Objectives

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

  1. Perform a battery state-of-charge test using open-circuit voltage
  2. Explain how a conductance (electronic) tester evaluates battery health
  3. Explain the procedure and pass/fail criteria for a carbon pile load test
  4. Determine when each test method is appropriate
  5. Identify the importance of removing surface charge before testing
  6. Interpret test results and determine necessary action

ASE A6 Alignment

This module directly addresses:

Introduction

A battery that looks fine on the outside can be completely failed on the inside. The only way to know a battery's true condition is to test it. In this module, you will learn three test methods — voltage testing, conductance testing, and load testing. Each reveals different information about the battery, and knowing when to use each one is a critical technician skill.

3D battery — testing and diagnosis label

3D Battery Label Used: Battery Testing & Diagnosis (case back panel)

Battery Testing & Diagnosis label — Voltage Test, Load Test, and Conductance Test

Battery Testing & Diagnosis — 3D Battery Label

Pre-Test Requirements

Real automotive battery — visual inspection starting point

Real automotive battery — visual inspection starting point

Before any battery test, two conditions must be met:

1. Visual Inspection

Check for obvious problems before connecting test equipment:

2. State of Charge Verification

A battery must be at least 75% charged (12.4V OCV or 1.225 SG) before load testing or conductance testing. A discharged battery will fail a load test even if it is perfectly healthy.

If the battery is below 75%, charge it first, then retest.

Test 1: Open-Circuit Voltage (State of Charge)

Voltage Test — from the Battery Testing & Diagnosis label

Voltage Test — 3D Battery Label

Fluke multimeter reading 12.60V DC on a battery — open-circuit voltage test

Fluke Multimeter — 12.60V DC Open-Circuit Voltage

What It Tells You

Open-circuit voltage (OCV) indicates how much charge the battery currently holds. It does not tell you whether the battery is healthy — only whether it is charged.

Procedure

  1. Turn off all electrical loads (headlights, radio, etc.)
  2. Disconnect the battery or ensure no parasitic loads are drawing current
  3. Remove surface charge — let the battery sit for 12 hours, or turn on the headlights for 2–3 minutes, turn them off, then wait 2 minutes before reading
  4. Set your multimeter to DC volts
  5. Connect red lead to positive post, black lead to negative post
  6. Read the voltage

Why Remove Surface Charge?

Surface charge is a temporary elevated voltage present immediately after charging. It can make a discharged battery appear fully charged and give a false reading.

Interpreting Results

OCV Reading State of Charge Action
12.6V+ 100% Proceed to load or conductance test if needed
12.4V 75% Acceptable for further testing
12.2V 50% Charge before load testing
12.0V 25% Charge before any further testing
11.9V or below Discharged Charge fully and retest; if still low, battery may be sulfated
3D Battery Component: Connect multimeter leads to the positive and negative posts on the 3D battery to demonstrate proper placement.

Test 2: Conductance Test

Conductance Test — from the Battery Testing & Diagnosis label

Conductance Test — 3D Battery Label

Viking Battery System Analyzer — conductance tester showing GOOD result

Viking Battery System Analyzer — Harbor Freight

Snap-on battery tester reading 12.6V — electronic battery analyzer

Snap-on Battery Tester

What It Tells You

A conductance tester sends a small AC signal through the battery and measures the internal resistance of the plates and electrolyte. Lower resistance = healthier battery. Higher resistance = degraded plates, sulfation, or corroded connections.

Advantages Over Load Testing

Advantage Detail
Non-invasive Does not draw heavy current from the battery
Safe No risk of overheating
Fast Results in seconds
Works at any charge state Can test partially discharged batteries (some testers)
Repeatable Can test multiple times without waiting

Procedure

  1. Enter the battery's CCA rating and type (flooded or AGM) into the tester
  2. Connect tester leads — positive to positive, negative to negative
  3. Press test — the tester sends an AC signal and analyzes the response
  4. Read the results

Interpreting Results

Most conductance testers provide one of three results:

Important:

A "GOOD" conductance result does not mean testing is complete. Conductance only measures internal resistance — it cannot verify that the battery can deliver its rated capacity under real current draw. A battery that passes the conductance test must still be load tested to confirm capacity. The value of conductance testing is as a quick screening tool: a FAIL means replace immediately, but a PASS means continue testing.

Limitations

Test 3: Load Test (Capacity Test)

Load Test — from the Battery Testing & Diagnosis label

Load Test — 3D Battery Label

Viking 1000 AMP carbon pile load tester connected to battery

Viking 1000 AMP Carbon Pile Load Tester

What It Tells You

A load test determines whether the battery can deliver adequate current under a heavy electrical load — simulating the demand of a starter motor. It reveals the battery's capacity — not just its charge level.

A battery can be fully charged but still fail a load test if its plates are damaged, sulfated, or worn out.

Equipment

Procedure

  1. Verify the battery is at least 75% charged (12.4V OCV)
  2. Connect the load tester — positive clamp to positive post, negative clamp to negative post
  3. Look up the battery's CCA rating on the label
  4. Calculate the test load: 50% of CCA (e.g., 600 CCA battery → apply 300 amps)
  5. Apply the load for 15 seconds
  6. Read the voltage at the end of the 15-second load
  7. Remove the load immediately after reading

Pass/Fail Criteria

Condition Result
Voltage stays above 9.6V at 70°F (21°C) during load PASS
Voltage drops below 9.6V at 70°F during load FAIL

Temperature correction: At lower temperatures, the minimum acceptable voltage is lower:

Temperature Minimum Voltage
70°F (21°C) 9.6V
60°F (16°C) 9.5V
50°F (10°C) 9.4V
40°F (4°C) 9.3V
30°F (-1°C) 9.1V
20°F (-7°C) 8.9V
10°F (-12°C) 8.7V
0°F (-18°C) 8.5V

Interpreting Results

Result Action
Pass (above 9.6V) Battery has adequate capacity
Fail (below 9.6V) Charge the battery fully and retest once. If it fails again, replace the battery
Voltage drops steadily Normal — plates are sustaining the reaction
Voltage drops then recovers Surface charge was present — remove and retest

Limitations

When to Use Each Test

Situation Recommended Test
Quick state-of-charge check OCV (voltage test)
Customer complaint of slow cranking Conductance test (screen), then load test (confirm)
Battery warranty claim Load test (industry standard for warranty decisions)
Fleet maintenance screening Conductance test (quick screen), load test if questionable
Battery is partially discharged Charge first, then conductance + load test
Verifying a new battery before installation OCV + conductance test

Decision Tree

Battery Testing Decision Tree — flowchart showing diagnostic steps

Battery Testing Decision Tree

Key Takeaways

  1. Always visually inspect before testing
  2. Verify state of charge is at least 75% (12.4V) before load testing
  3. Remove surface charge before OCV testing — wait 12 hours or apply a brief load
  4. Load test: apply 50% of CCA for 15 seconds — must stay above 9.6V at 70°F
  5. Conductance test: enter correct CCA and battery type for accurate results
  6. If a battery fails a load test, charge and retest once before condemning it
  7. Conductance testing is a fast screen, but load testing is always needed to confirm capacity

Vocabulary

Term Definition
Carbon pile Adjustable resistance device used to apply a controlled load to a battery
CCA Cold Cranking Amps — the battery's rated current at 0°F for 30 seconds
Conductance A measurement of how easily current flows through the battery (inverse of resistance)
Internal resistance Opposition to current flow within the battery, caused by plate condition and electrolyte
Load test Applying 50% of CCA for 15 seconds and measuring voltage under load
Open-circuit voltage Voltage measured with no load and surface charge removed
Surface charge Temporary elevated voltage after recent charging