Battery Life Calculator
Battery life (runtime) is the duration a battery can power a device before needing recharge or replacement. It depends primarily on the battery's capacity (measured in ampere-hours, Ah or mAh) and the device's current draw (amperes, A or mA). The basic formula is simple, but real-world factors like efficiency, discharge rate, temperature, and battery age affect actual performance.
Knowing expected battery life helps in designing portable products, selecting the right battery for a project, and managing power consumption. It's critical for consumer satisfaction in smartphones, laptops, wearables, and for reliability in medical devices, sensors, and emergency systems.
Key battery concepts:
- Capacity (mAh, Ah): Total charge stored.
- Current draw (mA, A): Rate at which device consumes charge.
- C‑rate: Discharge current relative to capacity (e.g., 1C = capacity in 1 hour).
- Efficiency: Derating factor for losses (converter, heat).
- Peukert effect: Effective capacity decreases at high discharge rates (important for lead‑acid).
This calculator solves for any one variable in the battery life equation given the other two:
- Find Time: Enter capacity and current → get runtime (with optional efficiency).
- Find Capacity: Enter desired runtime and current → get required battery capacity.
- Find Current: Enter capacity and desired runtime → get allowable current draw.
The calculator provides:
- Accurate conversions between mAh/Ah and mA/A.
- Optional efficiency factor to account for real-world losses.
- Common battery presets for quick reference.
Typical capacities for various battery types:
| Battery Type | Nominal Voltage | Typical Capacity (mAh) | Typical Current (mA) | Approx Runtime |
|---|---|---|---|---|
| AAA Alkaline | 1.5V | 1000 | 100 | 10 h |
| AA Alkaline | 1.5V | 2000 | 200 | 10 h |
| 9V Alkaline | 9V | 500 | 50 | 10 h |
| 18650 Li-ion | 3.7V | 2500 | 500 | 5 h |
| Smartphone | 3.8V | 3000 | 300 | 10 h |
| Tablet | 3.7V | 6000 | 1000 | 6 h |
| Laptop | 11.1V | 5000 | 2000 | 2.5 h |
Short runtime (<1h): High‑drain devices (drones, power tools).
Medium runtime (1‑10h): Typical electronics (phones, cameras).
Long runtime (>10h): Low‑power devices (sensors, clocks).
Below are answers to frequently asked questions about battery life:
1 Ah = 1000 mAh. So to convert mAh to Ah, divide by 1000. To convert Ah to mAh, multiply by 1000. Our calculator handles this automatically based on your unit selection.
If efficiency η = 90% (0.9), effective runtime = (Capacity × η) / Current. For example, 2000 mAh, 100 mA, 100% → 20 h; 90% → 18 h. Efficiency accounts for converter losses, battery internal resistance, etc.
Determine your device's average current draw and desired runtime. Use the capacity formula: Capacity (mAh) = Current (mA) × Time (h). Add margin for efficiency (e.g., 20%) and select a battery with at least that capacity at the required voltage.
For devices that are not always on, use duty cycle. Average current = (on‑current × on‑time + off‑current × off‑time) / total cycle time. Then apply the same formula.
Peukert's law states that a battery's effective capacity decreases as discharge current increases. It's significant for lead‑acid batteries, less for Li‑ion. For rough estimates, the simple formula works for moderate currents.
Low temperatures increase internal resistance, reducing effective capacity. High temperatures can accelerate self‑discharge and aging. For critical applications, consult battery datasheets.