Battery Life Calculator
Calculate how long any battery will last. Enter capacity (mAh or Wh) and current draw to get runtime in hours. Works for phones, laptops, power banks, and e-bikes.
Battery life depends on three things: how much energy the battery holds, how fast the device draws that energy, and the efficiency losses between the two. This calculator handles all three — enter your battery capacity and device current draw, and you'll get the runtime in hours.
How Long Will My Battery Last?
The core formula:
Battery life (hours) = Capacity (mAh) ÷ Current draw (mA) × Efficiency factor
The efficiency factor accounts for real-world losses from heat, voltage conversion, and battery chemistry degradation. For most lithium-ion batteries, use 0.85–0.90 (85–90% efficiency).
Example — smartphone:
- Battery: 4,500 mAh
- Average draw: 300 mA (moderate use: screen on, apps running)
- Efficiency: 0.85
4,500 ÷ 300 × 0.85 = 12.75 hours
That matches what most modern flagship phones deliver in real-world mixed use.
Common Battery Capacities and Expected Runtimes
| Device | Typical capacity | Typical draw | Expected runtime |
|---|---|---|---|
| AA alkaline battery | 2,500 mAh | 50 mA (TV remote) | ~42 hours |
| AA alkaline battery | 2,500 mAh | 500 mA (toy motor) | ~4 hours |
| Smartphone | 4,000–5,000 mAh | 200–400 mA | 10–20 hours |
| Wireless earbuds (case) | 500–800 mAh | 30 mA charging pods | 15–25 hours |
| Laptop | 45–100 Wh | 20–40W (light use) | 5–12 hours |
| Power bank (20,000 mAh) | 20,000 mAh | 500 mA per charge | ~28 phone charges (accounting for losses) |
| E-bike battery | 400–700 Wh | 200–500W motor | 40–100 km range |
mAh vs Wh — Which Should I Use?
For devices that run directly from a battery at a fixed voltage (like a torch from AA batteries), mAh is the right unit.
For devices that convert voltage internally (laptops, power banks, e-bikes), Wh (watt-hours) is more accurate, because the voltage step-up or step-down changes the effective capacity.
Converting between mAh and Wh:
Wh = mAh × Voltage ÷ 1,000
mAh = Wh ÷ Voltage × 1,000
Example — laptop battery:
- Stated capacity: 72 Wh at 11.1V
- In mAh: 72,000 ÷ 11.1 = 6,486 mAh
A 20,000 mAh power bank rated at 3.7V actually delivers:
- 20,000 × 3.7 ÷ 1,000 = 74 Wh of stored energy
- At 5V USB output: 74 Wh ÷ 5V = 14,800 mAh effective output
- After 85% conversion efficiency: ~12,580 mAh delivered to your device
This is why a "20,000 mAh" power bank only gives you 3–4 full phone charges, not 4–5.
How Many Charges From a Power Bank?
Charges = Power bank capacity (Wh) × Efficiency ÷ Phone battery (Wh)
Example — 20,000 mAh power bank charging a phone with a 4,500 mAh battery:
- Power bank stored energy: 20,000 × 3.7 ÷ 1,000 = 74 Wh
- Phone battery energy: 4,500 × 3.8 ÷ 1,000 = 17.1 Wh
- After 85% efficiency: 74 × 0.85 = 62.9 Wh delivered
- Charges: 62.9 ÷ 17.1 = 3.7 full charges
| Power bank | Phone charges (4,500 mAh phone) |
|---|---|
| 5,000 mAh | ~1 full charge |
| 10,000 mAh | ~2 full charges |
| 20,000 mAh | ~3.5 full charges |
| 26,800 mAh | ~4.5 full charges |
Why Does My Battery Die Faster Than the Rating?
Several factors reduce real-world battery life below the rated capacity:
Temperature: Cold degrades lithium-ion capacity significantly. At 0°C (32°F), you may get 20–30% less runtime. At −20°C, capacity can drop by 50%.
Age and charge cycles: Lithium-ion batteries lose capacity with each charge cycle. A battery at 500 cycles typically retains 80% of original capacity. At 1,000 cycles, it may be at 60–70%.
High discharge rates: Drawing more current than the battery's C-rating causes additional heating and efficiency losses. A 3,000 mAh battery at 3A discharge (1C) loses more capacity than at 300 mA (0.1C).
Voltage range: Battery capacity ratings are often measured to a lower cut-off voltage than the device actually uses. Some devices cut off at a higher voltage to protect the battery, using only 80–90% of rated capacity.
Extending Battery Life
For smartphones:
- Keep charge between 20–80% for daily use
- Avoid wireless charging for long sessions (generates more heat)
- Turn off background app refresh for apps you don't use live
- Reduce screen brightness — the display is typically the largest power draw
For laptops:
- Use battery health management settings (available in macOS, Windows 11, and most modern laptops)
- Keep the laptop cool — heat is the primary driver of long-term battery degradation
- Unplug external monitors and accessories when on battery
For power tools and e-bikes:
- Store batteries at 40–60% charge when not in use for extended periods
- Never store fully discharged — lithium-ion cells can be permanently damaged below a critical voltage threshold
Frequently Asked Questions
What does mAh mean?
mAh stands for milliampere-hours. It measures how much charge a battery can store. A 3,000 mAh battery can deliver 3,000 mA (3 amps) for 1 hour, or 300 mA for 10 hours, before it's depleted.
Why does my phone battery drain faster in cold weather?
Lithium-ion chemistry slows down at low temperatures, reducing the voltage and available capacity. The phone's battery management system may shut down the device early to protect cells from damage. The battery recovers when warmed up — cold weather doesn't permanently damage batteries unless the phone is charged while very cold.
How many charge cycles does a phone battery last?
Most lithium-ion batteries are rated for 300–500 full charge cycles before dropping to 80% of original capacity. Partial charges count as fractions of a cycle — charging from 50% to 100% counts as half a cycle. Apple, Samsung, and Google all recommend battery service around 500 full cycles.
Is it bad to leave my phone plugged in overnight?
Modern smartphones manage this automatically. iPhones use "Optimised Battery Charging" to pause charging at 80% and complete to 100% just before your usual wake time. Most Android phones have similar features. If your phone doesn't, charging overnight to 100% every night is harder on the battery than stopping at 80%.
What's the difference between capacity and runtime?
Capacity (mAh or Wh) is how much energy the battery can store. Runtime is how long the device runs on that energy. Runtime depends on how fast the device draws current — a high-drain device (gaming phone at full brightness) uses far more current than a standby device, giving much shorter runtime from the same capacity.
How do I measure my device's actual current draw?
A smart plug with energy monitoring (like TP-Link Kasa or Shelly) measures watts drawn from the wall. For battery-powered devices, a USB power meter (available for £5–£15/$5–$15) sits between the power bank and device and shows voltage, current, and watt-hours in real time.
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