What this calculator works out
This calculator divides battery capacity by the device's current draw and applies an efficiency factor, giving an estimated runtime in hours.
The efficiency factor matters. No battery delivers its full rated capacity in practice, and the shortfall grows with age, cold and high discharge rates.
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Before you rely on the figure
- Capacity in milliamp-hours and draw in milliamps must use the same units. Mixing mAh with amps is a factor-of-1,000 error.
- 85% is a reasonable efficiency for a healthy battery at moderate draw. Reduce it for an old battery, cold conditions or a high-drain device.
- Real devices do not draw a constant current. A phone idling draws a fraction of what it does under load.
- Rated capacity is measured under laboratory conditions and at a specified discharge rate.
Capacity in mAh only compares like with like if the voltage is the same. A 5,000 mAh battery at 3.7 V holds 18.5 watt-hours; the same mAh at 1.2 V holds 6 Wh — a third as much energy. Watt-hours are the meaningful comparison across different chemistries, which is why power banks are increasingly rated in Wh.
How this calculator works
Capacity divided by draw, adjusted for efficiency:
Hours = capacity in mAh ÷ draw in mA × (efficiency ÷ 100)The efficiency factor absorbs several real effects: conversion losses, the voltage falling as the battery discharges, and capacity that is not accessible without over-discharging. Treating them as one figure is a simplification that works well enough for planning.
Worked example: a 5,000 mAh battery
Using the default figures — 5,000 mAh capacity, 250 mA draw, 85% efficiency:
- Theoretical runtime: 20 hours
- With efficiency applied: 17 hours
Three hours lost to efficiency alone, before any allowance for cold or battery age. A battery three years old may retain only 80% of its original capacity, which takes the same device to under 14 hours. Both effects compound, and neither appears on the packaging.
Common mistakes
- Mixing mAh with amps. The draw must be in milliamps to match.
- Comparing mAh across different voltages. Use watt-hours for a fair comparison.
- Assuming constant draw. Most devices vary enormously between idle and load.
- Ignoring temperature. Lithium batteries lose substantial capacity in the cold.
- Using the rated capacity of an old battery. Capacity falls with cycles and age.
Frequently asked questions
Why does my power bank charge my phone fewer times than the numbers suggest?
Because of voltage conversion. A 10,000 mAh power bank at 3.7 V holds 37 Wh, but charging a phone requires stepping the voltage up to 5 V, which loses energy as heat. Real-world delivery is typically 60 to 70% of the rated capacity, so a 10,000 mAh bank charges a 3,000 mAh phone roughly twice rather than three times.
How should I look after a lithium battery?
Avoid extremes. Keeping charge between roughly 20% and 80% reduces wear compared with regular full cycles, heat is the main enemy, and storing a device long-term at around half charge is better than full or empty. Occasional full discharges are unnecessary for lithium chemistries, unlike older nickel-based ones.
Why is battery life worse in winter?
Chemical reaction rates fall with temperature, so the battery cannot deliver current as readily and its usable capacity drops. Phones commonly show sudden shutdowns in the cold at what they report as 20 or 30% charge. Capacity generally returns when the battery warms up — it is a temporary effect rather than permanent damage.
What does the efficiency figure represent?
Everything between the rated capacity and what you actually get: conversion losses, voltage sag under load, capacity reserved to protect the cells, and age. 85% suits a healthy battery in normal conditions; 70% or less is realistic for an older one or a high-drain application.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — battery safety, disposal and recycling requirements
- Health and Safety Executive — safe handling and charging of lithium batteries
Sources are checked at publication and can change — how I choose and check references.