Battery Life Calculator

Enter a battery's capacity and the current or power your device draws, and the calculator estimates how many hours it will run. A derating factor accounts for real-world losses, and a second calculator estimates charging time.

How battery life is calculated

At its simplest, battery life is capacity divided by load. A battery rated at 3,000 mAh can in theory supply 300 mA for 10 hours or 100 mA for 30 hours. Real batteries deliver less than their label, so the calculator multiplies by a usable-capacity factor, typically 70–90%.

1. Runtime (hours) = capacity (mAh) ÷ load (mA) × efficiency.
2. If you know watts instead: battery energy (Wh) = mAh × voltage ÷ 1,000, and runtime = Wh ÷ load (W) × efficiency.
3. For devices that are only on part of the time, multiply the load by the duty cycle to get the average current.

Working in watt-hours is more accurate whenever a voltage converter sits between the battery and the device — for example, a 3.7 V power bank charging a 5 V phone — because mAh figures at different voltages cannot be compared directly.

Choosing the efficiency factor

Duty cycle for sensors and gadgets

Many battery devices sleep most of the time. A sensor that draws 20 mA for 2 seconds every minute and 0.05 mA while asleep has an average current of about (20 × 2 + 0.05 × 58) ÷ 60 ≈ 0.72 mA. Enter the active current and the percentage of time it is active, or work out the average yourself and enter it with a 100% duty cycle. For very low average currents, the battery's self-discharge — a few percent per month for lithium, more for NiMH — may limit life more than the device does.

Worked example

A 3,000 mAh, 3.7 V lithium-ion cell powers a device that draws 250 mA continuously. The battery holds 3,000 × 3.7 ÷ 1,000 = 11.1 Wh. The load is 250 mA × 3.7 V = 0.925 W. At 85% usable capacity, runtime is 11.1 × 0.85 ÷ 0.925 ≈ 10.2 hours, or about 10 hours 12 minutes. Two identical cells in parallel double the capacity and the runtime; two in series double the voltage but keep the same mAh.

Typical battery capacities

Alkaline cells in particular deliver far less capacity at high current than at low current, which is why a flash-heavy camera empties AA batteries quickly while a wall clock runs for a year.

Reading a battery datasheet

Datasheets usually state a nominal capacity measured at a low discharge rate, such as 0.2C, and a nominal voltage. Look for the discharge curves: they show how much capacity the cell delivers at higher currents and at low temperatures, and the voltage at which the manufacturer considers it empty (the cut-off voltage). If your device stops working at a higher voltage than that cut-off — some electronics need at least 3.3 V from a lithium cell — you will get less than the rated capacity. Use the usable-capacity factor in the calculator to reflect this, and prefer the datasheet figure for your actual load over the headline number printed on the wrapper.

Series and parallel packs

Batteries in series add their voltages while the capacity in mAh stays the same; in parallel the voltage stays the same and the capacities add. Energy in watt-hours adds either way. Only combine identical cells of the same age and charge level, and use a battery management system with lithium cells.

Charging time

The second calculator divides the capacity you need to replace by the charger current, adjusted for charging efficiency. A 3,000 mAh battery at 20% charged with a 1 A charger needs about 2,400 mAh, which at 85% efficiency takes about 2.8 hours. Lithium-ion charging switches to a slower constant-voltage phase near the end, so full charges take longer than the estimate, while charging to 80% is comparatively quick.