Battery Life Calculator

Calculate battery runtime based on capacity and load. Essential for power supply troubleshooting.

Time = Capacity (Ah) / Current (A)

How to use:

Enter battery capacity and load current to calculate runtime. Or enter capacity and runtime to find average current draw. Optionally use power and voltage for power-based calculations.

Faultfinding tip: Shorter than expected runtime may indicate battery degradation, higher than expected current draw, or a short circuit. Check for parasitic loads when device is "off".

Published: December 2025 | Author: TriVolt Editorial Team | Last Updated: February 2026

Understanding Battery Life Calculations

Battery life (runtime) is the duration a battery can power a load before needing recharge or replacement. Calculating battery life is essential for designing portable devices, backup power systems, and any application where battery power is used. Understanding how capacity, current draw, and power consumption affect runtime helps engineers design efficient systems and helps users understand device performance.

Battery capacity is typically specified in ampere-hours (Ah) or milliampere-hours (mAh), representing the total charge a battery can deliver. The actual runtime depends on how quickly this charge is consumed - the current draw or power consumption of the load. Simple calculations provide estimates, though actual performance varies with temperature, discharge rate, and battery age.

Battery Life Formula

The fundamental formula for battery runtime is:

Runtime = Capacity (Ah) / Current (A)

Result is in hours

For power-based calculations, first convert power to current:

Current (A) = Power (W) / Voltage (V)

Then: Runtime = Capacity / Current

Combining: Runtime = Capacity × Voltage / Power

Battery Capacity Units

Battery capacity is specified in several ways:

  • Ampere-Hours (Ah): Common for larger batteries (automotive, solar, UPS)
  • Milliampere-Hours (mAh): Common for smaller batteries (phones, laptops, portable devices)
  • Watt-Hours (Wh): Energy capacity (Ah × Voltage) - accounts for voltage differences

Conversion: 1 Ah = 1,000 mAh. Watt-hours provide a more accurate comparison across different battery voltages.

Factors Affecting Battery Life

Discharge Rate (C-Rate)

Batteries have different capacities at different discharge rates. High discharge rates (high current) reduce effective capacity. A battery rated at 10Ah at 1A discharge may only provide 9Ah at 5A discharge.

Temperature

Battery capacity decreases at low temperatures. Lithium batteries may lose 20-30% capacity at freezing temperatures. High temperatures can reduce lifespan but may slightly increase capacity.

Battery Age and Cycles

Battery capacity degrades over time and with charge/discharge cycles. A battery with 500 cycles may retain only 80% of original capacity.

Cutoff Voltage

Batteries are considered "empty" at a cutoff voltage (e.g., 3.0V for lithium). Some capacity remains below cutoff, but using it can damage the battery.

Practical Applications

Portable Device Design

Engineers calculate battery life to ensure devices meet runtime requirements. This influences battery selection, power management, and device efficiency optimization.

Backup Power Systems

UPS systems and backup batteries must provide sufficient runtime during power outages. Calculations determine battery bank sizing for required backup duration.

Solar and Off-Grid Systems

Battery banks for solar systems must store enough energy for nighttime and cloudy periods. Runtime calculations help size battery banks appropriately.

Troubleshooting

Comparing calculated vs. actual runtime helps identify problems: battery degradation, higher-than-expected current draw, or parasitic loads.

Real-World Examples

Example 1: Smartphone Battery

Phone battery: 3,000 mAh (3.0 Ah) at 3.7V, average current draw: 0.3A:

Runtime = 3.0 Ah / 0.3 A = 10 hours

Energy capacity = 3.0 Ah × 3.7V = 11.1 Wh

Typical smartphone usage provides 8-12 hours

Example 2: Car Battery

Car battery: 50 Ah at 12V, load: 5A:

Runtime = 50 Ah / 5 A = 10 hours

Energy = 50 Ah × 12V = 600 Wh

This assumes constant load - actual varies with engine starts, etc.

Example 3: Power-Based Calculation

Battery: 20 Ah at 12V, load: 60W:

Current = 60W / 12V = 5A

Runtime = 20 Ah / 5 A = 4 hours

Or directly: Runtime = (20 Ah × 12V) / 60W = 4 hours

Important Considerations

Peukert's Law

For lead-acid batteries, capacity decreases with discharge rate according to Peukert's Law. High current draws reduce effective capacity more than simple calculations suggest.

Depth of Discharge

Many batteries shouldn't be fully discharged. Lead-acid batteries typically use only 50% of capacity for long life. Lithium batteries can use 80-90% but shouldn't go below cutoff voltage.

Efficiency Losses

Voltage converters, battery management systems, and wiring have efficiency losses. Actual runtime may be 5-15% less than calculated due to these losses.

Variable Loads

Most devices have variable power consumption. Use average current for estimates, or calculate for different operating modes (idle, active, peak).

Tips for Using This Calculator

  • Enter capacity in Ah (convert mAh by dividing by 1000)
  • Enter current in amperes (convert mA by dividing by 1000)
  • For power-based calculations, enter voltage and power
  • Results show runtime in hours
  • Remember: actual runtime may be 10-20% less due to various factors
  • For variable loads, use average current or calculate for different modes
  • Always verify critical calculations independently, especially for safety-critical applications

Disclaimer

This calculator is provided for educational and informational purposes only. While we strive for accuracy, users should verify all calculations independently, especially for critical applications. Actual battery performance varies with temperature, age, discharge rate, and other factors. We are not responsible for any errors, omissions, or damages arising from the use of this calculator.

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