Battery Life Calculator
Calculate how long your battery will last using capacity (mAh) and current draw (mA). Includes formulas, worked examples, and a charge time calculator.
Battery life (hours) = Battery capacity (mAh) ÷ Device current draw (mA). A 3,000mAh smartphone battery powering a device drawing 300mA of current lasts approximately 10 hours. Use this calculator to estimate runtime for any battery-powered device.
What Is Battery Life and How Is It Calculated?
Battery life is the amount of time a battery can power a device before it needs recharging. It is calculated by dividing the battery's energy capacity by the device's rate of energy consumption.
Formula:
Battery Life (hours) = Battery Capacity (mAh) ÷ Current Draw (mA)
Where:
- Battery Capacity (mAh) = the total charge stored in the battery, measured in milliampere-hours
- Current Draw (mA) = the amount of current the device consumes, measured in milliamperes
- Battery Life (hours) = the estimated hours of operation
How to Use the Battery Life Calculator
- Enter your battery capacity in mAh (check the battery label or device specs).
- Enter the device's current draw in mA (from the datasheet or measured with a multimeter).
- Read the estimated runtime in hours.
- For charge time, enter your charger's output current (mA) to see how long a full charge takes.
Worked Examples
Example 1 — Smartphone
- Battery: 4,000mAh
- Active screen-on current draw: 400mA
- Battery life = 4,000 ÷ 400 = 10 hours of screen-on time
Example 2 — Hearing Aid
- Battery: 100mAh (zinc-air cell)
- Current draw: 1.4mA
- Battery life = 100 ÷ 1.4 = 71 hours (approximately 6–7 days)
Example 3 — Laptop
- Battery: 72Wh at 11.4V = 6,315mAh
- Load at moderate use: 1,800mA (1.8A)
- Battery life = 6,315 ÷ 1,800 = 3.5 hours under moderate load
Example 4 — IoT Sensor (Low-Power Device)
- Battery: 2,000mAh AA batteries
- Average current draw: 5mA
- Battery life = 2,000 ÷ 5 = 400 hours (16.7 days)
Common Battery Capacities by Device Type
| Device | Typical Capacity | Avg Current Draw | Typical Battery Life |
|---|---|---|---|
| Smartphone | 3,000–5,000mAh | 300–500mA (active) | 6–12 hours screen-on |
| Laptop | 4,000–8,000mAh equiv. | 1,500–3,000mA | 3–8 hours |
| Wireless earbuds (each) | 50–100mAh | 10–20mA | 4–8 hours |
| Smartwatch | 200–400mAh | 30–60mA | 3–7 days |
| AA battery (1.5V) | 2,000–2,700mAh | Varies | Varies |
| Hearing aid | 80–150mAh | 1–2mA | 4–7 days |
| Smoke detector | 1,200mAh | 0.01mA (standby) | 5–10 years |
FAQ
How long will a 3000mAh battery last?
A 3,000mAh battery lasts approximately 6–10 hours in a smartphone under normal use (drawing 300–500mA). For low-power devices drawing 10mA, the same battery would last around 300 hours (12.5 days). Battery life depends entirely on the current draw of the device.
How do I calculate battery life in hours?
Divide battery capacity (in mAh) by the device's current draw (in mA). Battery life (hours) = mAh ÷ mA. For example, 5,000mAh ÷ 250mA = 20 hours.
What drains battery life the fastest?
Screen brightness, GPS, mobile data, and high processor loads are the biggest battery drains in smartphones. In general, the higher the current draw, the shorter the battery life.
Does a higher mAh rating always mean longer battery life?
Yes, all else being equal. A 5,000mAh battery will last roughly twice as long as a 2,500mAh battery with the same device. However, battery age, temperature, and discharge rate also affect real-world performance.
How long does it take to charge a battery from 0% to 100%?
Charge time = Battery capacity (mAh) ÷ Charger output (mA). A 4,000mAh battery with a 1,000mA (1A) charger takes approximately 4 hours for a full charge. Fast chargers (20W = ~4,000mA at 5V) can charge the same battery in approximately 1 hour.