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About battery life
A battery's capacity is printed in mAh (milliamp-hours). 2000 mAh means about 2000 mA for one hour, or 100 mA for 20 hours. So the basic formula is:
Battery life (hours) = capacity (mAh) ÷ current (mA)
Real batteries never give their full rating, so the calculator also uses a usable share — 85 % to start with. With 2000 mAh and 100 mA: 2000 × 0.85 ÷ 100 = 17 hours.
Many projects do not draw the same current all the time. A sensor may sleep most of the time and wake up for a few seconds to measure and send. For that, use Sleep and wake: the calculator works out the average current first.
Features
- Steady load — capacity and current give hours, days, months or years
- Sleep and wake — awake current and time, sleep current and how often it wakes up give the average current and the battery life
- Units — mAh or Ah; µA, mA or A; seconds, minutes or hours
- Typical batteries — CR2032, AAA, AA, 9 V and 18650 in one tap
- Usable share — room for real-world losses, 85 % to start with
- Self-discharge — optional, by battery type: alkaline, lithium, Li-ion, NiMH
- Where the battery goes — the share used while asleep, with a current-over-time drawing
Typical battery capacities
| Battery | Voltage | Typical capacity |
|---|---|---|
| CR2032 coin cell | 3 V | 220–240 mAh |
| AAA alkaline | 1.5 V | 1000–1200 mAh |
| AA alkaline | 1.5 V | 2000–2800 mAh (less at high current) |
| AA NiMH rechargeable | 1.2 V | 1900–2500 mAh |
| 9 V alkaline | 9 V | 400–600 mAh |
| 18650 Li-ion | 3.6–3.7 V | 2500–3500 mAh |
| LiPo pouch | 3.7 V | as printed (100–5000 mAh) |
Batteries in series (4 × AA = 6 V) keep the same mAh; only the voltage adds up. Batteries side by side (in parallel) add up their mAh.
Examples
| Project | Battery | Current | Life (85 % usable) |
|---|---|---|---|
| A device that draws 100 mA | 2000 mAh | 100 mA | 17 hours |
| Arduino Uno board, nothing attached | 9 V, 500 mAh | about 45 mA | 9.4 hours |
| 18650 cell, 150 mA load | 3000 mAh | 150 mA | 17 hours |
| Sensor: 80 mA for 2 s every 10 min, 10 µA asleep | 2000 mAh | 0.2766 mA average | 256 days |
Sleep current is often the real limit
The same sensor (80 mA for 2 s every 10 minutes, 2000 mAh, 85 % usable) with different sleep currents:
| Sleep current | Average current | Battery life | Used while asleep |
|---|---|---|---|
| 5 µA | 0.2717 mA | 261 days | 1.8 % |
| 50 µA | 0.3165 mA | 224 days | 15.7 % |
| 1 mA | 1.263 mA | 56 days | 78.9 % |
| 10 mA | 10.23 mA | 6.9 days | 97.4 % |
Many development boards draw several mA even in deep sleep, because of the USB chip, the power LED and the voltage regulator. A bare chip on a small board of its own can sleep at a few µA. That is the difference between one week and most of a year.
Self-discharge
A battery also empties by itself, even with nothing connected:
| Battery type | Self-discharge |
|---|---|
| Alkaline | about 2–3 % a year |
| Lithium coin cell (CR2032) | about 1 % a year |
| Li-ion / LiPo | about 2–3 % a month |
| NiMH, standard | about 20 % in the first month |
| NiMH, low self-discharge | about 15–20 % a year |
For a project that runs for months, pick the type under More options. The calculator adds self-discharge as an extra current: capacity × rate per month ÷ 730 hours.
Why use less than 100 %?
- The voltage drops as the battery empties. Your board may stop before the battery is really flat.
- High current and cold lower the usable capacity — a lot for alkaline cells.
- A voltage regulator or a boost converter loses some energy too.
- Rechargeable cells age and lose capacity over the years.
70–90 % is a fair guess for most projects.
Frequently asked questions
How do I calculate battery life?
Divide the capacity by the current: 2000 mAh ÷ 100 mA = 20 hours. Then multiply by a usable share, like 85 %, for a real-world number: 17 hours.
How long will a 2000 mAh battery last?
With 85 % usable: about 17 hours at 100 mA, about 3.5 days (85 hours) at 20 mA, and about 71 days at 1 mA.
How do I measure my device's current?
Put a multimeter in series with the battery, set to mA (or µA for sleep). For a sensor that sleeps and wakes, measure both currents and use Sleep and wake.
What is the difference between mAh and Wh?
mAh counts charge, Wh counts energy: Wh = mAh × voltage ÷ 1000. A 3000 mAh, 3.7 V cell holds 11.1 Wh. To compare batteries with different voltages, use Wh.
Does a bigger mAh number always last longer?
At the same voltage and the same load, yes. But a cheap cell may not reach its printed rating, and alkaline AA cells give much less than their rating at high current.
Related tools
- Ohm's Law Calculator — current from voltage and resistance
- LED Resistor Calculator — the current your LEDs draw
- Voltage Divider Calculator — measure the battery voltage with an ADC
- Electricity Cost Calculator — what mains devices cost to run
- Solar Panel Calculator — how many panels for your home
How to use it
- Pick Steady load or Sleep and wake
- Type the battery capacity, or tap a typical battery
- Steady load: type the current your device draws
- Sleep and wake: type the awake current and time, how often it wakes up, and the sleep current
- Keep the usable share at 85 %, or change it
- For long runs, pick the battery type under More options to count self-discharge
