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Investigating Science 11–12 · Year 11

The ‘life’ of an alkaline cell under different loads

Module 2: Cause and Effect – Inferences and Generalisations

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The idea

A battery’s life is not one number: an alkaline cell delivers less charge at higher current, so an inference drawn from one test holds only for the load and schedule tested.

What you need

  • fresh alkaline AA cells from one batch (the Energizer E91 datasheet is used for comparison), cell holders
  • resistors of 3.9 Ω (rated at least 2 W) and 10 Ω (rated at least 1 W)
  • a voltage data logger, or a multimeter read every 10 minutes
  • thermometer; a refrigerator for an optional cold run

How to do it

  1. Record each cell’s open-circuit voltage.
  2. Connect a cell across the 3.9 Ω resistor and log the voltage every minute until it falls to 0.8 V.
  3. Calculate the current at each reading, I = V/R, and the charge delivered, Q = Σ I Δt, in milliampere-hours.
  4. Repeat with the 10 Ω resistor, and if time allows with a cell and resistor kept at refrigerator temperature.
  5. Compare the charge delivered in each case with the datasheet and state what generalisation about battery life the data do and do not support.

What you should see

Read from the bar chart on the Energizer E91 datasheet (continuous discharge to 0.8 V at 21 °C): about 3050 mAh at 25 mA, 2530 mAh at 100 mA, 2000 mAh at 250 mA and 1540 mAh at 500 mA. Through 3.9 Ω a cell averaging 1.15 V carries 295 mA, so interpolation predicts about 1920 mAh and 6.5 hours of continuous running; the datasheet’s intermittent 3.9 Ω test (one hour a day) reaches 0.8 V after about 8 hours because the cell recovers between runs. Through 10 Ω at about 1.20 V the current is 120 mA and the prediction is about 2460 mAh and 20.5 hours. The claim “this battery lasts N hours” is therefore true only for a stated load, schedule and temperature.

What changes

What you change
load resistance (and temperature in the optional run)
What you measure
charge delivered to 0.8 V (mAh) and running time
What you keep the same
  • cells from one batch
  • same cut-off voltage
  • same logging interval
  • room temperature recorded

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • A battery holds a fixed amount of energy whatever the load.
  • A cell is flat when it stops working in one device.
  • One test of one brand shows which brand lasts longest.

Safety card

Low riskLearners carry it out

Hazards

  • resistors become warm
  • short-circuited cells heat rapidly
  • leaking exhausted cells

Controls

  • use resistors rated well above the power dissipated (up to 0.58 W at 3.9 Ω with a fresh 1.5 V cell)
  • never connect a cell directly across its terminals
  • recycle cells through a battery collection point

Note

Low-voltage electrical work with no hazardous chemicals: record a RiskAssess risk assessment following the NSW Department of Education Science safety and compliance page; use only cells or a laboratory low-voltage supply, never mains.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/investigating-science-stage-6-2017
  2. education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-11-12-curriculum-resources/investigating-science-year-11-and-12
  3. data.energizer.com/pdfs/e91.pdf
  4. phet.colorado.edu/en/simulations/circuit-construction-kit-dc

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