Battery reliability and safety (Part two)
Excerpted from The TAB Battery Book: An In-Depth Guide to Construction, Design, and Use (McGraw-Hill Professional; 2011) by Michael Root with permission from McGraw-Hill Professional.
Part one was previously published here on EDN
Battery Reliability and Safety
Early failures become evident soon after manufacturing and before, or at least not long after, a battery starts being used.
We won’t count the failures at various steps during manufacturing and before completion of the battery. These failures may count against the manufacturing yields if the cause for the failure cannot be fixed and the battery must be scrapped. This, in turn, results in fewer batteries produced and a higher cost of manufacturing a completed battery. Such failures also may point to a quality issue that is the result of a design weakness, defective components, or an errant manufacturing process step.
Many early failures are exposed before the batteries leave the manufacturing facility. Battery manufacturers perform a variety of tests designed to find early failures and help assure the batteries will function as advertised. Testing may include electrical measurements like open circuit voltage, loaded voltage, and impedance or internal resistance., as well as physical characteristics like dimensions and weight.
The seals may be tested for leaks. The entire cell may be inspected after assembly using X-ray imaging. Microcalorimetry is an instrumental method that measures very small amounts of heat released by a battery. The amount of heat released is related to the rate of self-discharge and the magnitude of an internal short circuit if one exists.
The rate of early failures starts at a relatively high level but rapidly decreases (see the top graph in Figure 8-5).
Figure 8-5 Top: Distributions of failure rates for different failure modes occurring throughout battery life—early failures, random defects, premature end of life, and normal depletion. Bottom: Combined failure rates for all failure modes.
Some examples of early failure causes are
- Human error Handling errors or accidents; for instance, batteries may be stored under too severe conditions (such as high vibration or temperature levels), dropped from a height onto a hard surface, or inadvertently short-circuited by connecting the negative and positive terminals.
- Design flaws These are usually found with accelerated stress tests performed during battery development, though. (See the next section, “Accelerated Testing.”)
- Nonconforming or missing components Component dimensions may be out of specification, or components made from the wrong material, resulting in chemical contamination, corrosion of metal parts, or mechanical weaknesses.
Components may be damaged or incorrectly made. Burrs on metal edges can break through separators and insulators to make direct contact between electrodes, causing an internal short circuit and early battery depletion.
- Contamination Foreign materials unintentionally allowed to get into the battery during manufacture may contain certain chemicals that are incompatible with other battery materials or otherwise decrease performance. Foreign material like bits of metal can puncture separators and insulators, causing an internal short circuit and early battery depletion.
The oxidation-dissolution-diffusion-plating reactions could persist until the contaminating metal is depleted. As the metal continues to plate on the negative electrode material, it may form dendrites or fingers of metal that grow toward the positive electrode. Eventually this process can create a direct connection between the negative and positive electrodes, causing an internal short circuit and early battery depletion (Figure 8-6).
Other contaminants may result in other parasitic reactions that cause gassing, like iron in the zinc electrode of an alkaline zinc–manganese dioxide cell. If the internal gas pressure is high enough, it could cause the cell to swell or force a vent to open with an attendant loss of electrolyte.
- Manufacturing variances Processes may not be in control, and the resulting battery may have low quality or could have widely variable performance. If welded electrical connections are weak, the weld can break and create an open circuit. Weak seals can lead to electrolyte leakage and ingress of moisture, leading to a loss of battery performance.
These may occur at any time during battery life after manufacture. This type of failure can occur during storage on the shelf or while in use. Some of the same modes that cause early failures, like weld or seal failure and internal short circuits, can also occur at different times throughout battery life.
The rate of random defects is usually relatively constant, but low, throughout battery life (see the top graph in Figure 8-5).
Premature End of Life
Premature battery end of life is caused by slow but steady loss of active materials in the battery or the degradation of other components, like insulators or seals. These types of failures generally show up later in battery life, depending on how fast the materials deteriorate. The rates of deterioration, and thus battery failures, may vary enough that batteries could fail over an extended period of time.
Figure 8-6 Dendrite growth from a contaminating metal becoming oxidized at the positive electrode, dissolving in the electrolyte, diffusing to the negative electrode, becoming reduced and depositing on the negative electrode, growing as dendrites through the separator as the process continues, then forming a short-circuit path between negative and positive electrodes.
Other reactions may lead to increased internal resistance, leaving the battery unable to support higher power demands. It may appear as though the battery were depleted even though there may be plenty of active material remaining in the battery for lower power discharge.
Regardless, the results are reduced battery capacity, compromised battery function, even an internal short-circuit condition, leading the batteries to prematurely reach end of life.
Examples of premature end of life causes are
- Excessive self-discharge and loss of active negative or positive electrode materials.
- Chemical breakdown of electrolyte, leading to gas buildup and drying out of the electrodes. The outcomes can include cell swelling and increased internal cell resistance.
- Electrolyte leakage from the cells through weakened seals or ruptured vents. Increased internal resistance from dried-out electrodes, ingress of moisture, and perhaps external corrosion of the metal battery case.
- Passivation or blockage of electrode surfaces from corrosion or deposition.
- Dendrites derived from foreign material (more details on this appear under “Early • Failures”). Some lithium ion cells may form dendrites if they are discharged below the recommended discharge voltage limit.
- Lithium plating on negative electrode surfaces can occur in lithium ion cells • when they are excessively charged. Similar to dendrites, the lithium may grow enough to directly contact the positive electrode and cause an internal short circuit that drains the battery.
- Other complications arise when considering batteries consisting of multiple cells • connected together. The performance characteristics of the individual cells are not the same; rather, they have a distribution of performance characteristics. As a result, the entire battery can be limited by the weakest cell. It may have the least capacity or highest internal resistance or some other limiting performance characteristic.
The cells with more capacity may not be completely discharged. This is a waste of battery capacity. Additionally, if the cells are connected in series and the battery is discharged beyond the capacity of the weakest battery, it can be driven by the higher-capacity cell or cells in the battery to a point where the polarity of the weak cell reverses. The negative electrode becomes positive and the positive electrode becomes negative. This situation can cause permanent damage to the cell that went into reversal and, thus, impact the overall battery performance.The ill effects of dissimilar cells in a battery can be minimized by closely matching, for example, the capacity and internal resistance of all the cells in the battery. However, even if the differences between cells in a rechargeable battery are small, they tend to become greater with the number of charge and discharge cycles.
A distribution of premature end of life failure rates may look like the top graph in Figure 8-5. There may be multiple failure modes that have their own distribution.