Battery Life Calculator
Estimate how many hours or days a battery lasts from its capacity and the load current, including Arduino and IoT devices that sleep most of the time.
For a device that is always on, set time active to 100%.
How it's calculated
The average current is I = active current × share + sleep current × (1 − share), and the usable capacity is divided by it. Cutoff voltage, temperature and ageing mean you rarely get the full rated capacity, so 70 to 90% is a realistic figure. Self-discharge is not included, so leave extra margin for devices meant to run for months.
Worked example: a 6,000 mAh battery at 500 mA
- Usable capacity: 6,000 mAh × 85% = 5,100 mAh
- Runtime: (5,100 mAh)/(500 mA) = 10.2 hours
- The same battery lasts 51 hours at 100 mA and 5.1 hours at 1 A. Halve the current and the time doubles.
How long a battery lasts, by capacity and current
| Capacity \ load | 10 mA | 50 mA | 100 mA | 250 mA | 500 mA | 1,000 mA |
|---|---|---|---|---|---|---|
| Capacity \ load500 mAh | 10 mA42.5 h | 50 mA8.5 h | 100 mA4.25 h | 250 mA1.7 h | 500 mA0.85 h | 1,000 mA0.425 h |
| Capacity \ load1,000 mAh | 10 mA85 h | 50 mA17 h | 100 mA8.5 h | 250 mA3.4 h | 500 mA1.7 h | 1,000 mA0.85 h |
| Capacity \ load2,000 mAh | 10 mA170 h (7.1 d) | 50 mA34 h | 100 mA17 h | 250 mA6.8 h | 500 mA3.4 h | 1,000 mA1.7 h |
| Capacity \ load3,000 mAh | 10 mA255 h (11 d) | 50 mA51 h | 100 mA25.5 h | 250 mA10.2 h | 500 mA5.1 h | 1,000 mA2.55 h |
| Capacity \ load4,000 mAh | 10 mA340 h (14 d) | 50 mA68 h | 100 mA34 h | 250 mA13.6 h | 500 mA6.8 h | 1,000 mA3.4 h |
| Capacity \ load5,000 mAh | 10 mA425 h (18 d) | 50 mA85 h | 100 mA42.5 h | 250 mA17 h | 500 mA8.5 h | 1,000 mA4.25 h |
| Capacity \ load5,100 mAh | 10 mA434 h (18 d) | 50 mA86.7 h | 100 mA43.4 h | 250 mA17.3 h | 500 mA8.67 h | 1,000 mA4.33 h |
| Capacity \ load6,000 mAh | 10 mA510 h (21 d) | 50 mA102 h (4.2 d) | 100 mA51 h | 250 mA20.4 h | 500 mA10.2 h | 1,000 mA5.1 h |
| Capacity \ load10,000 mAh | 10 mA850 h (35 d) | 50 mA170 h (7.1 d) | 100 mA85 h | 250 mA34 h | 500 mA17 h | 1,000 mA8.5 h |
| Capacity \ load20,000 mAh | 10 mA1,700 h (71 d) | 50 mA340 h (14 d) | 100 mA170 h (7.1 d) | 250 mA68 h | 500 mA34 h | 1,000 mA17 h |
Assumes 85% of the rated capacity is usable (h = hours, d = days). If the current varies, use the active share and sleep current in the calculator above.
How many hours of screen time from a phone battery
| Phone battery | Light use (messaging, reading) 1.5 W | Mixed use (video, social apps) 2.5 W | Heavy use (games, camera) 4 W |
|---|---|---|---|
| 3,000 mAh | 6.5 h | 3.9 h | 2.5 h |
| 4,000 mAh | 8.7 h | 5.2 h | 3.3 h |
| 5,000 mAh | 11 h | 6.5 h | 4.1 h |
| 5,100 mAh | 11 h | 6.7 h | 4.2 h |
| 6,000 mAh | 13 h | 7.9 h | 4.9 h |
| 7,000 mAh | 15 h | 9.2 h | 5.7 h |
Rough screen-on time, assuming a 3.85 V phone cell, 85% usable capacity and the average power shown. Brightness, signal and the phone model change it a lot.
How many phone charges from a power bank
- A 10,000 mAh power bank is rated at its 3.7 V cells, so it holds (10,000 mAh × 3.7 V)/(1000) = 37 Wh.
- With 85% efficiency through the 5 V converter and the phone's charger, it delivers 31.4 Wh.
- A 5,000 mAh phone battery (3.85 V) holds 19.2 Wh, so you get about 1.6 full charges. Dividing the mAh figures directly (2 charges) overestimates it.
Using it in practice
Step by step
- Enter the battery capacity in mAh (or Wh) and the average current your device draws.
- If the device sleeps most of the time, enter the active and sleep currents and the duty cycle instead of a single average.
- Apply a derating factor; the result is an estimate, not a guarantee.
Common mistakes
- Using the full printed capacity. Capacity is measured at a low discharge rate and room temperature down to a cutoff voltage. At high currents, in the cold, or if your circuit stops working above that cutoff, you get less. Plan on 70 to 80%.
- Forgetting the regulator and LEDs. A linear regulator's quiescent current, a power LED or a USB-serial chip can draw more than the microcontroller asleep. Measure the whole board's current, not just the chip's datasheet figure.
- Comparing mAh across voltages. A 3000 mAh 3.7 V cell holds about three times the energy of a 3000 mAh 1.2 V cell. Compare in Wh when voltages differ.
Measuring the average current
A multimeter shows a useful average only if the current is steady. For devices that wake briefly, measure the sleep current on a microamp range and the active current separately, time the active period, and let the calculator combine them. In battery-powered designs the sleep current usually decides the battery life: 1 mA of sleep current drains a 2000 mAh battery in under three months on its own.
Frequently asked questions
How long will a 2000 mAh battery last?
At a steady 100 mA, about 17 hours if you plan on 85% of the capacity (1700 mAh ÷ 100 mA). Enter your own device's current above.
How long does a 6000 mAh battery last?
Planning on 85% of the capacity: about 51 hours at 100 mA, 10.2 hours at 500 mA and 5.1 hours at 1 A. The time is the usable capacity divided by the current, so halving the current doubles it.
How long does a 500 mAh battery last?
About 42.5 hours at 10 mA, 8.5 hours at 50 mA and 4.2 hours at 100 mA, planning on 85% of the capacity. Small LiPo cells of this size suit sensors and wearables that sleep most of the time.
How many hours of screen time does a 5000 or 6000 mAh phone last?
Roughly 6.5 hours for 5000 mAh and 7.9 hours for 6000 mAh at a mixed-use average of 2.5 W, assuming a 3.85 V cell and 85% usable capacity. Games and the camera can draw 4 W or more and cut that by a third or more.
Why plan on less than the rated capacity?
Rated capacity is measured under ideal conditions. Real devices shut off at a cutoff voltage, run hot or cold, and batteries lose capacity with age, so 70 to 90% is a safer planning figure.