Battery Life Calculator
Enter your battery capacity (mAh) and device current draw (mA) to instantly estimate runtime. Switch to "Required Capacity" mode to find the minimum mAh needed for a target runtime. Adjust the efficiency slider to match your battery chemistry and operating conditions.
Real-world batteries deliver 70–90% of rated capacity. Default 80% accounts for discharge curve losses.
Frequently Asked Questions
How do you calculate battery life from mAh?
Battery life in hours = (Battery Capacity in mAh × Efficiency) ÷ Average Current Draw in mA. For example, a 2,000 mAh battery powering a 200 mA device at 80% efficiency lasts (2000 × 0.8) ÷ 200 = 8 hours. The efficiency factor (typically 0.7–0.9) accounts for discharge curve losses, temperature effects, and battery aging — real batteries never deliver 100% of their rated capacity.
What does mAh mean on a battery?
mAh stands for milliamp-hours, a unit of electric charge that indicates how much energy a battery stores. A 3,000 mAh battery can theoretically supply 3,000 mA for 1 hour, or 300 mA for 10 hours. Higher mAh means more energy stored and potentially longer runtime, assuming the same device current draw. Note that mAh alone doesn't tell you total energy — a 3,000 mAh battery at 3.7 V stores far more energy than the same capacity at 1.5 V.
Why is battery efficiency less than 100%?
Real batteries lose usable capacity due to several factors: (1) Discharge curve — voltage drops as the battery depletes, and most devices stop working before the battery is fully empty; (2) Temperature — cold weather can cut capacity by 20–50%; (3) Age and cycle count — rechargeable cells lose 20% capacity after ~500 full cycles; (4) Self-discharge — batteries lose charge while sitting unused. A default efficiency of 80% is a safe conservative estimate for most lithium-ion applications.
How long does an AA battery last?
A fresh AA alkaline battery holds roughly 2,400–3,000 mAh. At 80% efficiency and a 200 mA load, it would last about (2400 × 0.8) ÷ 200 = 9.6 hours. For very low-power devices like TV remotes (≈1 mA average), the same battery could last over 1,900 hours. Runtime depends heavily on current draw — use the calculator above with your device's actual consumption for an accurate estimate.
How many mAh do I need for 24 hours of battery life?
Required capacity (mAh) = ⌈ (Current Draw × Desired Hours) ÷ Efficiency ⌉. For a device drawing 100 mA and a target of 24 hours at 80% efficiency: (100 × 24) ÷ 0.8 = 3,000 mAh. Use the 'Required Capacity' mode in the calculator above to compute this automatically. Always add a 20–50% design margin to account for battery aging and worst-case temperature conditions.
What is the difference between mAh and Wh?
mAh (milliamp-hours) measures charge, while Wh (watt-hours) measures energy. To convert: Wh = mAh × Voltage ÷ 1000. A 3,000 mAh Li-Ion cell at 3.7 V stores 3000 × 3.7 ÷ 1000 = 11.1 Wh. Wh is the better measure for comparing batteries of different voltages, and it is what airlines use for carry-on battery size limits. For single-voltage systems, mAh is sufficient for runtime calculations.
How do I measure my device's current draw?
The most direct method is to place a multimeter set to DC mA in series with the power supply wire. For detailed profiling (including sleep modes and bursts), a dedicated power analyzer like the Nordic Power Profiler Kit II or Otii Arc captures microsecond-level current traces and calculates average consumption. Check your component datasheets for typical and maximum supply current figures, and sum all active components to estimate total draw before measuring.
Does battery capacity decrease in cold weather?
Yes, significantly. Lithium-ion cells lose roughly 20% capacity at 0°C and 40–50% at −20°C due to increased internal resistance slowing electrochemical reactions. Alkaline batteries are even more affected, losing up to 50% at freezing temperatures. For outdoor or cold-environment applications, use a lower efficiency value in the calculator (60–70%) and consider heated enclosures or lithium primary chemistries, which perform better in extreme cold than rechargeable types.
Battery Life Calculator: How to Estimate Runtime from mAh & Current
The Battery Life Calculator estimates how long a battery will power a device based on its capacity in milliamp-hours (mAh) and the device's average current draw in milliamps (mA). It also calculates the minimum battery capacity required to meet a target runtime — essential for embedded systems design, IoT deployments, portable electronics, and emergency backup planning.
Contents
Battery Life Formula
The core formula for estimating battery runtime is straightforward:
Runtime (hours) = (Capacity × Efficiency) ÷ Current Draw
- Capacity — battery rated capacity in mAh
- Efficiency — usable fraction (0.0–1.0, typically 0.8)
- Current Draw — average device consumption in mA
Example: A 3,000 mAh lithium-ion cell powering a 250 mA circuit at 80% efficiency:
To find the minimum capacity needed for a desired runtime, rearrange the formula:
Efficiency Factors
No battery delivers 100% of its rated capacity under real-world conditions. Several factors reduce usable capacity:
Discharge Curve Losses (5–15%)
Battery voltage drops as charge depletes. Most devices have a minimum operating voltage cutoff, leaving some charge inaccessible. This is the primary efficiency loss for lithium and alkaline chemistries.
Temperature Effects
Cold temperatures reduce capacity significantly — a lithium cell can lose 20–30% capacity at 0°C and up to 50% at −20°C. High temperatures accelerate aging and self-discharge. Use a lower efficiency figure for outdoor or cold-chain applications.
Battery Age & Cycle Count
Rechargeable batteries degrade over charge cycles. A lithium-ion cell at 500 cycles typically retains only 80% of original capacity. For aging cells, reduce the efficiency figure accordingly.
Self-Discharge
All batteries lose charge while idle. Alkaline cells self-discharge about 2% per year; NiMH up to 20% per month; lithium-ion about 2–5% per month. For long storage periods, account for self-discharge when sizing a battery.
Recommended Efficiency by Chemistry
| Chemistry | Typical Efficiency | Notes |
|---|---|---|
| Lithium-Ion (Li-Ion) | 80–90% | Flat discharge curve, good efficiency |
| Alkaline (AA/AAA/9V) | 70–85% | Voltage sags under load |
| NiMH | 75–85% | High self-discharge reduces effective capacity |
| LiFePO4 | 90–95% | Very flat curve, long cycle life |
| Lithium Primary (CR2032) | 75–85% | Very low self-discharge, good shelf life |
Common Battery Capacities
Use this reference table to quickly select the right battery for your application or to verify datasheet values when using the calculator.
| Battery Type | Typical Capacity | Voltage | Common Use |
|---|---|---|---|
| AA Alkaline | 2,400–3,000 mAh | 1.5 V | Remotes, clocks, toys |
| AAA Alkaline | 1,000–1,200 mAh | 1.5 V | Small remotes, LED lights |
| 9V Alkaline | 500–600 mAh | 9 V | Smoke detectors, effects pedals |
| CR2032 (Lithium coin) | 210–240 mAh | 3 V | Watches, RTC, key fobs |
| 18650 Li-Ion | 2,500–3,500 mAh | 3.6–3.7 V | Laptops, flashlights, power banks |
| 21700 Li-Ion | 4,000–5,000 mAh | 3.6–3.7 V | EVs, high-drain devices |
| Smartphone battery (typical) | 3,000–5,000 mAh | 3.7–3.85 V | Mobile phones |
| LiPo 1000 mAh | 1,000 mAh | 3.7 V | Drones, RC vehicles, wearables |
Capacities are typical rated values at 0.2C discharge rate at 25°C. Actual values vary by manufacturer and discharge rate.
Tips for Maximising Battery Life
Reduce Peak Current
Use sleep modes and duty cycling to minimise average current. A microcontroller drawing 50 mA active for 10% of the time averages only 5 mA — a 10x improvement.
Use Efficient Regulators
Linear regulators waste the voltage difference as heat. Switching regulators (buck/boost) achieve 85–95% efficiency. This directly improves effective battery life.
Measure Actual Current
Use a multimeter or power profiler (e.g. Nordic PPK2) to measure real average current draw. Datasheets list typical values; your circuit may differ.
Add a Safety Margin
Design for 120–150% of the minimum required capacity. This accounts for battery aging, temperature variation, and unexpected load spikes.