Battery Life Calculator Guide: Capacity, Runtime & Efficiency
Quick Answer
- *Runtime = Battery capacity (mAh) ÷ Load current (mA) = hours.
- *Multiply by 0.7–0.85 for real-world efficiency losses.
- *mAh = milliamp-hours. Higher mAh = longer runtime at same load.
- *Li-ion batteries lose ~20% capacity after 500 charge cycles.
The Basic Formula
Battery runtime calculation is simple in theory: divide capacity by load. A 5000mAh battery powering a device that draws 250mA should last 5000/250 = 20 hours. That’s the theoretical maximum. Real-world results are always lower.
Practical Runtime = (Capacity × Efficiency Factor) / Load
The efficiency factor accounts for voltage regulation losses, varying current draw, and battery chemistry limitations. For most applications, use 0.7 to 0.85.
Understanding mAh and Wh
mAh (milliamp-hours) tells you how much current a battery can deliver over time at its nominal voltage. But it doesn’t account for voltage. A 10,000mAh power bank at 3.7V stores much less energy than 10,000mAh at 12V.
Watt-hours (Wh) is the more accurate measure: Wh = mAh × V / 1000. A 10,000mAh / 3.7V battery stores 37Wh. Airlines limit carry-on batteries to 100Wh (about 27,000mAh at 3.7V) for safety reasons.
Why Theoretical and Actual Differ
| Factor | Typical Loss | Why |
|---|---|---|
| Voltage regulation | 10–20% | Buck/boost converters waste energy as heat |
| Variable load | 5–15% | Devices don’t draw constant current |
| Temperature | 10–30% | Cold reduces chemical reaction rates |
| Battery age | 0–20% | Capacity degrades with charge cycles |
| Cutoff voltage | 5–10% | Device shuts off before battery is fully drained |
Battery Chemistry Comparison
| Chemistry | Nominal Voltage | Energy Density | Cycle Life |
|---|---|---|---|
| Alkaline (AA) | 1.5V | ~130 Wh/kg | Single use |
| NiMH (AA) | 1.2V | ~80 Wh/kg | 500–1000 cycles |
| Li-ion (18650) | 3.7V | ~250 Wh/kg | 300–500 cycles |
| LiFePO4 | 3.2V | ~90 Wh/kg | 2000–5000 cycles |
| Lead acid | 2.0V/cell | ~35 Wh/kg | 200–300 cycles |
C-Rate and Discharge Curves
C-rate describes charge/discharge speed relative to capacity. 1C for a 3000mAh battery = 3A. Higher C-rates reduce effective capacity due to internal resistance and heat. A battery rated at 3000mAh at 0.2C might only deliver 2700mAh at 1C.
Discharge curves show how voltage drops over time. Lithium batteries hold relatively flat voltage until about 80% discharge, then drop sharply. This is why your phone shows 20% battery for a while, then dies quickly.
Extending Battery Life
For the battery itself: Keep lithium batteries between 20–80% charge when possible. Store at 40–60% in a cool place. Avoid fast charging unless necessary. Each fast-charge cycle degrades capacity slightly more than slow charging.
For device runtime: Reduce screen brightness (the biggest power draw on phones). Disable unused radios (Bluetooth, Wi-Fi, GPS). Use dark mode on OLED screens. Enable power-saving modes, which throttle CPU speed and reduce background activity.
Calculate battery runtime for your project
Use our free Battery Life Calculator →Frequently Asked Questions
How do I calculate battery life from mAh?
Divide capacity by load: Runtime = mAh / mA. A 3000mAh battery at 150mA = 20 hours theoretical. Apply 0.7–0.85 efficiency for real-world runtime.
What does mAh mean on a battery?
Milliamp-hours — how much current a battery delivers over time. 5000mAh = 5000mA for 1 hour, or 500mA for 10 hours. Higher mAh = longer runtime.
Why does my battery not last as long as calculated?
Voltage regulation losses (10–20%), temperature effects, battery aging, and variable load current all reduce real-world runtime. Use a 0.7–0.85 efficiency multiplier.
What is the C-rate of a battery?
C-rate = discharge speed relative to capacity. 1C for 2000mAh = 2A discharge. Higher C-rates reduce effective capacity and increase heat.
How do I extend battery life?
Keep lithium batteries between 20–80% charge, store at 40–60%, avoid extreme temperatures, reduce screen brightness, and disable unused radios.