Enrichment · STEM Explorations · Electronics

How long will
it run before
it dies?

Type the battery’s capacity and list what it powers, with the current each part draws and how much of the time it is on. The estimator gives the runtime, shows which part eats the battery and how much a change would buy.

Estimate the runtimeWhat you get

The battery runtime estimator

The worked example is made up: a weather station that wakes, reads a sensor and radios a reading, then sleeps. Nothing you type leaves this browser.

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What you get

Enrichment · STEM ExplorationsFree

What you finish with
The runtime in hours and days, the average current, the energy stored, each load’s share and the effect of halving the biggest one. It prints as a summary and downloads as a spreadsheet.
Who it is for
Students building electronics projects, robotics and maker clubs, science and engineering teachers and families doing STEM projects.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Up to 20 loads. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The STEM Explorations space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Name the project and type the battery’s capacity and voltage.
  2. Keep or change the usable share and regulator efficiency.
  3. Add each load with its current and the share of time it is on.
  4. Read the runtime and which load uses the most.
  5. Print the summary or download the spreadsheet for your design notebook.

Estimate the runtime

How the runtime is estimated

Usable charge divided by average draw.

The worked example: charge in, average draw outUsable charge2,160 mAh3,000 × 80% × 90%Average current12.60 mAeach load × its time onRuntime171 hours2,160 ÷ 12.60In days7.1hours ÷ 24The worked example: charge in, average draw outUsable charge2,160 mAh3,000 × 80% × 90%Average current12.60 mAeach load × its time onRuntime171 hours2,160 ÷ 12.60In days7.1hours ÷ 24
The station sleeps 80% of the time, so its average draw is only 12.60 mA, and 2,160 usable mAh last about 7.1 days.

The arithmetic

A milliamp-hour is one milliamp for one hour. Multiply the rating by the share you can use without damaging the battery and by the efficiency of any regulator to get the charge you can count on. Each load’s average current is its current while on times the share of time it is on; add them, then divide the usable charge by that total for hours.

Why real batteries fall short

Ratings are measured at a gentle discharge rate at room temperature. Drawing a large current, running in the cold and battery age all deliver less. Measure your parts’ currents with a meter if you can; data sheet figures are typical, and a status light left on can matter more than the parts you planned for.

Sources

  1. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy: batteries and energy storage basics (energy.gov).
  2. OpenStax. College Physics 2e (2022), chapter 20, Electric Current, Resistance, and Ohm’s Law, and section 21.2 on EMF and batteries (openstax.org).
  3. National Renewable Energy Laboratory: battery capacity, depth of discharge and temperature effects (nrel.gov).
  4. Next Generation Science Standards, HS-PS3 Energy and HS-ETS1 Engineering Design (nextgenscience.org).

The worked example is made up. The runtime is an estimate. Your figures stay in this browser. Support: team@caveatacademic.com.