Battery Life Explained: How to Estimate Runtime from mAh and Current Draw

What battery life actually measures

“Battery life” in this context means one specific thing: how many hours a charged battery will keep a device running before it goes flat. It is not the battery’s lifespan in years, and it is not how long the battery takes to charge — those are different questions. Here the number you want is runtime under a load, and it comes from two figures: how much charge the battery holds, and how fast the device pulls that charge out.

Charge capacity is quoted in milliamp-hours (mAh). One mAh is a current of one milliamp flowing for one hour, so a 3000 mAh cell can, in principle, supply 3000 mA for one hour, 300 mA for ten hours, or 100 mA for thirty. The device’s appetite is its current draw in milliamps (mA) — the average current it pulls while it runs. Put those together and the arithmetic is short. The battery life calculator does it for you and then applies a realistic derating so the answer is not the fantasy figure a battery never quite reaches.

How battery life is calculated

The core formula is a single division. Because charge equals current multiplied by time, dividing capacity by current leaves you with time:

Runtime (hours) = Capacity (mAh) ÷ Current draw (mA)

The “milli-” on the top and bottom cancels, so milliamp-hours divided by milliamps gives hours directly — no unit juggling required. A 2000 mAh battery driving a 200 mA load lasts 2000 ÷ 200 = 10 hours in the ideal case. That is the theoretical ceiling, the figure you would get if the battery handed over every last electron it is rated for. It never does, which is where the second half of the calculation comes in.

Real cells fall short of their rated capacity under load, so a good estimate multiplies the ideal runtime by an efficiency, or derating, factor between 0 and 1:

Estimated runtime = (Capacity ÷ Current) × Efficiency

A common default is around 0.70–0.85 — DigiKey’s widely used battery-life calculator, for example, defaults to 0.70. Lithium-ion cells tend to sit higher, around 0.90–0.95; alkaline and NiMH cells under a meaningful load sit lower. The battery life calculator lets you set that factor and shows both the ideal figure and the derated estimate side by side so you can see how much the losses cost you.

The same logic works in energy units, and for anything rated in watt-hours it is the cleaner path. Runtime in hours equals energy capacity in watt-hours (Wh) divided by power draw in watts (W). To move between the two systems you need the voltage: Wh = (mAh ÷ 1000) × volts. That voltage step is why you cannot compare a phone’s mAh rating with a power bank’s mAh rating unless both are quoted at the same voltage — more on that trap below. If your device’s consumption is easier to find in watts, the appliance wattage calculator and the Ohm’s law calculator help you get power, voltage and current onto the same footing before you divide.

Worked example

Suppose you are building a small sensor node around a single 2000 mAh lithium cell, and the electronics draw an average of 200 mA while awake. Start with the ideal runtime:

• Ideal: 2000 mAh ÷ 200 mA = 10 hours.
• Apply a 70% derating for real-world losses: 10 × 0.70 = 7 hours.
• Usable capacity after derating: 2000 × 0.70 = 1400 mAh.

Now watch how the two inputs move the answer. Halve the current draw to 100 mA and the ideal runtime doubles to 20 hours (14 after derating). Double the capacity to 4000 mAh and it doubles again. Runtime scales directly with capacity and inversely with current — twice the battery, twice the life; twice the load, half the life. That inverse relationship is the single most useful thing to internalise, because it tells you that shaving the average current is usually a bigger win than fitting a bigger cell. Drop the sensor into a sleep mode that pulls 1 mA for 90% of the time and the average current collapses, stretching a seven-hour runtime into weeks. Run the numbers for your own capacity and load in the battery life calculator and adjust the efficiency factor until it reflects your battery chemistry and load.

Why real battery life falls short of the maths

The plain mAh ÷ mA figure is an upper bound, and several real effects pull the true runtime below it. This is what the efficiency factor is standing in for, and it helps to know what is actually inside it.

The Peukert effect

A battery’s usable capacity is not fixed — it shrinks as you draw current faster. Pull hard and more of the stored energy is lost to internal resistance and slower electrochemistry instead of reaching your device. This is the Peukert effect, described by the Peukert exponent: close to 1.0–1.05 for lithium-ion (so barely noticeable), but 1.1–1.3 for lead-acid, where a heavy load can cost a large slice of the rated capacity. The practical upshot: a battery loafing at a gentle drain comes close to its rating; the same battery under a heavy load can fall well short.

Cut-off voltage

Devices do not run the battery to zero. They stop at a cut-off voltage — the point where the supply has sagged too low to work reliably — and there is usually real charge left above that line that never gets used. A cheaper device with a higher cut-off leaves more on the table than an efficient one that squeezes the cell down further.

Temperature

Batteries are rated at a comfortable room temperature, roughly 20–25 °C. In the cold, usable capacity falls — a lithium-ion cell can deliver noticeably less below freezing — and in the heat, self-discharge speeds up and long-term degradation accelerates. A device that lives outdoors or in a pocket in winter will not match its bench-test runtime.

Converter and regulator losses

Between the cell and the circuit there is usually a voltage regulator or DC-DC converter, and it is not perfectly efficient. Some of the battery’s energy is spent just transforming the voltage to what the device needs. The same physics governs energy lost along wiring — if you are running a load down a long cable, the voltage drop calculator shows how much you lose to resistance before the load even sees it.

Spiky, non-constant loads

The formula assumes a steady current, but few devices draw one. A phone screen on versus off, a drill under load versus idling, a radio transmitting versus sleeping — the real current jumps around. What matters for runtime is the average current over a full duty cycle, weighting each state by how long the device spends in it. Estimate that average honestly and the simple division still works; use the peak draw by mistake and you will badly underestimate battery life.

How to get a more accurate estimate

• Measure the real average current, not the headline draw. Add up the current in each state — active, idle, sleep — weighted by the fraction of time spent there. A device that sleeps 95% of the time is dominated by its sleep current, not its active peak.
• Work in watt-hours when voltages differ. If your capacity is in mAh at one voltage and your load is quoted in watts, convert both to the energy system (Wh and W) so you are comparing like with like. Mixing mAh from a 3.7 V cell with a load figured at 5 V is a classic source of wrong answers.
• Pick the efficiency factor to match the chemistry. Use roughly 0.90–0.95 for a gently loaded lithium-ion cell, 0.70–0.85 for alkaline or NiMH, and drop lower still for a high-drain load where the Peukert effect bites.
• Derate for age. A battery a few years old holds less than its label claims. If the cell has seen heavy cycling, knock the rated capacity down before you even start — treat a “2000 mAh” two-year-old cell as perhaps 1600–1800 mAh.
• Treat the result as a range. Because so many factors nudge the true figure, a sensible output is “about 6 to 8 hours”, not “7.0 hours”. The calculator gives you the central estimate; reality scatters around it.
• Match your wiring and cell to the current. For a high-drain build, undersized wiring wastes energy and heats up. The wire gauge calculator helps you size conductors so more of the battery’s charge reaches the load.

Common mistakes

Comparing mAh across different voltages. A 10,000 mAh power bank does not charge a 5000 mAh phone exactly twice. Power banks are usually rated at the cell’s 3.7 V, while the phone charges at 5 V (or higher), and the voltage conversion loses energy. Convert both to watt-hours before you compare — that is the only fair basis.

Using peak current instead of average. Plugging a device’s maximum draw into the formula makes battery life look far shorter than it is, because the device only hits that peak briefly. Runtime is governed by the time-weighted average current across the whole duty cycle.

Forgetting to derate at all. Taking the raw mAh ÷ mA figure at face value overstates runtime by 10–40%. The ideal number is useful as a ceiling, but never quote it as the answer.

Ignoring self-discharge on slow drains. For a device that sips a tiny current over months, the battery’s own self-discharge can rival the load. A coin cell “lasting ten years” on paper may be limited by shelf life, not by the circuit.

When an estimate is not enough

For most purposes — sizing a battery for a hobby project, sanity-checking a gadget’s advertised runtime, deciding whether a power bank will see you through a flight — the mAh ÷ mA method with a sensible derating is all you need. Where it stops being enough is safety-critical or commercial design: a medical device that must run for a guaranteed number of hours, a remote sensor that has to survive a winter on one charge, or any product where under-delivering on battery life is a warranty or liability problem. There the right move is to bench-test the real load profile across temperature, or to have an electronics engineer model the duty cycle properly rather than trust a single division. When the cost of being wrong is high, measure; when it is low, estimate.

Frequently asked questions

How do I calculate battery life from mAh?

Divide the battery’s capacity in milliamp-hours by the current your device draws in milliamps, and the answer is the runtime in hours. A 3000 mAh battery driving a 250 mA load lasts 3000 ÷ 250 = 12 hours in the ideal case. Because real batteries deliver less than their rated capacity, multiply that by an efficiency factor — usually 0.7 to 0.9 — for a realistic estimate.

Why is my real battery life shorter than mAh ÷ mA?

The division gives the theoretical maximum, which cells rarely reach. At higher discharge currents the usable capacity falls (the Peukert effect); devices shut off at a cut-off voltage while charge remains; and cold temperatures, self-discharge and regulator losses all take a share. Together these typically knock 10–40% off the ideal figure, which is what the efficiency factor is there to capture.

What efficiency or derating factor should I use?

It depends on the chemistry and the load. For a gently loaded lithium-ion cell, 0.90–0.95 is reasonable; for alkaline or NiMH, 0.70–0.85; and for a high current relative to the battery’s size, where the Peukert effect bites harder, lower still. If you have no better information, 0.80 is a sensible middle estimate. Set it to 1.0 only when you deliberately want the theoretical ceiling.

How do I calculate runtime from watt-hours (Wh)?

Runtime in hours equals the battery’s energy in watt-hours divided by the device’s power draw in watts. If your figures are in mAh and volts, convert with Wh = (mAh ÷ 1000) × volts. Working in watt-hours is the more reliable path whenever the battery and the load are quoted at different voltages, because it removes the voltage ambiguity that trips up raw mAh comparisons.

Why doesn’t a 10,000 mAh power bank fully charge my 5000 mAh phone twice?

Because the two mAh figures are quoted at different voltages and the conversion between them wastes energy. The power bank’s cells are typically rated at 3.7 V, while your phone charges at 5 V or higher, so the internal step-up converter loses perhaps 10–20% along the way. Compare in watt-hours instead: a 10,000 mAh bank at 3.7 V holds 37 Wh, and after conversion losses you realistically get around 28–32 Wh out at 5 V.

Does temperature affect battery life?

Yes. Most batteries lose usable capacity in the cold — a lithium-ion cell can deliver noticeably less below freezing — and high temperatures accelerate self-discharge and long-term wear. Manufacturers rate capacity around 20–25 °C, so if your device runs much colder or hotter, expect the real runtime to fall short and choose a lower efficiency factor to reflect it.

Is battery life the same as charging time?

No. This estimates how long a charged battery lasts under a load. Charging time is a separate calculation — roughly the capacity divided by the charger’s current, also adjusted for efficiency, since charging is not 100% efficient. A 3000 mAh battery charged at 1500 mA takes a bit over two hours, though the charge curve makes the final stretch slower.

Related calculators

• Battery Life Calculator — the parent tool: capacity and load in, runtime in hours, minutes and days out.
• Ohm’s Law Calculator — convert between volts, amps, resistance and power to pin down your load.
• Appliance Wattage Calculator — running cost and energy use from a device’s wattage and hours.
• Capacitor Energy Calculator — energy stored in a capacitor, for the other half of your power supply.
• Voltage Drop Calculator — voltage lost along a cable run before it reaches the load.
• Wire Gauge Calculator — size conductors to your current so less energy is wasted as heat.