Battery Capacity Calculator

How the Battery Capacity Calculator Works

The Battery Capacity Calculator is an essential tool that helps you convert between amp-hours (Ah) and watt-hours (Wh), and compute critical battery parameters such as voltage, C-rate, discharge current, and runtime. It is especially useful for professionals working with energy systems, electronics, and portable devices.

This tool is ideal for engineers, technicians, solar system designers, and anyone dealing with smartphones, laptops, electric vehicles, drones, or energy storage solutions who needs to properly size and evaluate battery performance.

Understanding the Basics of Battery Capacity

Battery capacity refers to the total amount of electrical energy a battery can store or deliver, commonly expressed in:

• Amp-hours (Ah): Current over time

• Watt-hours (Wh): Energy capacity

The relationship between them is governed by the formula:
E = V × Q
Where:

• E = Energy (Wh)

• V = Voltage (V)

• Q = Capacity (Ah)

This formula allows conversions between Wh and Ah when the battery’s voltage is known.

Example: Converting Wh to Ah

Using the calculator with:

• Voltage: 12V

• Stored energy: 600Wh

Formula used: Q = E ÷ V
Calculation: 600 Wh ÷ 12 V = 50.00 Ah

This means a 12V battery storing 600Wh has a total capacity of 50 amp-hours.

What Is C-Rate and Why It Matters?

C-rate indicates how quickly a battery is charged or discharged relative to its total capacity.

• 1C: Full capacity is used in 1 hour

• 2C: Full capacity is used in 30 minutes

• 0.5C: Full capacity is used in 2 hours

In our example, with a capacity of 50Ah and a C-rate of 1, the battery delivers:

• Discharge current: 50A

• Runtime: 1 hour or 60 minutes

This metric is critical for evaluating battery performance under load and sizing the correct battery for an application.

Formula Breakdown for Battery Calculations

The calculator uses well-established formulas:

1. Capacity (Ah) = Energy (Wh) ÷ Voltage (V)

2. Energy (Wh) = Voltage (V) × Capacity (Ah)

3. Discharge Current = Capacity × C-rate

4. Runtime = 1 ÷ C-rate (in hours)

These calculations are useful across a wide range of applications, from consumer electronics to industrial energy systems.

Common Battery Types and Their Specs

Here's a quick reference for popular batteries:

These values can help you quickly calculate runtime or energy storage for various applications using the calculator.

How to Use the Calculator for Different Needs

1. Converting Amp-Hours to Watt-Hours

If you have a battery rated at 20Ah and 24V:

• Energy = 20 × 24 = 480 Wh

2. Estimating Runtime

If a device consumes 10A and your battery has 50Ah:

• Runtime = 50 ÷ 10 = 5 hours

3. Determining Discharge Current

With a 100Ah battery and 2C rate:

• Discharge current = 100 × 2 = 200A

These insights are valuable for design, planning, and diagnostics in electrical systems.

What Affects Real Battery Performance?

While calculations give theoretical values, real-world performance can vary due to:

• Temperature changes

• Battery age and degradation

• Efficiency losses

• Internal resistance

• Discharge rates

For example, cold temperatures can reduce effective capacity by up to 30%, and aging batteries may have less than 80% of their original rated capacity.

Battery Runtime Table (12V Systems)

This table shows how scaling battery capacity affects runtime and discharge current.

Why Use a Battery Capacity Calculator?

This tool is especially useful when:

• Designing custom battery packs

• Sizing energy storage systems for solar panels

• Estimating how long a battery will last for a specific device

• Planning mobile or off-grid power solutions

• Comparing performance between different battery types

By using a calculator, you can avoid over-sizing (which wastes money and space) or under-sizing (which leads to poor performance or failure).

Tips for Accurate Battery Sizing

• Always check the nominal voltage before converting Wh to Ah

• Use the actual load current, not just theoretical maximums

• Factor in losses and real-world inefficiencies (10–20%)

• For critical systems, include a safety margin in capacity

• Regularly test battery health if used in long-term installations

A well-sized battery system ensures reliability, safety, and cost-effectiveness.