Battery reliability and safety (Part one)
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.
Editor's note: I found this book very informative regarding batteries and their construction, design and usage. In light of the recent Boeing 787 battery or charging system failures, I thought this would help us to understand some of the challenges and possible solutions on the battery side of things in designing a reliable power system. Please do comment and give our readers your expertise and thoughts about this problem and about this chapter 8 of the book. The following is part one of Chapter 8 and part two is posted here.
I will also be reviewing this book in the near future.
Important concepts from this chapter:
Battery reliability: The probability of failure
Battery safety: The consequences of failure
ReliabilityBattery reliability is the probability that a battery will deliver energy and power in a specified manner under a defined set of conditions.
Battery performance—the ability of a battery to deliver defined power and energy levels—is not constant. How it performs depends on a number of factors. The environmental conditions under which a battery operates usually have a large effect on performance, particularly temperature and mechanical vibration or shock. For some batteries, pressure (high and low) and humidity may also affect their performance.Batteries are designed and their materials are chosen to function under a range of conditions that are relevant to the intended applications. A lithium–manganese dioxide (Li/MnO2) coin cell that runs a wristwatch will mostly operate at room temperature, or about 21°C (70°F).
That same Li/MnO2 coin cell may instead be used to power a tire pressure sensor— devices mounted in an automobile wheel that measures the air pressure in the tire. Temperatures may range between –40°C (–40°F) and +120°C (+250°F), or even higher, in this application. Not to mention the severe shock and vibration it could experience on rough roads or hitting potholes.
So, battery manufacturers must either supply different batteries for different applications or design their batteries to reliably operate under as large of a range of conditions as possible.
Time is another factor. Some batteries may need to be stored unused for long periods of time, and then must function immediately when needed. Batteries typically lose some of their energy just sitting on a shelf for extended periods of time.
There are a number of ways a battery can fail to meet performance expectations, including self-discharge that decreases the energy output of a battery from parasitic chemical reactions depleting the active materials, or else increased internal resistance that reduces the power output of a battery.
Measuring VariabilityPhysical dimensions, discharge performance, internal resistance, and all of the other important characteristics of a single battery type vary somewhat from cell to cell and battery to battery. These variations are the cumulative result of all the variabilities associated with all of the individual cell components. Not only that, each of the many manufacturing process steps that are necessary to assemble the battery can have their own variability that affects the overall cell variability.
Battery manufacturers strive to minimize the sources of variability of cell components and manufacturing processes, but variability cannot be eliminated altogether. Raw material, component, and battery specifications are written with this fact in mind by including tolerances—an acceptable range of variability for each component part and for the battery itself.
Battery manufacturers also perform an assortment of quality assurance tests on incoming raw materials and purchased components, as well as the completed batteries during and after manufacturing to verify they meet specifications within the range of specified tolerances. There is variability in the equipment used to test batteries that may affect, however slightly, their measured performance outputs.
Batteries may be shipped to and stored in warehouses or other storage facilities following manufacture. They are shipped to the customer, who may store the batteries for unpredictable periods of time. During storage and while shipping, batteries may be exposed to different environmental conditions, especially high temperatures, which may measurably degrade performance. Finally, just the elapsed time between manufacture and use may also affect battery operation.