Battery Life

How long will a battery last — a phone, a torch, a laptop, an e-bike? Or how big a battery do you need to run something for a day? Solve for the runtime, the capacity or the load, and see how long it takes to charge.

ElectricityBatteriesEnergy

Inputs

Input examples

The device uses

Click Solve next to the quantity you want to find: it moves into the result below. Capacity is printed on the battery in mAh or Ah; the usable share allows for the fact that batteries are never drained completely.

Battery capacity C

Current drawn I

Runtime t

Battery voltage V

Usable share — 80–95% is realistic

Charger power — optional, for the charging time

mAh, Wh and Battery Life

A battery’s capacity in mAh says how much charge it holds: 2500 mAh can deliver 2500 mA for one hour, or 250 mA for ten. To compare batteries of different voltages, use the energy in watt-hours: Wh = Ah × V. A 5000 mAh phone battery at 3.85 V holds 19 Wh; a laptop holds around 60 Wh.

Real runtimes are a bit shorter: batteries are not drained to zero, they lose capacity when cold and as they age, and high currents waste some energy as heat in the battery’s internal resistance — Ohm’s law again.

Wh= Ah × V

I = P/Vpower → current

coldless capacity

100 Whairline limit

Battery Level

over time

charge left

low-battery warning (20%)

Result

Enter the values above

Typical Batteries

Battery	Capacity	Energy
AA alkaline	2500 mAh · 1.5 V	3.8 Wh
Smartwatch	300 mAh · 3.8 V	1.1 Wh
Smartphone	5000 mAh · 3.85 V	19 Wh
Power bank	20 000 mAh · 3.7 V	74 Wh
Laptop	5400 mAh · 11.1 V	60 Wh
E-bike	14 Ah · 36 V	500 Wh
Electric car	— · 400 V	60 000 Wh

Worked Techniques

Runtime from mAh

Divide the usable capacity by the current. Watch the units: mAh divided by mA gives hours.

Example:

Devices Rated in Watts

If you know the power, either turn it into a current with Ohm’s power law, or divide the energy in Wh directly by the watts.

Example:

Charging Time

Some energy is lost as heat while charging. Phones also slow down for the last 20% to protect the battery, so a full charge takes longer than this estimate.

Example:

Sources & Verification

References

A. R. Hambley, Electrical Engineering: Principles and Applications, 7th ed., Pearson, 2018 — Sec. 1.3 (Power and Energy).