Battery Life Calculator – Calculate Runtime Using t = C / I with Step-by-Step Solutions
Battery life is one of the most common questions in electronics, engineering, and everyday life. How long will this battery last? Will my device run long enough? What size battery do I need?
The fundamental formula is simple: t = C / I, where t is runtime, C is battery capacity, and I is the current draw. But real-world factors like efficiency, discharge rates, and temperature can make a significant difference.
This calculator handles three different calculation modes based on what information you have available. Whether you need to find battery life from capacity and current, required capacity from desired runtime, or maximum current draw from capacity and runtime, this tool handles the math and unit conversions automatically.
Quick access: Use our free battery life calculator here
What Does This Calculator Do?
This tool calculates battery-related values using the fundamental formula: t = C / I.
Three calculation modes:
Calculate Battery Life (t = C / I) – Find runtime from capacity and current draw
Calculate Battery Capacity (C = t × I) – Find required capacity from desired runtime and current
Calculate Current Draw (I = C / t) – Find maximum current from capacity and desired runtime
Plus an efficiency factor – Apply a real-world efficiency percentage to account for losses.
Plus voltage input for Wh/kWh – When you use energy units (Wh or kWh), the calculator prompts for battery voltage so it can convert them to charge units (Ah) correctly.
Here's a quick example:
A 2.5 Ah battery powering a device drawing 0.5 A:
- Battery life: 5 hours
- Capacity: 2.5 Ah
- Current draw: 0.5 A
The calculator shows you exactly how it got the answer, including any unit conversions needed.
Understanding Battery Life Calculations
The Fundamental Formula
t = C / I
Where:
- t = Battery life / runtime (hours)
- C = Battery capacity (Ah or mAh)
- I = Current draw (A or mA)
Rearranged Formulas
- t = C / I – Find runtime from capacity and current
- C = t × I – Find required capacity from runtime and current
- I = C / t – Find maximum current from capacity and runtime
Key Concepts
- Capacity (C): Total charge a battery can store, measured in Ah (amp-hours) or mAh (milliamp-hours)
- Current (I): Rate of energy consumption, measured in A (amperes) or mA (milliamperes)
- Runtime (t): How long the battery can power the device, measured in hours, minutes, or seconds
Real-World Factors
- Peukert Effect: Higher current reduces effective capacity
- Temperature: Cold reduces capacity; heat degrades battery life
- Depth of Discharge (DoD): Deeper discharge reduces cycle life
- Efficiency: Real-world efficiency is rarely 100%
- Battery Chemistry: Different chemistries have different characteristics
Unit Support
This calculator handles all common battery units automatically:
Capacity Units
| Unit | Symbol | Conversion to Ah |
|---|---|---|
| Amp-hours | Ah | 1 |
| Milliamp-hours | mAh | 0.001 |
| Kiloamp-hours | kAh | 1000 |
| Watt-hours | Wh | Requires voltage: Ah = Wh / V |
| Kilowatt-hours | kWh | Requires voltage: Ah = (kWh × 1000) / V |
Note: Watt-hours (Wh) and kilowatt-hours (kWh) measure energy, not charge. To convert Wh to Ah, divide by the battery's voltage: Ah = Wh / V. The calculator provides a voltage input (default 3.7 V for Li-ion) that appears automatically when you select Wh or kWh as your capacity unit.
Current Units
| Unit | Symbol | Conversion to A |
|---|---|---|
| Amperes | A | 1 |
| Milliamperes | mA | 0.001 |
| Microamperes | µA | 0.000001 |
| Kiloamperes | kA | 1000 |
Time Units
| Unit | Symbol | Conversion to hours |
|---|---|---|
| Hours | hours | 1 |
| Minutes | minutes | 1/60 ≈ 0.01667 |
| Seconds | seconds | 1/3600 ≈ 0.000278 |
| Days | days | 24 |
How to Use the Calculator
Step 1: Choose Your Mode
Select one of three calculation modes:
- Battery Life – Use t = C / I
- Capacity – Use C = t × I
- Current – Use I = C / t
Step 2: Enter Your Values
Depending on the mode, enter the required values with their units:
For "Battery Life" mode:
- Battery capacity (C) with unit
- Current draw (I) with unit
For "Capacity" mode:
- Desired runtime (t) with unit
- Current draw (I) with unit
For "Current" mode:
- Battery capacity (C) with unit
- Desired runtime (t) with unit
Step 3: (Optional) Enter Battery Voltage for Wh/kWh
If you select Wh or kWh as your capacity unit, a voltage input appears automatically. Enter the battery's nominal voltage (e.g., 3.7 V for Li-ion, 12 V for lead-acid, 5 V for USB) to convert between energy and charge correctly.
Step 4: Optional Efficiency
Toggle the efficiency factor on and enter a percentage (e.g., 85%). This accounts for real-world losses in the battery system.
Step 5: Select Result Unit
Choose your preferred unit for the result.
Step 6: Calculate
Click the "Calculate" button. The results appear instantly.
Step 7: Review the Solution
The calculator shows detailed steps explaining how the result was derived, including all unit conversions and intermediate calculations.
Step-by-Step Examples for Each Mode
Example 1: Calculate Battery Life
Problem: A 2.5 Ah battery is powering a device that draws 0.5 A. How long will the battery last?
Step 1: Identify the given values
- C = 2.5 Ah
- I = 0.5 A
Step 2: Apply the formula
- t = C / I
- t = 2.5 / 0.5
- t = 5 hours
Result: The battery lasts 5 hours.
Example 2: Calculate Required Capacity
Problem: You need a device to run for 8 hours drawing 0.5 A. What battery capacity do you need?
Step 1: Identify the given values
- t = 8 hours
- I = 0.5 A
Step 2: Apply the formula
- C = t × I
- C = 8 × 0.5
- C = 4 Ah
Result: You need a 4 Ah battery.
Example 3: Calculate Maximum Current Draw
Problem: A 10 Ah battery needs to last 5 hours. What is the maximum current draw?
Step 1: Identify the given values
- C = 10 Ah
- t = 5 hours
Step 2: Apply the formula
- I = C / t
- I = 10 / 5
- I = 2 A
Result: The maximum current draw is 2 A.
Example 4: Battery Life with Efficiency Factor
Problem: A 2.5 Ah battery powers a device drawing 0.5 A, but the system has 85% efficiency. What is the actual runtime?
Step 1: Identify the given values
- C = 2.5 Ah
- I = 0.5 A
- Efficiency = 85% = 0.85
Step 2: Calculate theoretical runtime
- t_theoretical = C / I = 2.5 / 0.5 = 5 hours
Step 3: Apply efficiency
- t_actual = t_theoretical × efficiency
- t_actual = 5 × 0.85 = 4.25 hours
Result: Actual runtime is 4.25 hours.
Example 5: Using Wh with Voltage Input
Problem: A laptop battery has 60 Wh of energy at 11.1 V. It powers a device drawing 2 A. How long will it last?
Step 1: Identify the given values
- C = 60 Wh
- Voltage = 11.1 V
- I = 2 A
Step 2: Convert Wh to Ah using voltage
- C = 60 Wh / 11.1 V ≈ 5.41 Ah
Step 3: Apply the formula
- t = C / I
- t = 5.41 / 2
- t ≈ 2.70 hours
Result: The battery lasts about 2.7 hours.
Understanding Battery Capacity
What is Ah (Amp-hour)?
An amp-hour (Ah) is a unit of electric charge. It represents the amount of charge a battery can deliver over time. A 1 Ah battery can deliver 1 A for 1 hour, or 0.5 A for 2 hours.
What is Wh (Watt-hour)?
A watt-hour (Wh) is a unit of energy, not charge. It equals voltage × amp-hours: Wh = V × Ah. Two batteries with the same Wh rating but different voltages will have different Ah ratings. For example:
- 3.7 V, 2.5 Ah → 9.25 Wh
- 12 V, 2.5 Ah → 30 Wh
This is why Wh-to-Ah conversion requires knowing the voltage. The calculator provides a voltage input that appears automatically when you select Wh or kWh as your capacity unit.
Typical Battery Capacities
| Device | Typical Capacity | Notes |
|---|---|---|
| AA alkaline | ~2.5 Ah | 1.5 V nominal |
| Smartphone | 3–5 Ah (11–19 Wh) | 3.7–3.85 V nominal |
| Tablet | 6–10 Ah (22–38 Wh) | 3.7–3.85 V nominal |
| Laptop | 3–6 Ah (40–70 Wh) | 11.1–15.4 V nominal |
| Car battery | 50–70 Ah | 12 V nominal |
| Electric vehicle | 150–270 Ah (60–100 kWh) | 350–400 V pack |
Note: Capacity values vary widely by model and chemistry. The Ah/Wh pair depends on the nominal voltage of the battery.
Capacity vs. Runtime
- Larger capacity → Longer runtime (all else equal)
- Higher current draw → Shorter runtime (all else equal)
- Runtime = Capacity ÷ Current
When to Use Each Mode
| Mode | Formula | When to Use | Typical Scenario |
|---|---|---|---|
| Battery Life | t = C / I | You have capacity and current | Estimating how long a device will run |
| Capacity | C = t × I | You have runtime and current | Sizing a battery for a target runtime |
| Current | I = C / t | You have capacity and runtime | Determining the sustainable current draw |
Common Questions About Battery Life
Q: What is battery life?
Battery life (or runtime) is the amount of time a battery can power a device before needing recharge. It depends on battery capacity and the device's current draw.
Q: What's the formula for battery life?
The formula is t = C / I, where t is runtime, C is capacity in Ah, and I is current in A.
Q: What is the difference between Ah and mAh?
Ah (amp-hours) and mAh (milliamp-hours) are both units of battery capacity. 1 Ah = 1000 mAh. mAh is often used for smaller batteries like in smartphones and AA batteries.
Q: What is Wh (watt-hour)?
Watt-hour (Wh) measures battery energy capacity, not charge. It's calculated as voltage (V) × Ah: a 3.7 V, 2.5 Ah battery has 9.25 Wh of energy. Because Wh depends on voltage, converting Wh to Ah requires dividing by the battery's nominal voltage — there is no single fixed conversion factor. The calculator provides a voltage input (default 3.7 V) that appears when you select Wh or kWh as your capacity unit.
Q: What voltage should I use for Wh-to-Ah conversion?
Use the battery's nominal voltage — the average voltage during discharge. Common values:
- Li-ion: 3.7 V
- LiFePO₄: 3.2 V
- Lead-acid: 12 V (per cell: 2 V)
- USB power: 5 V
- Laptop battery pack: 11.1 V or 14.8 V
If you're unsure, check the label on the battery or the device's specifications.
Q: What is the Peukert Effect?
The Peukert Effect describes how higher discharge rates reduce a battery's effective capacity. Drawing more current means the battery delivers less total capacity than its rating suggests. Peukert's equation is t = C / I^k, where k is the Peukert constant (about 1.1–1.4 for lead-acid, closer to 1.05 for lithium). For high-current draws, the simple t = C / I formula overestimates runtime.
Q: How does efficiency affect battery life?
Real-world efficiency is rarely 100%. Factors like heat, internal resistance, and conversion losses reduce the effective runtime. This calculator includes an efficiency factor to account for these losses.
Q: Why consider Depth of Discharge (DoD)?
Depth of Discharge (DoD) affects battery cycle life. For lead-acid batteries, keeping above 50% DoD significantly extends lifespan. For lithium batteries, 80% DoD is common.
Q: What current do common devices draw?
- LED: 20–50 mA
- Arduino: 50–200 mA
- Smartphone: 500–2000 mA
- Laptop: 1–4 A
- Motor (small): 1–10 A
- Motor (large): 10–100+ A
Tips for Getting the Best Results
Choose the right mode. Make sure you're calculating what you need — battery life, capacity, or current.
Check your units. The calculator handles conversions automatically, but make sure you're entering the right values with the right units. Remember that Wh and kWh require a voltage to convert to Ah — the calculator prompts for it automatically.
Use the correct voltage for Wh/kWh. For Li-ion, use 3.7 V; for lead-acid, use 12 V; for USB, use 5 V. The default is 3.7 V (Li-ion), but you can change it.
Consider efficiency. Real-world efficiency is rarely 100%. Use the efficiency toggle for more accurate results.
Double-check your inputs. A single digit error changes everything. Take a moment to verify each number.
Review the steps. The step-by-step solution helps you understand the process and verify the calculation.
Account for Peukert Effect. For high-current applications, remember that effective capacity decreases with higher current draw. Use t = C / I^k instead of t = C / I when accuracy matters.
Consider temperature. Battery performance changes with temperature — cold reduces capacity, heat degrades battery life.
Final Thoughts
Understanding battery life is essential for anyone working with electronics, designing battery-powered devices, or simply choosing the right battery for a project. The formula t = C / I is simple but powerful — it connects capacity, current, and runtime in a way that applies to everything from smartphones to electric vehicles.
This calculator handles all three variants of the battery formula, with full unit support (Ah, mAh, kAh, Wh, kWh for capacity; A, mA, µA, kA for current; hours, minutes, seconds, days for time), an automatic voltage input for Wh/kWh conversions, an efficiency factor for real-world losses, and step-by-step solutions.
Whether you're estimating runtime, sizing a battery, or checking current draw, this tool handles the math while you focus on understanding the power system.
