Battery Life Calculator

Written byOleh Soloninupdated 20 August 2026

Capacity, mAh

Draw, mA

Voltage, V

Advanced options

Converter efficiency, %

Result

Runtime17 h 0 min
Hours17
Days0.71
Stored energy, Wh11.1
Charges per day1.41
Worth knowingCapacity drops in the cold and with age, so expect 10-20% less in practice

Capacity divided by consumption

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

Capacity is quoted in milliamp-hours: a 3,000 mAh cell will in theory deliver 3,000 milliamps for one hour, or 300 for ten. Divide capacity by consumption and you have the runtime.

Theory and reality diverge noticeably. A converter costs 10 to 20 percent, cold weather takes another third, and after a couple of hundred charge cycles the cell has lost a fifth of its rated capacity. Hence the efficiency field.

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

Consumption is treated as constant. Real devices swing between sleep, screen-on and radio activity.
Efficiency defaults to 85 percent, typical for a switching converter.

FAQs

How do I convert mAh to watt-hours?

Multiply by voltage and divide by a thousand. A 3,000 mAh cell at 3.7 volts holds 11.1 Wh. Watt-hours compare fairly across different voltages.

Why does a power bank deliver less than its label?

The rating refers to internal 3.7 volt cells while USB outputs 5 volts. Conversion costs energy, so a 10,000 mAh bank puts about 6,500 mAh into a phone.

How does cold affect a battery?

At −10 °C a lithium cell gives about a third less, at −20 °C roughly half. Capacity returns on warming, but a frozen cell must never be charged.

What is a charge cycle?

One full discharge and recharge. Lithium cells are rated for 300 to 500 before losing twenty percent of capacity; partial discharges wear them more slowly.

Worked example

A gadget on a lithium cell

Input: 3,000 mAh, 150 mA draw, 85 percent efficiency

Output: 17 hours, from 11.1 Wh of stored energy

Note: Seventeen hours under constant load. If the device sleeps most of the time at 5 mA, the same cell lasts weeks.

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