How battery life is calculated
Battery runtime is determined by dividing usable battery capacity by the rate at which the device consumes power.
**Core formula:** \`\`\` Runtime (hours) = Battery Capacity (mAh) × Efficiency Factor / Current Draw (mA) \`\`\`
For example, a 5,000 mAh battery powering a device that draws 250 mA at 85% efficiency gives: \`\`\` Runtime = 5,000 × 0.85 / 250 = 17 hours \`\`\`
This calculator applies an efficiency factor automatically to account for real-world losses that reduce usable capacity below the rated value. Results appear as you type — no button needed.
Understanding mAh and mA
**mAh (milliampere-hours)** is the unit used to measure battery capacity. It tells you how many milliamps a battery can supply for one hour. A 5,000 mAh battery can theoretically deliver 5,000 mA for one hour, or 500 mA for ten hours.
**mA (milliamperes)** is the unit for electrical current draw. Every device consumes current while running. The higher the current draw, the faster the battery drains.
The relationship is simple: large mAh combined with small mA gives long runtime. Small mAh with large mA gives short runtime.
To find current draw for a device, check the datasheet, measure with a USB power meter, or calculate from wattage: mA = (Watts × 1000) / Volts.
What is efficiency factor?
Rated battery capacity is measured under ideal laboratory conditions. In real-world use, several factors reduce how much of that capacity is actually usable:
- **Voltage sag:** Battery voltage drops as charge decreases. Devices often shut off before the battery is fully depleted. - **Temperature:** Cold temperatures significantly reduce capacity. A battery rated 5,000 mAh at room temperature may deliver only 3,500 mAh at 0°C. - **Battery age:** Lithium-ion cells lose roughly 20% capacity after 300-500 charge cycles. - **Heat:** High ambient temperatures during discharge increase internal resistance and losses. - **Variable load:** Devices rarely draw constant current. Bursts of high current (like starting a motor or transmitting data) reduce overall efficiency.
An efficiency factor of 85% is a reasonable default for most lithium-ion batteries in normal conditions. Use 70-75% for aged batteries or cold weather, and 90-95% for new batteries in light, steady-load applications.
Watt-hours and why they matter
Milliamp-hours (mAh) describe charge capacity but do not capture the full picture of stored energy because energy depends on voltage. Two batteries with the same mAh rating but different voltages store different amounts of energy.
**Energy formula:** \`\`\` Energy (Wh) = Capacity (mAh) × Voltage (V) / 1000 \`\`\`
For example: 5,000 mAh at 3.7V = 18.5 Wh, while 5,000 mAh at 12V = 60 Wh.
Watt-hours are more useful when comparing batteries of different voltages, estimating solar panel charging times, or checking carry-on airline limits (typically 100 Wh without approval, 160 Wh with airline permission). Enter the battery voltage in the optional field to see the Wh equivalent.
Common device current draws
Typical current consumption by device type (approximate values):
- **Phone on standby**: 2-10 mA - **Phone screen on, light use**: 150-300 mA - **Phone gaming or video**: 400-600 mA - **Tablet browsing**: 300-700 mA - **Bluetooth earbuds**: 20-50 mA - **Smart watch**: 5-25 mA - **Arduino Uno**: 40-60 mA - **Raspberry Pi Zero**: 100-160 mA - **Raspberry Pi 4 (under load)**: 600-1,200 mA - **LED strip per meter**: 200-1,200 mA - **Laptop (light use)**: 1,500-3,000 mA - **Laptop (heavy use)**: 3,000-6,000 mA - **GPS tracker**: 20-60 mA - **Security camera**: 200-500 mA
These are rough estimates. Always measure your specific device with a USB power meter or refer to the device datasheet for accurate results.
How to use
1. Enter your battery capacity in mAh. This value is printed on the battery label or listed in the product specifications. 2. Enter the device current draw in mA. Check the device datasheet, USB power meter reading, or manufacturer specs. 3. Adjust the efficiency slider. Leave at 85% for typical use. Lower it for aged or cold batteries; raise it for new batteries under light loads. 4. Optionally enter the number of devices if multiple devices share the same battery. The calculator multiplies per-device draw by the device count automatically. 5. Optionally enter battery voltage to see capacity expressed in watt-hours (Wh). 6. Results appear instantly. The main result shows runtime at your selected efficiency. The table below shows runtime across a range of efficiency levels (70% through 95%) so you can plan for best-case and worst-case scenarios.
FAQs
Q: What does mAh mean on a battery? A: mAh stands for milliampere-hours. It measures how much charge a battery holds. A 5,000 mAh battery can deliver 5,000 milliamps for one hour, or 1,000 milliamps for five hours.
Q: Why is my actual battery life shorter than the calculator shows? A: The efficiency factor accounts for common losses, but actual runtime depends on temperature, battery age, variable load, and how much capacity your device leaves unused before shutting off. Lower the efficiency factor to get a more conservative estimate for harsh conditions or older batteries.
Q: How do I find the mA draw for my device? A: Check the device datasheet or product page. Alternatively, plug it into a USB power meter (available for under \$15) which displays current in real time. You can also find wattage on the label and divide by voltage: mA = (watts / volts) × 1000.
Q: Can I use this for power banks? A: Yes. Enter the power bank's rated capacity in mAh and your device's average current draw. Power banks typically have 80-90% efficiency due to conversion losses, so adjust the efficiency slider accordingly.
Q: What efficiency factor should I use for an old battery? A: For a battery that has gone through 300 or more charge cycles, use 70-80%. For a battery over 3 years old with heavy use, use 60-70%. A new lithium-ion battery in normal conditions is typically 85-90% efficient.
Q: Does temperature affect battery life? A: Yes, significantly. At 0°C (32°F), a lithium-ion battery may deliver only 70-80% of its rated capacity. At -20°C (-4°F), capacity can fall to 50% or less. Use a lower efficiency factor in cold-weather deployments.
Q: How does the parallel devices option work? A: When multiple devices share a single battery, total current draw increases proportionally. Enter your per-device current draw and the number of devices. The calculator multiplies them to find total draw before computing runtime. For example, three sensors at 30 mA each = 90 mA total.
Q: What voltage should I use for lithium-ion? A: Use 3.7V for a single lithium-ion cell (nominal). Multi-cell packs multiply this: 7.4V for 2S, 11.1V for 3S, and so on. USB power banks are commonly 5V at the output. Car batteries are 12V.