Battery Life From mAh and Current Draw: Estimating Real Runtime

Every battery prints a capacity on its label, and every datasheet lists a current draw, yet the gap between those two numbers and how long the thing actually runs trips up beginners and seasoned makers alike. The math itself is a single division. The trouble is that the clean number it gives you is almost always optimistic. This guide walks through the formula, the derating factor that separates the lab figure from the field figure, and how I use the math when I am sizing a pack for a real project.

The One Formula You Need

Runtime comes from dividing how much charge a battery holds by how fast a device pulls it out:

hours = capacity (mAh) / current draw (mA)

A 2000 mAh cell feeding a steady 100 mA load gives 2000 / 100 = 20 hours. That is the answer you get if everything behaves perfectly: a fresh cell, a flat load, room temperature, and no losses anywhere in the circuit. Nothing in the real world is that tidy, which is why this raw number is a ceiling, not a prediction.

The units have to match. Milliamp-hours pair with milliamps. If your draw is listed in amps, multiply by 1000 first, or you will be off by three orders of magnitude. A 5200 mAh drone pack pulling 18 A in hover is 5200 / 18000 = 0.29 hours, about 17 minutes before derating. Keep the unit discipline tight and the formula never lies to you about the order of magnitude.

Why You Multiply by Roughly 0.8

The printed capacity is measured under conditions chosen to flatter the cell: a gentle discharge rate, a brand-new battery, a warm room. Your project rarely matches any of that. Voltage regulators throw away 10 to 20 percent of the energy as heat. Cells lose capacity with every charge cycle. Cold weather raises internal resistance and hides charge you paid for. To fold all of that into one honest number, multiply the runtime by an efficiency factor:

real hours = (capacity / current) × 0.8

The 0.8 is a working default, not a law of physics. It bakes in a typical 20 percent margin so the estimate lands closer to what the device delivers on a Tuesday afternoon two years into its life. Raise it toward 1.0 for an efficient load on a fresh cell. Drop it to 0.6 or 0.7 for an aged pack, a spiky load, or a project that lives outdoors in winter. The factor is the knob that turns a textbook answer into a planning answer.

A Worked Example

Take that 2000 mAh battery and that 100 mA device again, but this time stop pretending the world is perfect.

20 hours × 0.8 = 16 hours

Sixteen hours is the number I would write on the project sheet. Twenty is the absolute best case you will never quite reach; sixteen is where you plan your battery swap. If the device has to survive an 18-hour shift, 16 hours tells you immediately that one pack will not make it, and you can decide between a bigger cell, a second pack, or shaving the load before you are stuck in the field. That single multiply turns a hopeful guess into a decision.

You can run these numbers by hand, but the battery life calculator does the division, the derating, and the unit handling in one place, and it hands you a shareable link so a teammate sees the exact same estimate. It also reverses the math: tell it the runtime you need and it solves for the capacity you have to source.

Why Real Life Runs Shorter Than the Math

Even with the 0.8 factor, devices die sooner than expected, and the reasons are worth knowing so you can adjust.

First, the load is rarely flat. A wireless sensor that idles at a few milliamps but wakes to transmit pulls a brief burst many times its average. If you feed the calculator the peak instead of the time-weighted average, your runtime comes out far too short; if you feed it only the idle figure, far too long. The honest input is the average over a full duty cycle.

Second, the battery has aged. A cell labeled 3000 mAh may only hold 2400 after a couple of years of cycling. Enter the real, measured capacity rather than the optimistic nameplate, or lean harder on a lower efficiency factor to account for the loss.

Third, voltage matters in ways mAh hides. Milliamp-hours describe charge, not energy. A pack at a higher cell voltage carries more usable energy for the same mAh, which is exactly why power banks rated in watt-hours are easier to compare honestly. When you are pushing serious current, the cell sags under load and the regulator drops out earlier than the flat math predicts. If you are also reasoning about the resistance and voltage in that circuit, the Ohm's Law calculator pairs naturally with this one to keep current, voltage, and resistance consistent.

Sizing a Battery for a Project

The most useful trick is to run the formula backwards. Instead of asking how long a known battery lasts, ask how big a battery a known runtime needs:

capacity (mAh) = target hours × current (mA) / 0.8

Say you are building a soil-moisture sensor that must run a full month between charges, with an averaged draw of 0.5 mA after accounting for sleep and brief radio bursts. A month is roughly 720 hours, so you need 720 × 0.5 / 0.8 = 450 mAh. A small coin cell will not carry that comfortably, but a single 18650 at 2500 to 3000 mAh leaves generous headroom. Now you are choosing a cell from a number, not a hunch.

The same backward math settles the classic power-bank argument before a flight. One bank prints 20000 mAh, another prints 74 Wh; they are not the same unit, and the bigger sticker does not automatically win. Convert your need to watt-hours, enter your laptop's draw in watts, and read the hours each can actually deliver. The watt-hour figure is the fair one because it already accounts for the cell voltage the mAh number quietly leaves out.

The Takeaway

The runtime formula is just capacity over current, but the value lives in the 0.8 you tack on afterward and the honesty of the inputs you feed it. Use the average load, not the peak. Use the real capacity, not the nameplate. Match your units, and reach for watt-hours whenever voltages differ. Do that and your battery estimates will start matching reality closely enough to plan around, whether you are sizing a sensor for a season in the dirt or deciding which pack survives the mission.

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Made by Toolora · Updated 2026-06-13