Battery Life Calculator

Written byOleh Soloninupdated 25 September 2026

Capacity, mAh

Draw, mA

The load's current at its own voltage.

Battery voltage, V

Advanced options

Converter efficiency, %

Load voltage, V

When a converter sits in between — 5 V from a 3.7 V cell, say. 0 means the load runs at the battery voltage.

Result

Runtime17 h
Hours17
Days0.71
Stored energy, Wh11.1
Charges per day1.41
Worth knowing
The figure uses the rated capacity. It falls with age, and heat speeds that up for good; in the cold the battery gives less for a while and recovers once it warms up
Working3,000 mAh × 85% ÷ 150 mA = 17 h

Capacity divided by consumption

Estimates how long a device will run on a battery of a given capacity, allowing for converter efficiency.

Runtime is battery capacity divided by the current drawn: 3,000 mAh at 150 mA is 20 hours. Through a converter, part of the energy is lost in it, so the runtime is multiplied by its efficiency: at 85% it comes to 17 hours. That holds while the load runs at the battery voltage. If the converter changes it — 5 V from a 3.7 V cell, say — the runtime follows the energy: 3,000 mAh × 3.7 V × 0.85 ÷ (5 V × 150 mA) = 12.6 hours; the load voltage field does that.

Stored energy in watt-hours is the capacity in amp-hours times the voltage: 3 Ah at 3.7 V is 11.1 Wh. That figure lets you compare batteries of different voltages.

How to use

Enter the battery capacity in milliamp-hours and the device's current draw in milliamps.
Set the battery voltage, and a converter's efficiency under the advanced fields if there is one.
Read the runtime, the number of days, the stored energy and how many charges a day you will need.

Runtime at 150 mA

Each row is a separate run: 150 mA, 85% efficiency, 3.7 V.

All rows come from the calculator on this page.
Capacity	Runtime	Days	Energy
2,000 mAh	11 h 20 min	0.47	7.4 Wh
3,000 mAh	17 h	0.71	11.1 Wh
5,000 mAh	28 h 20 min	1.18	18.5 Wh
10,000 mAh	56 h 40 min	2.36	37 Wh
20,000 mAh	113 h 20 min	4.72	74 Wh

Rated capacity and real capacity

The calculator uses the capacity on the label. Capacity fades with age, and heat speeds that up for good — even in storage. Cold works differently: a chilled battery gives less for a while and recovers once it warms up (Apple, "Maximizing Battery Performance"). An old battery runs for less than the sum says.

What this calculator shows

Runtime in hours and in a readable hours-and-minutes form
Stored energy in watt-hours, the fair way to compare cells of different voltage
How many charges a day continuous use would require

What to keep in mind

Capacity is the rated figure; ageing and temperature are not modelled.
The current is constant; for a varying load, enter the average.

FAQs

How long does a 3,000 mAh battery last?

At 150 mA, 20 hours with no losses or 17 hours through an 85% converter; at 50 mA with the same efficiency, 51 hours.

How do I convert mAh to Wh?

Multiply the amp-hours by the voltage: 3,000 mAh = 3 Ah, and at 3.7 V that is 11.1 Wh.

What is the circuit efficiency?

The share of the battery's energy that reaches the device through a voltage converter. If the device runs straight off the battery, set 100%.

How many charges a day will I need?

24 hours divided by the runtime: with 5.1 hours of runtime (3,000 mAh at 500 mA), 4.71 charges a day.

How long will an AA cell last?

The same way: capacity over current. A 2,500 mAh nickel–metal hydride cell at 20 mA with no converter runs for 125 hours.

What if the device sleeps and wakes?

Enter the average current: the calculator treats the draw as constant. The average is each mode's current times its share of the time, added up.

Worked example

A sensor drawing 150 mA

Input: 3,000 mAh, 150 mA, 3.7 V

Output: 17 hours, 11.1 Wh stored

Note: With the default 85% efficiency; without a converter it would be 20 hours.

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