What is system efficiency in this context?

System efficiency accounts for energy losses between the battery and your load. These include DC-DC converter losses (typically 5–20%), wiring resistance losses, and battery internal resistance losses. A 90% efficiency means only 90% of stored charge reaches the load; you must store 100/90 ≈ 11% more to compensate.

Why add a safety margin?

Battery capacity degrades over charge cycles and at temperature extremes. A 20–30% safety margin ensures your device still meets its runtime target when the battery has aged or when operating in cold conditions. New batteries used at room temperature can use a smaller margin (10–15%).

What is the formula used?

Required capacity (mAh) = (Load current (mA) × Runtime (h)) / (Efficiency / 100). The recommended capacity then adds the safety margin: Recommended = Required × (1 + Safety margin / 100).

Should I enter peak or average current?

Use the average current draw over the full runtime. If your device has sleep/active cycles, calculate the duty-cycle-weighted average: (I_active × t_active + I_sleep × t_sleep) / total_time. Peak current affects component ratings but not total energy storage.

How does this differ from the Battery Life Calculator?

The Battery Life Calculator tells you how long a given battery will last. This tool works in reverse — you specify the desired runtime and it tells you how large a battery you need. Use this tool during the design phase; use the Battery Life Calculator to verify an existing battery.

What efficiency value should I use for a Li-ion direct load?

If your load connects directly to the battery terminals with no converter, use 95–98% to account only for internal resistance and wiring losses. If you use a boost or buck converter, use the converter efficiency rating, typically 85–92% for common modules.

Battery Capacity Calculator

Name: Battery Capacity Calculator
Availability: InStock
Author: Nham Vu

Enter your device current draw, desired runtime, and efficiency to calculate the minimum battery capacity (mAh) required.

Device & Runtime Parameters

Load Current (mA)

Average current drawn by the device over its operating cycle.

Desired Runtime (hours)

How long the device must run on a single charge. Use decimals for minutes (1.5 = 90 min).

System Efficiency (%) (70–98%)

Account for converter, wiring, and internal resistance losses. 90% is a good default.

Safety Margin (%) (0–100%)

Extra headroom for aging, temperature, and tolerance. 20% is recommended for most designs.

Recommended Battery Capacity

—

mAh

Formula: Minimum (mAh) = (I × t) / η
Recommended (mAh) = Minimum × (1 + margin / 100)
Where I = current (mA), t = runtime (h), η = efficiency (0–1).

Summary