Most battery manufacturers prefer not to discuss or talk about the pos- sible failure modes of battery failure. Rather they are interested in talking about the length and the dependable life of the batteries. However, it remains to be a fact that batteries do fail.
In VRLA lead-acid batteries, grid corrosion is the leading cause of battery failure. It is the corrosion of the positive grid that limits the life expectancy of the battery. As the battery cycles and ages, the grid grad- ually begins to deteriorate losing its conductivity. In addition, the adhe- sion of the active paste materials also decreases. The material lost from the plates, lead/sulphur salt falls to the bottom of the cell container and gradually build up. The build up eventually grows to form a conductive path leading to shorting off the battery. This leads to reduction of the cell capacity. Analogous to deposits in a water pipe that restricts the flow of water through a pipe, corrosion on the plate restricts the flow of current in the battery.
Another common failure of VRLA batteries is grid growth. During the life of the battery, the plates tend to expand dimensionally. This change in size exerts pressure on the casing, vents, and the terminal posts. Since the weakest link in the battery assembly remains to be the terminal posts, the expansion forces them upwards to crack the casing-to-cover seal thus leading to premature battery failure.
As the battery ages, the grid deteriorates inwardly and loses its ability to support the current. The corrosion is analogous to a 4 American Wire Gauge (AWG) wire being reduced down to 12 AWG wire. Eventually, the electrical path to the terminal post is lost. In order to prevent pre- mature battery failure owing to grid corrosion, the traction battery manufacturers must provide grid thickness and cycle life data for EV applications. The thicker the battery grid is, the longer the usable life of EFFECTS OF CURRENT DENSITY ON BATTERY FORMATION 31
the battery is. Furthermore, the thicker grid provides better mechanical support for the active material and reduces the porosity of the plate material.
Thicker grids for VRLA batteries are manufactured using bottom-pour casting resulting in low porosity that dramatically reduces the rate of grid corrosion. However, users of VRLA batteries have witnessed prema- ture battery failures owing to the defects formed during production, application environment designs, charging control mechanisms, and manufacturing techniques.
After years of development and perfecting design attributes, manu- facturers have found one area of VRLA battery design that they have not been able to control—the self-discharge tendency of the negative plates of the battery. In addition, the capacity performance of the battery tends to “fall off” prematurely. This “fall off” in battery capacity happens in ideal float operating conditions because of discharged negative plates.
Another common failure is due to discharge of the negative plate over a period of time. This occurs owing to the high recombination efficiency of the battery. As the battery continues to age, the recombination effi- ciency continues to improve (greater that 98% in some cases). The oxygen released at the positive plate, during the charging process, is absorbed by the negative plate. This reduces the spongy lead active material and prevents the negative plates from reaching the fully charged state.
The failure modes due to battery operating conditions and their effects are illustrated in Figure 2–3.
In some studies, it was believed that as long as enough negative active material existed, the negative plates could remain in a slightly dis- charged state without substantial loss of battery performance. Water loss in the VRLA battery is mainly attributed to the loss of hydrogen that is given off by the corrosion process at the positive grid. The water loss occurs when some of the oxygen created at the positive plate does not diffuse into the negative plate. Instead the oxygen is depleted by the corrosion of the positive grid.
The resulting depolarization or loss of charge of the negative plate allows self-discharge. Self-discharge is a chemical reaction that leads to the discharge of the negative plate. The reaction removes the sulphate and lowers the electrolyte specific gravity. This lowering in the specific gravity in turn leads to lowering the open circuit voltage (OCV).
In some cases a boost charge may be applied to the discharged neg- ative plate. The boost charge is also referred to as an equalization charge for battery packs. Typically, this boost charge requires to be closely mon- 32 ELECTRIC VEHICLE BATTERY EFFICIENCY
itored and varies from 2.45 V/cell to 2.5 V/cell. However, this equaliza- tion charge results in gassing and generates heat for a short time.
In VRLA batteries, the oxygen given off is at the top of the charge cycle. The oxygen off the positive plates causes heating and gassing of the VRLA battery. Since all the oxygen off the positive plates is either recombined at the negative plates or used to corrode the positive grid.
The hydrogen is either recombined at the negative plates or used to corrode the positive grid. The hydrogen that is given off as a by-product of the positive plate corrosion is vented out of the cell in periodic inter- vals. In addition, hydrogen is also vented out due to battery self- discharge at the negative plates. The combined loss of hydrogen is the main reason for the battery water loss.
The new design of VRLA battery includes a catalyst in the cell. The catalyst recombines the oxygen and the hydrogen to recover water lost during electrolysis. In VRLA battery cells, the catalyst attracts and recom- bines some of the oxygen that would normally be diffusing to the neg- ative plates. This oxygen recombines with the hydrogen present in the headspace of the battery.
As a result, the negative plate is slightly polarized and obtains a full charge. A slight amount of hydrogen evolved during the process, recom- bines at the catalyst surface. Here are some of the benefits of the catalyst:
EFFECTS OF CURRENT DENSITY ON BATTERY FORMATION 33
Figure 2–3 Battery failure modes due to operating conditions.
Significant Charging and
Discharging Depth Significant Vibration
High Temperature
Deterioration of +ve Plate
Low Temperature Grid Expansion
Early Separation of +ve Plate Active Material
Separation of +ve Plate
Active Material Deterioration of +ve Plate
Active Material
Consistent performance throughout the battery life The polariza- tion on the negative plates is maintained at full state of charge.
As a result, the battery capacity performance does not fall off prema- turely.
Longer battery life The polarization on the positive plates is lowered, reducing the battery corrosion rate at the positive grid.
Reduction in float current Since the plates are polarized, a lower float current is required to maintain full charge on the battery cells.
Reduction in the battery thermal runaway potential Since there is a lower current through the cell, there is less internal heating and thus a lower thermal runaway potential.
Reduction in the battery impedance Since the battery functions at an improved state of charge, the cell’s impedance is lowered.
Reduction in the battery water loss Less venting causes less water loss from the battery.
Under high temperature conditions, the float charge increases. The higher float charge causes higher heating and gassing of the battery.
Higher gassing results in more release of hydrogen. The excess release of hydrogen in turn causes water loss. In addition, the positive grid cor- rosion rate is accelerated and leads to the temperature derating of the battery. For example, 50% reduction in life for each 15°F increase in the battery temperature above 77°F. Loss of water due to high-temperature operation also accelerates the dryout of battery cells. This condition shortens the battery life even further for both AGM and gel based VRLA products.
The developments in charger technology include temperature com- pensation, fold back protection to reduce the effects of higher tempera- tures, excessive gassing, and thermal runaway. Provisions for airflow between cells allows for more uniform thermal distribution from cell to cell thus maintaining a uniform temperature gradient in the battery pack.
Repeated cycling of a battery is essential for a long life. Users often mistake the expected float warranty to be the actual battery life under cycling conditions. In either case, the VRLA batteries can provide excel- lent cycle life along with a cycle warranty. Users should always request that a 20-year VRLA battery make 1,200 cycles to 80% depth of discharge at C/8 rate to 1.75.
In several battery designs, antimony serves to provide a higher energy density.
However, calcium on the other hand when used in float applications batteries tends to improve the battery life. This is owing to the slower 34 ELECTRIC VEHICLE BATTERY EFFICIENCY
positive grid corrosion and battery requires less maintenance. The addi- tion of tin (Sn) to the calcium (Ca) alloys in VRLA batteries results in excellent cycling capability.
Antimony additives in the VRLA battery are similar to the flooded cells. The antimony additives in the VRLA battery cause the float current and gassing to increase as the battery undergoes cycling. At the end of its life, an antimony battery’s float current is almost six times greater than that of a battery with calcium as an additive. Thus calcium-based batteries are suited for telecom based applications since they do not dry out as easily and exhibit lower positive grid corrosion.