Electronics and projects
LED resistor
Supply, LED colour and current in; the exact resistor, the next E12 value and its power out.
Class 6 to 12, makers, Arduino users
Electronics and projects
Battery life calculator
Find out how long a battery will run your project before the exhibition day. Enter its capacity and the current your circuit draws, and allow for real-world losses to get a safer estimate.
- Made forClass 6 to 12, makers, robotics
- Shows the workingEvery step, ready for your practical file
- Instant answersResults change as you type
How long will the battery last?
Enter the capacity printed on the battery and the current your project draws. A multimeter on the mA range in series with the circuit shows the current.
Estimated running time (using 80% of the capacity)16 hours16 hours; the ideal figure is 20 hours
16 hours estimate20 hours ideal
- Average current
- 100 mA
- Ideal time
- 20 h20 hours
- With losses
- 16 h80% of label
Working
- Ideal time = capacity ÷ current = 2,000 mAh ÷ 100 mA = 20 hours.
- Allowing for losses: 20 h × 80% = 16 hours (16 hours).
- Check this: Real running time is shorter than this sum. Capacity falls at high current, in the cold and as a battery ages, and most circuits stop working before the battery is completely flat.
- Tip: For a project that must last, measure the real current with a multimeter and test a full run before the exhibition day.
What it works out
Understanding battery life
A battery's capacity, printed in mAh (milliamp-hours), is roughly the current it can supply multiplied by the hours it lasts. In theory a 2,000 mAh battery gives 100 mA for 20 hours, or 200 mA for 10 hours.
In practice a battery delivers less than its label, more so at high currents, in the cold and as it ages, and most circuits stop working before the battery is completely flat. That is why the calculator lets you count only a share of the capacity, and says so beside the answer.
Formula and worked example
Working out battery life by hand
The calculator uses exactly these formulas. Write them in your notebook and check one answer by hand: it is the surest way to remember them.
The formula
- Time (h) = capacity (mAh) × usable share ÷ current (mA)
- Average current = Iworking × duty + Isleeping × (1 − duty)
- Energy (Wh) = capacity (mAh) × voltage (V) ÷ 1,000
- usable share
- the fraction of the label capacity you expect to get, for example 0.8
- duty
- the fraction of the time the circuit is working, for example 0.1 for 10%
- Wh
- watt-hours, for comparing batteries of different voltages
Worked example
A 2,000 mAh battery pack running a project
- A steady 100 mA, in theory: 2,000 ÷ 100 = 20 hours.
- Counting only 80% of the label: 2,000 × 0.8 ÷ 100 = 16 hours.
- If the project instead works at 60 mA for 10% of the time and sleeps at 0.5 mA otherwise, the average is 60 × 0.1 + 0.5 × 0.9 = 6.45 mA.
- Then 2,000 × 0.8 ÷ 6.45 = 248 hours, a little over 10 days.
AnswerAbout 16 hours steady, or about 10 days with sleep
In projects and exams
How students use it
Capacity and load current to running time, with an allowance for real-world losses. Here is where it earns its place in a school year.
Exhibition day planning
Know whether the model will run through a whole day of judging or needs a spare battery at lunch.
Choosing a battery
Compare a 9 V battery, a pack of AA cells and a Li-ion cell by their mAh and voltage before you buy.
Sleep modes in IoT projects
See how much longer a sensor project lasts when it sleeps between readings.
Solar models
Work out how much battery a solar street-light model needs to stay lit through the night.
Check your work
Common mistakes to avoid
These are the slips that cost marks in exams and burn out parts on the bench.
Adding mAh for cells in series
Cells in series add their voltage, not their mAh. Two 2,000 mAh cells in series are still 2,000 mAh, at twice the voltage.
Using the wrong current
Motors draw far more when loaded or starting than when spinning freely. Measure the current while the project does its real job.
Forgetting the board itself
A microcontroller board, its power LED and its regulator draw current even when your code does nothing.
Trusting the ideal figure
The ideal figure is the upper limit. Plan with the derated estimate and test a full run before the big day.
Questions
Questions about battery life
Stuck on a project or a sum? Ask the institute on WhatsApp.
Ask on WhatsAppDo two batteries in series last twice as long?
No. In series the voltage doubles and the mAh stays the same. Identical cells in parallel add their mAh, which is what makes a battery last longer (connect only matching cells at the same charge).
What is the difference between mAh and Wh?
mAh measures charge; Wh measures energy and includes the voltage: Wh = mAh × V ÷ 1,000. Use Wh to compare batteries of different voltages, such as a 3.7 V cell and a 9 V battery.
Why does my 9 V battery die so fast with motors?
Motors draw a high current and small 9 V batteries hold much less charge than their size suggests, so the voltage collapses quickly. Motors are usually better served by a pack of AA cells or a suitable rechargeable pack with the right charger and protection.
How do I find out how much current my project draws?
Set a multimeter to mA, break one battery wire and connect the meter in series, then run the project normally. Note the highest and the usual readings.
Free ebooks
Read more about battery life
Free PDF ebooks in easy English: download, print and share them.
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