How to Calculate Battery Run Time?

This guide explains how to calculate battery run time using mAh, Wh, and watts, and how to use a battery run time calculator for real-world conditions.

It also explains why battery longevity calculators produce different results, focusing on depth of discharge, temperature, discharge rate, battery chemistry, and system efficiency.

Key Takeaways (Battery Run Time & Longevity)

• Battery run time depends on usable energy (Wh), not just mAh ratings.
• mAh ÷ mA and Wh ÷ W formulas are equivalent only when voltage and units are aligned.
• Depth of discharge, temperature, and system efficiency typically reduce ideal runtime by 10–40%.
• Lithium and LiFePO₄ batteries allow higher DoD and more stable runtime than lead-acid.
• Battery longevity calculators estimate lifespan, while runtime calculators estimate hours per charge—they solve different problems.

Part 1. What does battery run time mean?

Battery run time is the amount of time a battery can continuously power a device before it reaches its usable discharge limit. It is typically measured in hours and depends on both battery capacity and load demand.

For example, if a device draws 500 mA from a 4000 mAh battery, the theoretical runtime is 8 hours. However, real-world runtime is usually shorter due to efficiency losses, temperature, and discharge limits.

Part 2. How to calculate battery run time (accurate methods)

1 Current-based battery run time formula

Runtime (hours) = Battery Capacity (mAh) ÷ Load Current (mA)

Use this method for constant-current devices where current draw is known and relatively stable.

2 Power-based battery run time formula (Watts)

Runtime (hours) = (mAh × Voltage ÷ 1000) ÷ Load Power (W)

This method is preferred for devices specified in watts, such as inverters, laptops, and industrial equipment.

3 Practical battery run time estimate (recommended)

Runtime ≈ [(mAh × V ÷ 1000) ÷ W] × DoD × η

• DoD = Depth of Discharge (usable percentage)
• η = System efficiency (DC-DC, inverter, BMS losses)

Typical values: Li-ion / LiFePO₄ DoD 0.8–0.95, lead-acid ~0.5; system efficiency 0.85–0.95.

Part 3. Battery run time calculator (mAh, Wh, Watts)

This battery run time calculator supports current-based and power-based inputs and applies DoD and efficiency for realistic results.

Part 4. Understanding battery run time formulas

The mAh-based and watt-based formulas are mathematically equivalent when units are consistent:

• mAh → Ah = ÷1000
• Wh = Ah × V
• W = V × A

For engineering estimates, always convert to watt-hours (Wh) first. This avoids voltage-related errors and aligns with inverter and system-level calculations.

Part 5. Battery run time calculation examples

Example: 12V Battery with Inverter

• Battery: 100Ah @ 12V → 1200Wh
• Load: 400W AC
• DoD = 0.8, inverter η = 0.9

Usable energy ≈ 1200 × 0.8 × 0.9 = 864Wh
Runtime ≈ 864 ÷ 400 = 2.16 hours

Part 6. Why battery run time and longevity calculator results vary

Many users confuse a battery run time calculator with a battery longevity calculator. They answer different questions:

1 Battery chemistry

• Lithium-ion / LiFePO₄: higher usable DoD, stable voltage, longer cycle life
• Lead-acid: sensitive to deep discharge, lower usable capacity

2 Depth of discharge (DoD)

Higher DoD increases usable runtime per cycle but shortens overall battery lifespan.

Learn more: What Does Depth of Discharge Mean?

3 Charge & discharge rate (C-rate)

High discharge rates reduce effective capacity due to internal resistance and heat.

Reference: C-Rate Explained

4 Temperature

• Cold reduces available capacity (−10–30%)
• Heat accelerates degradation and reduces longevity

5 Charging behavior & maintenance

Overcharging, undercharging, and poor maintenance explain why real systems often underperform calculator estimates.

Part 7. FAQs: Battery run time & longevity