Power & Energy
How to calculate battery life from mAh
To estimate how long a battery lasts, divide its capacity by the current the load draws: runtime = mAh ÷ mA. A 2000 mAh battery running a 100 mA load lasts about 20 hours. Real batteries give a little less, so plan on 70–80% of the label.
Capacity in mAh
A battery holds a fixed amount of charge, written in milliamp-hours. 2000 mAh can supply 2000 mA for an hour, or 200 mA for 10 hours: current times hours stays the same.
That's a phone cell under a moderate load. Halve the current and it lasts twice as long, which is why phones dim their screens to save battery. A drone with a 2200 mAh pack drawing 13 A flies for about 2200 ÷ 13,000 ≈ 0.17 h: ten minutes.
Live experiment
Run a cell flat
A big capacitor plays a lithium cell, full at 4.2 V and flat at 3.0 V, running Pip through a resistor. Pick the cell, 0.047 to 0.22 F, and the resistor, 100 Ω or 220 Ω, and time how long it lasts. In the simulator, hours pass in seconds.
“How long will it last?” is lesson 1 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Watt-hours: the energy
mAh counts charge, not energy. A 3.7 V lithium cell gives each bit of charge more push than a 1.5 V AA, so the same mAh does more work. Multiply by the voltage to get watt-hours:
Use Wh when a converter sits between battery and load. A power bank's 10,000 mAh is measured at its 3.7 V cells; boosted to 5 V USB, with some loss, it delivers more like 6,000 mAh at 5 V. Airlines limit power banks by watt-hours, usually 100 Wh, because energy is what makes a fire.
Devices that sleep
Most battery gadgets don't draw a steady current. Average it over a cycle:
A sensor that wakes for 1 s a minute at 20 mA, and sleeps at 0.1 mA, averages about 0.43 mA. On 2000 mAh that's about 4,600 hours, over six months. Notice the sleep current is nearly a quarter of the total: shaving it matters.
Why real batteries give less
- C-rate: 1C is the current that empties a cell in an hour, 2.5 A for 2500 mAh. Capacity is quoted at a gentle rate, often 0.2C; pull harder and internal resistance turns more energy into heat.
- Cut-off voltage: the load stops working before the cell is truly empty, and lithium cells must stop around 3.0 V anyway.
- Cold and age: both cut the capacity you can get out.
- Converter losses: a regulator between battery and load wastes some energy.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“How long will it last?” is lesson 1 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Or start from the very first lesson, no account neededWork it out
- Battery life calculatorHours from mAh and milliamps, for a steady load or a device that sleeps.
- Electrical power practice problemsPower three ways, energy in kWh, battery life and running costs, all worked.
- Battery voltage and capacity chartFull, nominal and flat voltages, typical mAh, and how full a lithium cell is from its voltage.
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 2.5 A. 1C empties the cell in one hour: 2500 mA, or 2.5 A. Half of that, 0.5C, is 1.25 A.
- Why not “2500 A”? That mixes up milliamps and amps. 2500 mA is 2.5 A, not 2500 A.
- Why not “1 A”? 1 A would be 0.4C for this cell. 1C empties it in one hour, so it's 2500 mA.
- Why not “25 mA”? 25 mA would take 100 hours to empty a 2500 mAh cell. That's 0.01C.
Show the answer
Answer: How fast energy is being used. Power is how fast energy is used or delivered. In a resistor, that energy turns into heat, so more power means hotter, faster.
- Why not “The colour of the glow”? Power has nothing to do with colour. It's energy per second, in watts.
- Why not “How much charge is stored”? Stored charge is capacity, in mAh. Power is a rate: how fast energy is used.
- Why not “How long a battery lasts”? How long a battery lasts depends on its capacity and the load. Power is how fast the load uses energy.
Show the answer
Answer: 100 mA. One loop, one current. Every cell in the chain carries the same 100 mA, so each drains at the same rate.
- Why not “400 mA”? No cell can carry more than the loop's current. Each carries exactly 100 mA.
- Why not “25 mA”? The current doesn't split between cells in series. It's one loop, so each carries all 100 mA.
- Why not “Zero”? Every cell is part of the one loop, so all of them carry the current and drain together.
“How long will it last?” is lesson 1 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Questions people ask
How do you calculate battery life?
Divide the capacity by the current: hours = mAh ÷ mA. A 2000 mAh battery running a 100 mA load lasts about 20 hours. Real batteries usually give somewhat less, so many designers plan on 70–80% of the rated capacity.
What is the difference between mAh and Wh?
mAh measures charge; watt-hours measure energy. Multiply amp-hours by the voltage to convert: a 4000 mAh, 3.7 V cell holds 4 × 3.7 = 14.8 Wh. Use Wh to compare batteries of different voltages, or when a converter sits between the battery and the load.
What does 1C mean for a battery?
The current that would empty it in one hour. For a 2500 mAh cell, 1C is 2.5 A and 0.5C is 1.25 A. Ratings like 'charge at 0.5C' or 'discharge up to 10C' use it.
How do I estimate battery life for a device that sleeps?
Work out the average current. A sensor that draws 20 mA for 1 s each minute and 0.1 mA the rest averages (20 × 1 + 0.1 × 59) ÷ 60 ≈ 0.43 mA. A 2000 mAh battery then lasts about 4,600 hours, over six months.
Related topics
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.
- How to charge a lithium-ion batteryConstant current, then constant voltage at 4.2 V: the two-stage curve every lithium charger follows.
- Electrical power explained: P = V × IP = V × I: how fast energy turns into heat, and why a resistor has a power rating.
- How does a buck converter work?Switch, coil, diode and capacitor: the step-down converter that halves a voltage with almost no heat.