Battery Life Calculator

How Battery Life Is Estimated

Battery capacity in milliamp-hours (mAh) describes how much current a battery can theoretically supply over one hour before its rated charge is used up. Dividing capacity by the device's average current draw gives a straightforward runtime estimate:

$$\text{Battery Life (hours)} = \frac{\text{Capacity (mAh)}}{\text{Current Draw (mA)}}$$

For example, a 3,000 mAh battery powering a device that draws 150 mA on average gives 3000 ÷ 150 = 20 hours of estimated runtime.

Why Real Runtime Is Usually Lower Than This Estimate

This simple division assumes the battery delivers its full rated capacity at a perfectly constant rate, which isn't how real batteries behave. Voltage sags as a battery discharges, and many devices become less efficient (or stop working entirely) once voltage drops below a threshold, even though some rated capacity technically remains. Higher discharge currents also reduce a battery's effective capacity below its nameplate rating (a phenomenon called the Peukert effect in lead-acid batteries, and a milder version of the same idea applies to lithium-ion cells), and temperature — especially cold — further reduces usable capacity. Treat this calculator's result as an optimistic upper bound, not a guarantee.

Finding a Device's Average Current Draw

Check the device's datasheet or product specifications for a typical or average operating current — many devices list this directly. Where it isn't published, a USB power meter or multimeter can measure actual current draw directly. Average draw is usually meaningfully lower than peak draw (the current spikes a device pulls momentarily, e.g. during a wireless transmission or screen wake), so using a peak figure here would significantly underestimate runtime.

A Brief History of Battery Capacity Ratings

The amp-hour as a way of describing how much charge a battery can store dates back to the earliest practical rechargeable batteries. Gaston Plante is generally credited with inventing the lead-acid battery in 1859, and Camille Faure's pasted-plate improvements in 1881 made it practical to manufacture at a scale where a standardized capacity rating actually meant something when comparing different battery models.

The milliamp-hour became the everyday consumer unit much later, as portable electronics grew small enough to need battery specs that shoppers could compare on a shelf. Nickel-cadmium (NiCd) rechargeable batteries popularized mAh ratings on consumer packaging through the mid-to-late 20th century, and the rating carried over directly when Sony commercialized the first mass-market lithium-ion battery in 1991 — the chemistry that now powers most of the phones, laptops, and other portable devices this calculator is typically used for.

Common Battery Life Calculation Mistakes

Using peak current instead of average current is the most common input mistake, producing an artificially pessimistic runtime estimate. Confusing mAh (a capacity/charge unit) with mA (a current/rate unit) is another frequent mix-up — they're related but not interchangeable, and mixing them up in a spreadsheet formula silently produces a nonsense number. Ignoring temperature and discharge-rate effects, as covered above, is the main reason a calculated estimate and real-world runtime diverge.

Battery Terms You Should Know

mAh (Milliamp-Hour) — a unit of electric charge capacity; a 2000 mAh battery can theoretically supply 2000 mA for one hour, or smaller currents for proportionally longer.

mA (Milliamp) — a unit of electric current, describing a rate of charge flow rather than a total capacity.

Peukert Effect — the phenomenon where a battery's effective usable capacity decreases at higher discharge currents, most pronounced in lead-acid batteries but present to some degree in most battery chemistries.

Discharge Curve — the graph of a battery's voltage over time as it drains, rarely flat in practice, which is why a linear capacity/current calculation is an approximation rather than an exact prediction.