## Battery Life and Exponential Distributions

### Topic

Battery Life and Exponential Distributions

### Vocabulary

- Exponential Distribution
- Continuous Data
- Coefficient of Determination

### Student Exploration

#### Sick of replacing the batteries in your household objects? How often do you have to replace the batteries in your video game controllers? In your camera?

People use AA and AAA batteries for all types of household objects. Batteries power video game controllers, cameras, toys, calculators, portable radios, flashlights, and small appliances. Unfortunately batteries don’t last very long. The battery sales across the globe make about $50 billion (U.S.) each year, and is a growing industry, see http://batteryuniversity.com/learn/article/battery_statistics. The life of a battery can be represented as an exponential distribution, when measuring the probability of a battery dying over time. Check out the following table tracking the days of use of batteries and the probability of failure over that time, note that the probability of failure is in decimal form, such that 0.095 means 9.5%.

Days in Use | Probability of Failure |
---|---|

0 | 0 |

1 | 0.00995 |

2 | 0.0198 |

5 | 0.04877 |

10 | 0.095 |

18 | 0.16473 |

32 | 0.27385 |

43 | 0.34295 |

99 | 0.62842 |

Use the table above to answer the follow questions.

- Use your TI-84 calculator to create an exponential regression equation to represent the exponential distribution.
- What is the coefficient of determination?
- How likely is it that the exponential equation is correct?
- What is the probability that a battery will fail after 150 days?
- What are the implications of this information when thinking of your uses of batteries?

### Extension Investigation

Other applications of exponential distributions include representing the traffic patterns in a city, the length of support phone calls, life of many electrical items, or the number of customer transactions at a store. Auto insurers use exponential distributions to determine the risk of a client getting in an accident. This is how they determine how much to charge individuals for insurance. Check out this practice problems for the test for people who want to become risk assessors for insurance companies, “Example P Practice Problems” 7, 6, 4, 3, 2, and 1 at http://probabilityexam.wordpress.com/tag/exponential-distribution/.

### Resources Cited

http://batteryuniversity.com/learn/article/battery_statistics

http://probabilityexam.wordpress.com/tag/exponential-distribution/

### Connections to other CK-12 Subject Areas

- Binomial Distributions and Probability
- Multinomial Distribution
- Linear Regression Equations.

Image from: http://commons.wikimedia.org/wiki/File:Duracell-AA.png