Battery Life Calculator
Calculate how long a battery will last under a given load, or determine what capacity you need for a target runtime. Adjust efficiency to account for real-world losses in voltage regulators and converters.
Wh to mAh Converter
Convert between watt-hours (Wh) and milliamp-hours (mAh) using the battery's nominal voltage.
Formula: mAh = (Wh / V) x 1000 | Wh = (mAh x V) / 1000
Common Battery Types Reference
Click any row to load that battery's capacity into the runtime calculator.
| Battery Type | Chemistry | Voltage (V) | Capacity (mAh) | Energy (Wh) | Common Uses |
|---|---|---|---|---|---|
| AAA | Alkaline | 1.5 | 800-1200 | 1.2-1.8 | Remotes, toys, clocks |
| AAA (rechargeable) | NiMH | 1.2 | 800-1100 | 1.0-1.3 | Low-drain devices |
| AA | Alkaline | 1.5 | 2000-3000 | 3.0-4.5 | Flashlights, controllers |
| AA (rechargeable) | NiMH | 1.2 | 1900-2800 | 2.3-3.4 | Cameras, game controllers |
| C Cell | Alkaline | 1.5 | 7000-8500 | 10.5-12.8 | Radios, lanterns |
| D Cell | Alkaline | 1.5 | 12000-18000 | 18-27 | Large flashlights, boomboxes |
| 9V Battery | Alkaline | 9.0 | 200-550 | 1.8-5.0 | Smoke detectors, multimeters |
| CR2032 | Lithium | 3.0 | 210-240 | 0.6-0.7 | Watches, key fobs, BIOS |
| 18650 | Li-ion | 3.7 | 2500-3500 | 9.3-13.0 | Laptops, flashlights, EVs |
| 21700 | Li-ion | 3.7 | 4000-5000 | 14.8-18.5 | Tesla EVs, e-bikes, power tools |
| 26650 | Li-ion / LiFePO4 | 3.2-3.7 | 4000-5500 | 12.8-20.4 | High-drain flashlights |
| LiPo (small) | Li-Polymer | 3.7 | 500-2000 | 1.9-7.4 | Drones, RC, wearables |
| LiPo (large) | Li-Polymer | 3.7 | 3000-10000 | 11.1-37 | RC aircraft, robots |
| Lead-Acid (small) | SLA | 12 | 7000 | 84 | UPS, alarm systems |
| Smartphone (typical) | Li-ion | 3.85 | 3000-5000 | 11.6-19.3 | Mobile phones |
Series vs. Parallel Battery Configurations
Parallel Connection
Batteries share the same voltage but add their capacities.
- Voltage stays the same
- Capacity (mAh) multiplied by number of batteries
- Runtime increases proportionally
- Example: 2x 3000 mAh 3.7V = 6000 mAh at 3.7V
Series Connection
Batteries stack voltages but capacity stays the same.
- Voltage adds up
- Capacity (mAh) remains unchanged
- Total energy (Wh) increases
- Example: 2x 3000 mAh 3.7V = 3000 mAh at 7.4V
How Battery Life Calculation Works
Basic Formula
Runtime (hours) = (Battery Capacity in mAh / Load Current in mA) x Efficiency
Required Capacity Formula
Capacity (mAh) = (Runtime in hours x Load Current in mA) / Efficiency
Why Efficiency Matters
The rated capacity of a battery (e.g., 3000 mAh) represents the theoretical maximum under ideal conditions. In practice, several factors reduce the usable energy:
- Voltage regulation losses: Buck, boost, and linear regulators convert battery voltage to the level your circuit needs, losing 5--30% as heat.
- Internal resistance: All batteries have internal resistance that causes voltage drop under load, reducing effective capacity.
- Temperature: Cold temperatures can reduce lithium battery capacity by 10--20%. Alkaline batteries lose even more in cold weather.
- Discharge rate: High current draw reduces the effective capacity of most battery chemistries. A battery rated at 3000 mAh at a 0.2C discharge rate may only deliver 2400 mAh at 1C.
- Cut-off voltage: Devices typically stop functioning before the battery is fully depleted, leaving some unusable energy.
Choosing an Efficiency Factor
| Scenario | Efficiency | When to Use |
|---|---|---|
| Direct drive (no regulator) | 0.95-1.0 | LED + resistor, simple motor circuits |
| LDO regulator | 0.85-0.95 | Low dropout, small voltage difference |
| Switching regulator (good) | 0.85-0.92 | Modern buck converters, quality boards |
| Boost converter | 0.75-0.85 | Stepping up voltage (e.g., 3.7V to 5V) |
| Worst case / unknown | 0.70 | Conservative estimate for planning |
Real-World Examples
Example 1 -- ESP32 IoT Sensor
Setup: ESP32 drawing 80 mA average, powered by a single 18650 battery (3000 mAh) through a 3.3V LDO regulator (90% efficiency).
Runtime = (3000 / 80) x 0.90 = 33.75 hours (~1.4 days)
Example 2 -- Trail Camera
Setup: Trail camera using 8x AA batteries (parallel pairs in series = 5000 mAh effective at 6V), drawing 50 mA average with 85% efficiency.
Runtime = (5000 / 50) x 0.85 = 85 hours (~3.5 days)
Example 3 -- How much capacity for a 7-day sensor?
Requirement: Low-power sensor drawing 5 mA needs to run for 7 days (168 hours) with a boost converter (80% efficiency).
Capacity = (168 x 5) / 0.80 = 1050 mAh -- a single AAA NiMH battery would suffice.
Frequently Asked Questions
How do I calculate battery life from mAh?
Divide the battery capacity (mAh) by the device current draw (mA), then multiply by the efficiency factor (typically 0.7-0.9). For example, a 3000 mAh battery powering a 500 mA device at 85% efficiency gives: (3000 / 500) x 0.85 = 5.1 hours.
What is the difference between mAh and Wh?
mAh (milliamp-hours) measures charge capacity and depends on voltage. Wh (watt-hours) measures energy and is voltage-independent. To convert: Wh = (mAh x V) / 1000. Wh is more useful for comparing batteries of different voltages because it represents actual energy stored.
Why does my device not last as long as calculated?
Several factors reduce real-world runtime: varying current draw (WiFi/GPS spikes), self-discharge over time, aging batteries with reduced capacity, temperature effects, and voltage regulation overhead. The efficiency factor helps account for some of these, but actual usage patterns always vary.
Can I use batteries in series to increase runtime?
Not directly. Series connection increases voltage but keeps the same mAh capacity. However, higher voltage can improve regulator efficiency (less voltage drop across an LDO, for instance), which may slightly increase runtime. For longer runtime, use parallel connections to add capacity.
What current draw does my device use?
Check the device datasheet or measure it with a multimeter in series. Note that current draw often varies significantly -- a phone might draw 50 mA idle but 500 mA+ with the screen on and WiFi active. Use the average current for runtime calculations.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
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Privacy
This calculator runs entirely in your browser. No battery data, device specifications, or calculation results are transmitted or stored anywhere.
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Battery Life Calculator FAQ
How do I calculate battery life from mAh?
Divide the battery capacity (mAh) by the device current draw (mA), then multiply by the efficiency factor (typically 0.7-0.9). For example, a 3000 mAh battery powering a 500 mA device at 85% efficiency gives: (3000 / 500) x 0.85 = 5.1 hours.
What is battery efficiency factor?
The efficiency factor accounts for energy lost as heat in voltage regulators, boost/buck converters, and internal battery resistance. Typical values are 0.7-0.8 for buck/boost converters, 0.85-0.95 for linear regulators, and 0.9 for well-designed direct-drive circuits.
How do I convert Wh to mAh?
Divide the energy in watt-hours (Wh) by the nominal voltage (V), then multiply by 1000. Formula: mAh = (Wh / V) x 1000. For example, a 10 Wh battery at 3.7V = (10 / 3.7) x 1000 = 2703 mAh.
Do batteries in parallel increase runtime?
Yes. Batteries connected in parallel add their capacities together while keeping the same voltage. Two 3000 mAh batteries in parallel give 6000 mAh total capacity, doubling the runtime. Batteries in series increase voltage but keep the same mAh capacity.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.