Battery maintenance and repair (I)

Large-capacity lead-acid batteries (hereinafter referred to as "batteries") are the guarantee of base station power supply. When there is a "power shortage" in China, the reliability of backup power supply is particularly important. In the Yangtze River Delta and Pearl River Delta regions, there are many times of stopping three and supplying four every week, and even more serious situations such as stopping four and supplying three. Most base stations in the wild are difficult to guarantee power supply and use Class I and Class II power supplies. In this way, the reliability problem of batteries is particularly serious.

At present, science and technology are developing rapidly. In recent years, the development of lead-acid batteries is also relatively fast. Large valve-controlled sealed lead-acid batteries have basically replaced acid-proof and explosion-proof batteries. Even large valve-controlled sealed lead-acid batteries have been developing in recent years. However, lead-acid batteries are still the only choice for large-capacity fixed batteries. How to extend the normal service life of lead-acid batteries has always been the main issue discussed by industry insiders.

The same battery has a very different service life under different equipment conditions, different usage conditions and different maintenance conditions. This requires finding the differences in equipment conditions, usage conditions and maintenance conditions. The main phenomena of battery failure are:

a. Softening of positive plate;

b. Corrosion of positive plate grid;

c. Sulfidation of negative plate;

d. Loss of water;

e. A small number of batteries experienced thermal runaway (including battery swelling).

Next, we will use battery failure modes to explore the impact of equipment conditions, usage conditions, and maintenance conditions on battery failure and how to deal with it.

1. Battery failure modes and causes

1. The positive plate of the battery softens

The positive plate of the battery is composed of a grid and an active material, in which the effective component of the active material is lead oxide. When discharging, lead oxide is converted into lead sulfate, and when charging, lead sulfate is converted into lead oxide. Lead oxide is composed of α-lead oxide and β-lead oxide. Among the two types of lead oxide, α-lead oxide has a small charge capacity but a large volume, is harder than β-lead oxide, and mainly plays a supporting role; β-lead oxide is just the opposite, with a large charge capacity but a small volume, is softer than α-lead oxide, and mainly plays a charging role. α-lead oxide is generated in an alkaline environment. Once it participates in discharge inside the battery, charging can only produce β-lead oxide. The active material of the positive plate is a porous structure. In terms of the contact area with the electrolyte-sulfuric acid, the porous structure is dozens of times that of the plane. If α-lead oxide participates in discharge, it can only generate β-lead oxide after recharging, which loses support. Not only will the active material of the positive plate fall off, but the falling active material will also block the micropores of the positive plate, resulting in a decrease in the actual area of ​​the positive plate participating in the reaction, resulting in a decrease in battery capacity. The battery life requirements for backup power supplies are relatively strict, and the battery capacity requirements are relatively wide. Therefore, the ratio of α-lead oxide and β-lead oxide in the backup power supply is larger than that of deep-cycle power batteries. In order to reduce the participation of α-lead oxide in discharge, the depth of discharge is generally controlled to be only 40%. As the battery is used for a longer time, the capacity of the battery decreases. A new battery discharges 40% of the power, which is bound to exceed 40% for an old battery. Therefore, the old battery is equivalent to a deep discharge depth, and the softening of the positive plate of the battery will also be accelerated. Therefore, the rate of decline in the later stage of the battery capacity life curve is much higher than that in the middle stage. The smaller the battery capacity, the deeper the discharge depth, the more α-lead oxide is lost, and the more serious the softening of the positive plate is, resulting in a faster decline in battery capacity, forming a vicious circle.

Therefore, the depth of battery discharge needs to be strictly controlled. This control is achieved by the power management system settings of the base station. Currently, the main criteria for controlling the depth of battery discharge are still the discharge amount and discharge voltage. In this way, forced discharge in emergency situations should be avoided as much as possible, and the battery capacity should be increased according to the discharge amount.

2. Corrosion of the positive plate of the battery

The lead in the grid of the positive plate may be oxidized to lead oxide during the charging process, and cannot be reduced to lead, resulting in corrosion of the positive plate. The volume of lead oxide is larger than that of lead, forming a linear volume increase deformation, causing the positive plate active material to separate from the grid, resulting in the failure of the positive plate. Overcharging will seriously accelerate the corrosion of the positive plate. We generally think that there will be no overcharge state. In fact, if the floating charge voltage of the base station cannot keep up with the increase in ambient temperature and compensates for the decrease, overcharging will occur. If the air conditioning of the base station is insufficient or damaged, overcharging of the battery will also occur. In this way, the positive plate grid of the battery will have different corrosion rates under different conditions of use. The corrosion of the positive plate in the Yangtze River Delta and Pearl River Delta regions will also be more serious than in the mainland, which is closely related to the ambient temperature of the battery.

3. Sulfation of the negative plate of the battery

After the battery is discharged, the lead on the negative plate is converted into lead sulfate. If it is not charged in time or the charging time is relatively long, these lead sulfate crystals will gradually accumulate and form coarse lead sulfate crystals, which cannot be restored by ordinary charging methods. Therefore, it is called irreversible lead sulfate saltization, or sulfidation for short.

Under floating charge with a cell voltage of 2.25V, it takes a week to fully charge the battery, and 28 days to fully charge it. During this period, the battery is in an undercharged state. Coarse lead sulfate crystals can be found 12 hours after the battery is discharged. In areas with power shortages, battery sulfidation is quite serious.

When used in a normal floating charge state, as the ambient temperature changes during the day and night, lead sulfate crystals will accumulate to form coarse lead sulfate crystals and cause sulfidation.

When the ambient temperature is relatively low in winter, the floating charge voltage of the battery should be increased accordingly. If the floating charge equipment does not rise accordingly according to the room temperature, the battery will be under-charged and battery sulfation will occur.

A battery that loses water is equivalent to an increase in the sulfuric acid concentration in the electrolyte, which also creates conditions for accelerating the sulfidation of the battery.

Faster charging can inhibit battery sulfation. The charging current of base stations is relatively small, so the degree of sulfation is more serious than that of batteries with large charging current. In addition, the smaller the floating charge voltage fluctuation, the smaller the disturbance of the floating charge current, which also forms the conditions for battery sulfation.

Batteries using low antimony alloy positive plates have a relatively low float charge voltage and are more prone to sulfation than other lead-calcium-tin-aluminum alloy batteries.

From the above reasons for sulfation failure, it can be seen that many batteries cannot be avoided. Especially when the battery pack has a single cell lagging behind, some of the lagging single cells are in an undercharged state, so the battery is more susceptible to sulfation than other batteries.

Once the battery is sulfided, it cannot be solved by simple floating charge and equalizing charge, and other measures must be taken. At present, our company's technology is mainly to eliminate the sulfidation of the battery, restore the original nominal capacity, and put it into use again.