium battery enhanced, and the battery is revived.


INTRODUCTION
In recent years, high needs in the maintenance of batteries is emerging due to the energy transition from vehicles that used combustion as energy to electrical energy [1][2][3][4][5]. With this the emission of greenhouse gases is reduced as it become major concern issues nowadays [ [6][7][8][9]. Battery is a technology that capable to transform chemical energy to electrical. The energy stored in the battery is used through the process of oxidation-reduction where the movement of the electron produced an electromotive force which also known as voltage. Batteries are generally classified as either primary or secondary. Primary batteries are the type that may only use one time because the active materials are used up when the cell discharges. Once the primary battery is discharged completely, it is discarded. Meanwhile, the secondary batteries may be used repeatedly because the chemical reaction which produces electrical energy can be reversed by recharging the battery. Ni-Cd batteries is one of the secondary batteries that have used in massive applications in different industrial area especially in transportation and UPS due to their high performance and able to recharge rapidly [10]. Moreover, the Ni-Cd battery have high energy density.
In the Ni-Cd cell, nickel oxyhydroxide (NiOOH) is the charged active material in the positive plate. During discharge, the charge nickel hydroxide goes to a lower valence state by accepting electrons from external circuit Ni-Cd battery. Meanwhile, cadmium (Cd) metal is the charge active material in the negative plate. During discharge it is oxidize to cadmium hydroxide and release electron to the external circuit. also use aqueous solution of potassium hydroxide as their electrolyte. These reactions occur in the electrolyte which act as medium of ion transfer only, therefore there is no significant changes that can observe during the charge or discharge cycle.
However, the performance of the Ni-Cd batteries degraded over period after undergo charging and discharging process due to the formation of crystalline [11]. Crystallization is an electrochemical reaction that occurs when the Ni-Cd battery in discharge state that build gradually on the battery cell plates [12][13]. During the discharging process, the Nickel-Hydroxide in state of liquid is transform into solid crystalline and formed on the plate surface of the battery. As the formation of the crystalline is accumulated, the size of the surface area of the plate decrease and the internal impedance of the Ni-Cd battery is higher. Thus, the battery temperature also increased [14]. This condition cause the Ni-Cd battery suffered from capacity loss as the life expectancy to be correspondingly reduced [15][16][17]. The formation of the crystalline also contribute as factor that cause the memory effect in the Ni-Cd battery [18][19].
In order to overcome this issue, the process of de-crystallization is required. There are few researches have been conducted to improve the performance of Ni-Cd battery. One of the ways is using the conventional way where additives such as carbon powder, carbon nanotubes, titanium dioxide, glass fibres, silicon dioxide, aluminium oxide and boric acid [20][21][22] is added. Other than that, the application of low current and high current pulses method to restore the capacity of the batteries is also possible [23,24]. However, these methods have own merits and demerits in terms of the effectiveness of the technique. Therefore, in this study, de-crystallization with technique of high current pulses is chosen as study focus where the injected high current pulses force the electrons at the battery plate move and chemical reaction occurs. To generate high current pulses during de-crystallization, a power converter [25,26] is considered to generate DC pulses as the main power supply from the main grid in AC state.
There are two types of crystalline which are soft crystalline and hard crystalline Normally, most of the soft crystalline able to break down into electrolyte through normal process of discharging. But, softcrystalline that unable to break during the discharging process will form hard-crystalline on the battery plate after period of time. Therefore, high current pulses are required injected to the Ni-Cd battery so that some of the hard crystalline in solid state is able to break down and transform into liquid state of Nickel-Hydroxide. Hence, the aim of this study is to observe the significant parameters of the Ni-Cd battery such as battery group voltage, discharging time and battery capacity during before and after de-crytallization by using technique of high current pulses.
The remainder of the paper is organized as follows. A briefly technical framework and experimental block diagram description is discussed in Section 2. The results and analysis are deliberated in Section 4. Lastly, Section 5 summarizes all the findings.

DESCRIPTION OF DE-CRYSTALLIZTION SYSTEM 2.1. Battery Regenerator
Battery regenerator is used to help the battery undergo capacity restoration (reviving battery) in order to improve the performance of the degraded batteries and extend the battery life. For Ni-Cd battery, decrystallization process is one of the refurbishment techniques to reverse the crystallization of the Ni-Cd battery. De-crystallization is achieved by sending high current pulses that break down the crystalline layer formed by amorphous Nickel-Hydroxide. The battery regenerators restore the lost battery capacity through carefully controlled current pulses. These current pulses sent by the rectifier controller is sufficient to break soft-crystallization and hard-crystallization of Nickel-Hydroxide. By this, the capacity of the regenerated battery is enhanced, and the life cycles of the battery extend. Besides, the process of the de-crystallization also can achieve by sending low frequency of electrical pulses. This result the capacity of the regenerated battery and the life span of the battery is increased.

High Current Pulses
During the process of de-crystallization, current pulses with high-powered is applied to the Ni-Cd battery to break the formed crystalline according to the feedback of the battery. High battery voltage indicates high internal impedance which indicate the crystalline formation on the surface of the plate is high. Therefore, high current pulses in range of 100 A to 300 A is required. However, if the voltage detection is low, the working principle that apply is vice versa. The block diagram on the working principle of high current pulses in de-crystalline the Ni-Cd battery is shown in following Figure 1.  Figure 1: Block diagram on working principle of the revived Ni-Cd battery system.
From Figure 1, the buck-boost transformer is supplied with 3-phase AC power supply to powered up the thyristor. As the battery is connected to the system, the related data such as battery cell temperature, voltage across the reference cell, transformer temperature, battery voltage and battery current is send to the controller of the system. Then, the thyristor controller analyze the signals feedback from the controller to regulate the suitable current pulses to break the crystalline. The thyristor generate current pulses based on the suitable fire switching scheme. This process is repeated until the referenced battery cell is achieved it desired voltage. In the battery technology, temperature is one of the important parameter to observe in order to preserve the performance of the regenerated battery [27][28]. Therefore, the regenerator machine must capable to detect the cell temperature of reference battery. If the temperature is reached the reference temperature, the used machine will increase the period of the pulsation in order to allow the temperature of the reference battery below than the reference temperature.

Experimental Setup
The flowchart of the experiment process on the reviving the Ni-Cd batteries by restoring charge capacity of the battery is shown in Figure 2. Meanwhile, the experimental setup of the research is shown in Figure 3. The selected Ni-Cd battery bank have been used in railway transportation as an emergency back-up source for 5 years and the specifications of the battery is given in Table 1. The specifications used during the Ni-Cd battery regeneration is stated as in Table 2.    When the Ni-Cd batteries is received, proper inspections are performed on the battery by filling the service form, battery cleaning and repair any visible external damage. Then, the received Ni-Cd batteries will undergo complete charging process by connecting it to the charger machine with set parameters. After the charging process is complete, the Ni-Cd batteries undergo for discharging process by using discharger machine with set parameters. During the discharging process, the Ni-Cd batteries undergo deep discharge where the discharge voltage is set to 0.6 V to clear the memory effect [10]. After completing the discharging process, the batteries were ready to enter regeneration process. The process is repeated until the desired battery voltage which is above 0.8 V is achieved.

RESULTS AND ANALYSIS
In this section, results analysis on de-crystallization of Ni-Cd battery is discussed. Before the decrystallization process is start, capacity test is performed on the Ni-Cd battery with 80% of depth of discharge to identify the initial performance of the Ni-Cd batteries. The observed parameters are battery group voltage, discharged time and capacity of the battery. The graph of discharged battery group voltage before and after de-crystallization is shown in Figure 4. From the graph that presented in the Figure 4, it is observed that the trend of the Ni-Cd battery total voltage is decrease against time. Before the process of decrystallization, most of the Ni-Cd battery voltage is on the average of 1.3 V and the total Ni-Cd battery voltage is 26.95 V. After an hour of discharged before decryatallization, the battery group voltage dropped 10.9% from the initial battery group voltage and the time taken for the Ni-Cd battery bank complete discharged is 95 minutes. This happened due to the accumulation of crystalline at the surface of the Ni-Cd battery electrode which result the internal resistance of the Ni-Cd battery is high and create memory effect. The high internal resistance in the Ni-Cd battery cause the battery unable to deliver the current to the load effectively. Moreover, high internal resistance in the Ni-Cd battery leads to increment of temperature and voltage to drop. Therefore, the Ni-Cd battery is required to undergo the de-crystalline process to break the crystalline formation on the battery plate and deep discharged to clear the memory effect.
After the process of de-crystallization with high current pulses technique, the battery voltage dropped 7.2% from the initial battery group voltage after an hour and the total discharged time is 300 minutes. From this results, the difference percentage of the battery group voltage at 4.5% between before decrystallization and after de-crystalline after an hour is observed. Moreover, the discharged time after the Ni-Cd battery bank undergo de-crystallization is extended for 205 minutes. This indicated that the technique of high current pulses is able to enhance the capacity of the Ni-Cd battery and thus the life cycles of the Ni-Cd battery is improved. This is because the crystalline that formed on the surface of the electrode is broke by the injected high current pulses which up to 150 A into the Ni-Cd battery. Hence, the discharge capacity of the Ni-Cd battery is increased from 22 AH to 58 AH. The performance parameter of the Ni-Cd batteries before and after regeneration is tabulated as in Table 3.  Table 3, it is observed that the time of discharge is enhanced from 95 minutes to 255 minutes and capacity percentage of the Ni-Cd battery is increased for 41%. Besides, the total discharge capacity of the Ni-Cd battery is improved from 22 AH to 58 AH after the decrystallization. With this, it is proved that the technique of high current pulses in restoring the Ni-Cd battery capacity is reliable.

CONCLUSION
Based on the results discussed, the de-crystallization of Ni-Cd batteries with high current pulses managed enhanced the performance of the Ni-Cd battery as the discharge capacity is increased for 41% and the life cycles of the Ni-Cd battery is extended as the discharging time is improved 160 minutes.
At first, the period of the Ni-Cd battery usage is short due to the formation of crystalline that have accumulated on the surface of the battery plate during the process of charging and discharging process. Due to this crystalline formation, the internal resistance of the Ni-Cd battery is high as the discharging time of the battery shorten and thus the performance of the battery declined.
However, with high current pulses the formation of the crystalline is capable to break. Consequently, the parameters of the Ni-Cd battery such as battery capacity, battery cell voltage and life cycle of the Ni-Cd battery is improved and the Ni-Cd battery is revived. Last but not least, the used technique is environmental friendly as it use electrical energy during the process of decrystallization.