Influence of charging voltage on capacity reduction of lead-acid batteries

Introduction. Energy storage devices are widely used in power supply for regulation and load balancing of consumers, in alternative power engineering [1, 3], in industrial, and agricultural power units [2, 5]. Lead-acid batteries (LAB) are the most widespread and widely used . The lead-acid battery, having been invented back in 1859, is still the most common type of accumulator batteries due to its low cost, ease of manufacturing, and a well-established method of utilization [4].

The energy performance of lead-acid batteries, and their evaluation, depends largely on their charge-discharge mode and application. For example, renewable energy storage requires partial charge-discharge of the battery. In automotive starting and power consumption systems, the battery mainly operates in floating charge and shallow discharge modes. It is well known that different charge-discharge modes affect battery wear and shorten the battery life [6].

According to State Standard GOST R 53165-2008, the voltage of fully charged batteries with a nominal voltage of 12 volts, after not more than 24 hours of standstill at a temperature of 25°C, at open circuit for sealed lead-acid batteries should not be less than 12.8 volts. Since such a lead-acid battery contains 6 accumulators, the open circuit voltage of a single accumulator shall not be less than 2.13 volts.

A charging voltage of 2.3 to 2.4 volts is recommended for charging a battery with a nominal voltage of 2 volts. For a deeply discharged battery it is acceptable to charge with a higher charging voltage of 2.5 to 2.7 volts. Charging with a higher voltage is accompanied by active electrolysis, which leads to crumbling of the active mass of the plates and battery failure [6].

Despite the large number of studies on different charging modes, the effect of charging voltage on battery performance needs further investigation. This paper is devoted to the study and analysis of the effect of reduced charging voltage (below the recommended charging voltage of 2.3-2.4 volts per battery, but exceeding the battery open circuit potential of 2.13 volts, according to State Standard GOST R 53165-2008), on the characteristics of batteries and their service life.

The purpose of the research is to evaluate the effect of charging voltage on capacity reduction of lead-acid batteries.

Materials and methods of research. Six sealed lead-acid batteries manufactured by the AGM technology with a nominal capacity of 5 amp-hours (Ah) and a nominal voltage of 6 volts were used in this work. Each battery consists of 3 batteries with a nominal voltage of 2 volts. The batteries under study were divided into 3 groups, with 2 batteries per group.

Based on the requirements of State Standard GOST R 53165-2008, the voltage of a fully charged battery with a nominal voltage of 6 volts, after not more than 24 hours of standstill at a temperature of 25°C, at open circuit for sealed lead-acid batteries should not be less than 6.4 volts. To evaluate the effect of reduced charging voltage on the performance of lead-acid batteries, the charging voltage of batteries with a nominal voltage of 6 volts should exceed 6.4 volts but be less than 6.9-7.2 volts [6].

The studies were conducted at these values of charging voltage: for the first group of batteries, 6.5 volts (101.5%); for the second group, 6.75 volts (105.5%); and for the third group,

7 volts (109.5%).

To charge the batteries, the device "Kulon - 715d" was used. The initial charging current equal to 0.5 amp was selected based on the recommendation of the battery manufacturer. In the process of charging the batteries with a current equal to 0.5 amp, when reaching the specified charging voltage at the battery terminals (respectively for the groups: 6.5, 6.75 and 7.0 volts), the mode of charging at constant current automatically changed to the mode of charging at a constant (specified for this group) charging voltage. Charging was stopped in 10 minutes after the current decreased to zero, according to the device display.

To determine the capacity reduction of the batteries, they were discharged using the electronic load module "WEL3005" . Before starting the tests, all 6 batteries were fully charged at the specified charging voltage for each group. In the first cycle of tests, the batteries were discharged with a current of 0.25 amp until the voltage dropped to 5.25 volts at the battery terminals and their actual capacity was measured in amp-hours, according to State Standard GOST R 53165-2008. In the next eight cycles, the batteries were charged to the specified group charge voltages, and then discharged with a current of 0.25 amp to a voltage drop to 6 volts, without measuring the capacity of the batteries. In cycle 10, after charging, the batteries were discharged until the voltage dropped to 5.25 volts, i.e., the tests were performed similarly to the tests in the first cycle.

Results and their discussion. According to the results of the tests in the 1st and 10th charge-discharge cycles of the batteries, in accordance with the measured actual capacities of the batteries, Table 1 was compiled for each of the three groups of batteries, and the average capacities (Ah) were calculated, at charging voltages of: 6.5 volts (101.5%); 6.75 volts (105.5%) and 7 volts (109.5%), respectively.

Table 1 – Decrease of battery capacity after ten charge-discharge cycles and charging voltages lower than recommended by the batteries instructions

Cycle No.	Capacity (Ah) of the battery when charged at 6.5 volts (101.5% of open circuit voltage)		Capacity (Ah) of the battery when charged at 6.75 volts (105.5% of open circuit voltage)		Capacity (Ah) of the battery when charged with 7 volts (109.5% of open circuit voltage)
	LAB No.1	LAB No.2	LAB No.3	LAB No.4	LAB No.5	LAB No.6
1	4.93	4.95	4.93	4.96	4.92	4.94
10	4.33	4.37	4.44	4.48	4.61	4.64
Cycle No.	Average capacity of the batteries No.1, 2 (Ah)		Average capacity of the batteries No.3, 4 (Ah)		Average capacity of the batteries No.5, 6 (Ah)
1	4.94		4.945		4.93
10	4.35		4.46		4.625

According to the average values of the battery capacity, we determined the reduction of the battery capacity:

△ С=С1 - С10;(1)

and degradation of battery capacity, in %:

D = C1 - C10 ,100%.

E

Where C 1 , C 10 are the average values of the batteries capacity determined, respectively, in the 1st and 10th charge-discharge cycles of the batteries.

The results of calculations: capacity reduction Δ С (according to formula 1) and degradation of battery capacity D e (according to formula 2), depending on the value of reduced charging voltage, relative to the recommended by the batteries operating instructions are presented in Table 2.

Table 2 – Characteristics of batteries after ten charge-discharge cycles and charging voltages lower than recommended by the batteries operating instructions

LAB metrics	Average reduction of batteries capacity Ah, when charged at 6.5 volts (101.5%)	Average reduction of batteries capacity Ah, when charged at 6.75 volts (105.5%)	Average reduction of batteries capacity Ah, when charged at 7 volts (109,5%)
ΔС, Ah	0.59	0.485	0.305
D E , %	11.94	9.81	6.19

According to the measured and calculated average values of the batteries capacity in the 1st and 10th cycles (table 1), graphs of the change in the batteries capacity were plotted (Figure 1).

—•— Charge up to 7 volts

Charge up to 6.5 volts

—Й—Charge up to 6.75 volts

Number of charge-discharge cycles

Figure 1 – Variation of battery capacity (with nominal capacity of 5 Ah), at different charging voltages

Conclusions. Based on the results obtained, it can be concluded that at low charging voltage, the capacity of rechargeable batteries decreases rapidly. When the charging voltage increases, on the contrary, this trend decreases. It can be assumed that the decrease in battery capacity at low voltage charge is due to plate sulfation.

At the same time, it can be assumed that when the charging voltage is increased, electrolysis is more active, which contributes to the mixing of the electrolyte, and the gas bubbles formed as a result of electrolysis destroy the sulfate particles and separate them from the battery plates, which reduces the intensity of the initial capacity loss of the battery.

To improve the performance of the battery, it is necessary to charge the battery at a charging voltage higher than the fully charged battery voltage recommended by State Standard GOST R 53165-2008. This minimizes the reduction of the battery capacity and increases the number of charge-discharge cycles allowed.