Answer to sTEm–at-Work Puzzle #45: Battery Recharge Expectations

The visual challenges for students in this puzzle generate the best value for the puzzle. Most students don't want to see more than one curve, expect that curve to be a straight line, and always like to see the plots meet, or cross the abscissa. This puzzle provides no satisfaction on any of these fronts.

The three different test regions can generate a battery performance discussion with a bit of web work to learn what that lithium redox chemistry is. (If that is too much of a challenge for your students, just spring board over to the car battery since that is well documented on the web.) The puzzle can also lead to a discussion of deviations from expected behavior. Although no values are provided, a qualitative discussion of variance and standard deviation can be introduced. As for this graphic, the tested battery had these recovery characteristics:
  • It recovered 80 percent after it was 100% discharged and recharged 500 times
  • It recovered 75 percent after it was 50% discharged and recharged 600 times and
  • It was 30% discharged in less than 30 minutes and recovered 70 percent after 1,200 charge/discharge cycles
The battery failed the 30% discharge in less than 54 minutes. There was acceptable recovery percent within that expected recycle window, but the battery did not hold its charge for the expected 0.9 hrs. Since one of the tested batteries in the sample did not meet the graphically displayed minimal charge/discharge specifications below, none of the batteries in that manufactured lot will be shipped to this customer.    

 The Tech did not authorize shipment to the customer. YES or NO?       

Answer: YES

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