How Long Will a Battery Last? Turning mAh into Real Runtime

You see “3,000 mAh” on a power bank and “5,000 mAh” on a drone battery and assume the bigger number lasts longer. Sometimes it does, often it doesn’t, and the reason trips up almost everyone: milliamp-hours are not a measure of energy. They’re a measure of charge, and charge only becomes runtime once you know the voltage and the load. Get those two extra numbers and you can estimate how long almost anything will run — phone, e-bike, drone, or a string of LEDs on an AA cell.

Why mAh on its own is a half-finished number

A milliamp-hour says a battery can deliver a certain current for a certain time: 3,000 mAh can push 3,000 mA for one hour, or 300 mA for ten hours. What it deliberately leaves out is the pressure behind that current — the voltage. A 3,000 mAh cell at 3.7 V holds far less energy than a 3,000 mAh pack at 36 V, even though the mAh figure is identical. Comparing two batteries by mAh alone is like comparing two fuel tanks by litres without asking whether they’re full of petrol or water.

The fix is to convert charge into energy, measured in watt-hours (Wh), which folds the voltage in:

Wh = (mAh × V) / 1000

That 3,000 mAh phone cell at 3.7 V holds about 11.1 Wh. The 36 V e-bike pack at 14,000 mAh holds about 504 Wh — roughly 45 times the energy, which the raw mAh comparison completely hides. Once everything is in watt-hours, batteries of any chemistry and voltage finally sit on the same scale. The Battery Life Runtime Calculator does this conversion first, every time, which is why it asks for voltage even when you enter capacity in mAh.

Putting the load on the same scale

Energy in the tank is only half the equation; you also need to know how fast the device drains it. Device draw can be quoted as current (mA or A) or directly as power (watts), and to divide cleanly you want it in watts too. Current becomes power the same way capacity became energy — by multiplying in the voltage:

W = V × A

Now both sides speak the same language, and runtime is just energy divided by power:

runtime (hours) = (Wh × efficiency) / load watts

A phone holding 11.1 Wh driving a 300 mA load at 3.85 V (about 1.16 W) would, on paper, run for roughly 9–10 hours. That matches the ballpark of light real-world use — and the gap between “on paper” and “in your hand” is the next piece.

The efficiency factor is where spec sheets lie

If you stop at energy ÷ power you’ll always overestimate, because no battery delivers 100% of its rated energy to the load. Voltage regulators burn a slice as heat, boost and buck converters are imperfect, and the battery’s own internal resistance wastes more the harder you pull. A realistic system efficiency is 80–90%, and the calculator defaults to 85%. That single multiplier is the difference between a manufacturer’s optimistic claim and what your stopwatch actually shows.

It’s worth seeing the runtime at several efficiencies side by side, because the spread is large. At 100% efficiency a pack might show 10 hours; at 85% it’s 8.5; at 60% — a cold drone battery under heavy throttle — it could be just 6. The tool prints a small table across 100/90/85/75/60% precisely so you can pick the row that matches your real conditions rather than trusting one rosy number.

Why real life is always shorter still

Even a careful efficiency estimate is an upper bound, because three things conspire against you that a single formula can’t fully capture:

• Voltage sags as the battery drains. A “3.7 V” Li-ion cell is 4.2 V full and drops toward 3.0 V empty, so its real average voltage — and the energy you actually extract — is lower than the nominal label.
• Cold weather steals capacity. Lithium chemistry slows down in the cold; a pack that runs an e-bike 40 km in summer might manage 28 km in winter with nothing else changed.
• Loads are rarely constant. A phone idles, then spikes when the screen lights or the radio transmits. The formula assumes a steady draw, so spiky devices drain faster than the average suggests.

Treat the calculated figure as a best case under steady conditions and shade it down for cold, age, and bursty use.

Working it through

The honest way to estimate battery life is to stop staring at the mAh number and run the three steps: convert capacity to watt-hours so the voltage is baked in, convert the load to watts so it’s on the same scale, then divide and knock off 10–20% for efficiency. Do that and a 3,349 mAh phone, a 5,000 mAh drone and a 2,500 mAh AA cell all become directly comparable in the only unit that matters — hours of actual use.

When you’d rather not juggle the conversions by hand, enter your capacity, voltage, draw and efficiency into the Battery Life Runtime Calculator and read the runtime plus the watt-hour and milliamp-hour equivalents. If you’re sizing the load side of a circuit — say a current-limiting resistor for an indicator LED — pair it with the LED Series Resistor Calculator so the draw you feed in is grounded in the real components, not a guess.