Objectives

By the end of this lesson, students will be able to:

  1. Explain the role of the electrolyte in energy storage and release
  2. Define specific gravity and explain its relationship to state of charge
  3. Use a specific gravity/voltage chart to determine battery condition
  4. Use open-circuit voltage to determine battery state of charge

ASE A6 Alignment

This module directly addresses:

Introduction

The Three Active Components — Positive Plate (PbO2), Negative Plate (Pb), and Electrolyte (H2SO4)

Plates label — positive plate (PbO2) and negative plate (Pb) active materials

In Module 1, you learned what is inside a battery. Now we answer the question: how does it actually work? The electrolyte — a mixture of sulfuric acid and water — is not just a passive liquid that sits in the battery. It is an active participant in every chemical reaction that stores and releases energy. Understanding the electrolyte is essential for diagnosing battery problems, because every test you perform is really measuring what is happening inside this liquid.

3D Battery Labels Used: Cell 1 (Electrolyte States), Cell 3 (Specific Gravity)

The Role of the Electrolyte

The electrolyte is not just a passive liquid that sits in the battery. It is an active participant in the chemical reaction.

During discharge:

During charging:

This is why the concentration of the electrolyte tells you the battery's state of charge. A fully charged battery has strong acid. A discharged battery has weak acid — mostly water.

Top view of battery showing 6 electrolyte cells

3D Battery Component: Cell 1 labels (Electrolyte States) explain this role — how the electrolyte carries ions, changes concentration, and serves as a direct indicator of battery condition.

Specific Gravity

State of Charge meter — SG 1.265 (fully charged) to SG 1.120 (discharged)

Battery front view — 6 cells with electrolyte labels

Specific gravity (SG) is the density of a liquid compared to water. Pure water has an SG of 1.000. Sulfuric acid is heavier than water, so battery electrolyte has an SG greater than 1.000.

SG and State of Charge

Because the acid concentration changes with charge state, measuring SG tells you how charged the battery is:

State of Charge Specific Gravity Open-Circuit Voltage
100% (fully charged) 1.265 12.6V
75% 1.225 12.4V
50% 1.190 12.2V
25% 1.155 12.0V
0% (discharged) 1.120 11.9V

Memory anchors:

Open-Circuit Voltage as a Charge Indicator

Since most modern automotive batteries are sealed (maintenance-free), you cannot directly measure the electrolyte's specific gravity. Instead, open-circuit voltage (OCV) is used to determine state of charge:

OCV State of Charge
12.6V or higher 100%
12.4V 75%
12.2V 50%
12.0V 25%
11.9V or lower Discharged

Important: The battery must sit for at least 12 hours (or have surface charge removed) for an accurate OCV reading. A battery fresh off a charger or just driven will show a higher voltage that does not reflect its true state of charge.

Interactive Animation: Built-in Hydrometer

See how specific gravity relates to charge state, then step through the three hydrometer readings: green (charged), dark (discharged), and clear (low electrolyte). Open in new tab ↗

Key Takeaways

  1. The electrolyte is an active participant, not just a conductor
  2. Specific gravity directly indicates state of charge (1.265 = full, 1.120 = dead)
  3. Open-circuit voltage is used to check state of charge (12.6V = full, 11.9V = dead)
  4. Accurate OCV requires the battery to sit for at least 12 hours

Vocabulary

Term Definition
Specific gravity Density of a liquid compared to water; used to measure electrolyte strength
Open-circuit voltage Battery voltage measured with no load connected and surface charge removed
Surface charge Temporary elevated voltage immediately after charging that does not reflect true state of charge