Title: Module 6: Battery Failure & Diagnosis - Lesson
Course: ASE A6 Battery Service > Section: Module 6: Battery Failure & Diagnosis

Objectives

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

1. Identify the primary causes and effects of battery sulfation
2. Describe plate shedding and its role in internal short circuits
3. Explain cell imbalance and how the weakest cell limits overall battery performance
4. Perform a parasitic (key-off) battery drain test using a DMM
5. Identify common sources of parasitic draw and apply the fuse-pulling isolation method
6. Read battery date codes and determine warranty eligibility

ASE A6 Alignment

This module directly addresses:

- TASK 5 Confirm parasitic (key-off) battery drain; determine necessary action

Introduction

In Module 3, you learned how to test a battery and determine whether it passes or fails. In Module 4, you learned how to charge a battery correctly. But what happens when a battery fails, and why? This module answers those questions. Understanding failure modes helps you explain to a customer why their battery died and prevents you from simply replacing a battery when the real problem is something else entirely.

A battery that keeps dying is not always a bad battery. It may be a parasitic draw, a charging system problem, or a wiring issue draining it overnight. Diagnosing the root cause is what separates a parts replacer from a technician.

3D Battery Labels Used: Sulfation (Cell 5), Failure Modes (Cell 6)

Sulfation

What Is Sulfation?

During normal discharge, lead sulfate (PbSO4) forms on both the positive and negative plates. This is part of the normal chemical reaction. When the battery is recharged, the lead sulfate converts back to active material — lead dioxide on the positive plate and sponge lead on the negative plate.

Sulfation occurs when lead sulfate is allowed to remain on the plates for extended periods without recharging. The soft, amorphous lead sulfate gradually transforms into hard, crystalline lead sulfate that resists conversion back to active material.

3D Battery Component: Examine the electrolyte cells and the plate paste on the plate grids. The paste represents the active material that converts during charge and discharge. When sulfation occurs, this active material is replaced by hard, white crystal deposits that no longer participate in the chemical reaction.

Causes of Sulfation

Cause | How It Happens
Chronic undercharging | Battery never reaches full charge; residual sulfate hardens over time
Prolonged storage without a maintainer | Self-discharge allows sulfate to accumulate unchecked
Short trip driving | Alternator does not run long enough to fully recharge the battery
High temperatures | Accelerates the crystallization of lead sulfate
Low electrolyte level | Plates exposed above the electrolyte sulfate rapidly and permanently

The most common real-world cause is chronic undercharging. A vehicle driven only on short trips, a battery left sitting for months, or an alternator that is slightly underperforming will all produce the same result: lead sulfate that never gets fully converted, hardens, and permanently reduces the battery's ability to store energy.

Effects of Sulfation

- Reduced capacity — hard sulfate occupies plate surface area that would otherwise participate in the chemical reaction
- Increased internal resistance — sulfate crystals are poor conductors, reducing the battery's ability to deliver current
- Higher charging voltage required — the charger must work harder to push current through the resistant sulfate layer
- Premature failure — a sulfated battery may test good on a voltage check but fail under load

Can Sulfation Be Reversed?

Sulfation Type | Reversible? | Method
Soft sulfation (early stage) | Sometimes | Slow charge at low amperage over 24-72 hours
Hard sulfation (crystallized) | Rarely | Desulfation chargers may help slightly, but usually the battery is beyond recovery

Key point: Prevention is far more effective than cure. Keeping a battery fully charged prevents sulfation from forming in the first place.

3D Battery Component: The Cell 5 label (Sulfation) illustrates the progression from normal plate material to sulfated plates. Compare this to the clean plate paste on the plate grids.

Plate Shedding

What Is Plate Shedding?

Over time, the active material (plate paste) on the positive and negative plates physically breaks away from the plate grids and falls to the bottom of the cell. This accumulated material is called sediment or mud.

3D Battery Component: Look at the plate grids in the 3D battery. The grid is the structural framework — a lattice of lead alloy. The plate paste is pressed into this grid. When plate shedding occurs, the paste separates from the grid, leaving bare lattice with reduced active surface area.

Causes of Plate Shedding

Cause | Mechanism
Vibration | Road vibration loosens paste from the grid over time
Overcharging | Excessive gassing creates mechanical stress on the paste
Deep cycling | Repeated deep discharges expand and contract the paste, weakening adhesion
Age | Natural degradation of the bond between paste and grid
Manufacturing defects | Poor paste adhesion from the factory

Effects of Plate Shedding

1. Reduced capacity — less active material on the plates means less chemical reaction surface area
2. Sediment buildup — shed material collects at the bottom of each cell
3. Internal short circuits — if sediment builds high enough to touch the bottom of the plates, it creates a conductive path between the positive and negative plates within a cell
4. Cell failure — a shorted cell drops from 2.1V to near 0V, reducing the battery from 12.6V to approximately 10.5V

Identifying Plate Shedding

- Battery fails a load test but may show acceptable OCV
- One cell reads significantly lower voltage than the others
- Battery case may feel warmer on one side (shorted cell generates heat)
- In flooded batteries with translucent cases, sediment may be visible at the bottom

Cell Imbalance

What Is Cell Imbalance?

A 12-volt lead-acid battery contains six cells connected in series. Each cell produces approximately 2.1 volts when fully charged. In a perfect battery, all six cells age identically. In reality, manufacturing tolerances, temperature differences across the case, and uneven electrolyte mixing cause cells to age at different rates.

The weakest cell limits the entire battery.

How Cell Imbalance Develops

Factor | Effect
Uneven temperature exposure | Cells closer to the engine run hotter, aging faster
Manufacturing variation | Slight differences in plate thickness or paste density
Electrolyte stratification | Acid concentration varies within a cell, causing uneven plate wear
One cell shorting partially | A developing short pulls down the entire battery

Diagnosing Cell Imbalance

Individual cell voltage testing is the most direct method. On flooded batteries with removable caps, a cadmium probe test or individual cell voltage reading can identify a weak cell:

Cell Condition | Expected Voltage
Healthy, fully charged | 2.10-2.15V
Weak or aging | 1.95-2.05V
Failing or shorted | Below 1.90V or 0V

If five cells read 2.12V and one reads 1.95V, the battery will underperform despite having five good cells. The weak cell limits the total voltage and available capacity.

Open-circuit voltage check is a simpler screening method:

OCV Reading | Likely Condition
12.6V | All cells balanced and healthy
12.4V | Slight imbalance or partial discharge
10.5V | One cell likely shorted (5 cells x 2.1V = 10.5V)

3D Battery Component: Count the six electrolyte cells in the 3D battery. Each one is an independent electrochemical unit connected in series. When one fails, the entire battery suffers — just like one weak link in a chain.

Parasitic Draw Testing

What Is Parasitic Draw?

A parasitic draw — also called key-off drain or dark current — is the small amount of electrical current that flows from the battery when the vehicle is off and the key is removed. Every modern vehicle has some parasitic draw because certain modules require constant power: the clock, the radio memory, the body control module, the anti-theft system, and others.

Acceptable vs Excessive Draw

Draw Level | Interpretation
25-50 mA (0.025-0.050A) | Normal for most vehicles
50-85 mA | Borderline — check manufacturer specs
Above 85 mA | Excessive — diagnose and repair

A 50 mA draw on a 60 Ah battery would theoretically drain the battery in approximately 50 days (60,000 mAh / 50 mA = 1,200 hours). In practice, batteries reach an unusable state well before full discharge, so a borderline draw combined with short-trip driving can kill a battery in weeks.

Parasitic Draw Test Procedure

Equipment needed: Digital multimeter (DMM) set to DC amps (milliamp range)

Step-by-step procedure:

1. Turn off all accessories — lights, radio, HVAC, everything
2. Remove the key from the ignition (or ensure the vehicle is fully off for push-button start)
3. Close all doors and ensure interior lights are off (use a door latch tool or tape the door switch)
4. Wait for modules to go to sleep — this can take 20-60 minutes on modern vehicles. Some luxury vehicles require up to 90 minutes
5. Disconnect the negative battery cable
6. Connect the DMM in series between the negative cable and the negative battery post

- Set the meter to the highest amperage scale first (10A or 20A) to avoid blowing the milliamp fuse
- Once you confirm the draw is low, switch to the milliamp scale for accurate reading

1. Read the draw — record the steady-state value after modules settle
2. Compare to specification — typical acceptable range is 25-50 mA

Critical warning: Do NOT open doors, turn on lights, or activate any systems during the test. Any activation will wake modules and produce a false high reading.

Fuse-Pulling Isolation Method

When the parasitic draw is excessive, you need to find which circuit is responsible:

1. With the ammeter still connected and showing the excessive draw, begin pulling fuses one at a time from the interior and underhood fuse boxes
2. After pulling each fuse, observe the ammeter reading
3. When the draw drops to an acceptable level, the circuit protected by that fuse contains the source of the draw
4. Check the fuse label to identify which components are on that circuit
5. Investigate each component on the circuit individually
6. Reinstall each fuse before pulling the next one (unless the draw dropped)

Common Sources of Excessive Parasitic Draw

Source | Why It Happens
Body control module (BCM) not sleeping | Software glitch, faulty door/hood switch keeping module awake
Aftermarket accessories | Poorly wired stereo, alarm, dash cam, or LED lights not switching off
Trunk or glove box light staying on | Faulty switch — light stays on when closed
Faulty relay | Relay contacts welded closed, keeping a circuit energized
Infotainment system | Module fails to enter sleep mode after shutdown
Seat module | Power seat module stays active due to faulty position sensor
Charging port | USB or 12V accessory port providing power when vehicle is off

3D Battery Component: The 3D battery does not contain electrical circuits, but it clearly shows why parasitic drain is destructive. A slow, continuous drain prevents the electrolyte cells from maintaining full acid concentration. Over time, the plate paste sulfates, and the battery's capacity diminishes irreversibly.

Battery Age and Warranty

Reading Battery Date Codes

Most batteries have a date code stamped or stickered on the case or terminal. The most common format uses a letter-number system:

Code Element | Meaning
Letter (A-L) | Month (A = January, B = February, ... L = December)
Number (0-9) | Last digit of the year

Examples:

Date Code | Translation
C5 | March 2025
J4 | October 2024
A6 | January 2026
F3 | June 2023

Some manufacturers use different formats — always check the battery label or manufacturer documentation for the specific format.

Expected Battery Lifespan

Climate | Typical Lifespan
Hot climate (southern states) | 3-4 years
Moderate climate | 4-5 years
Cold climate (northern states) | 4-6 years

Heat is the primary enemy of battery longevity. High temperatures accelerate plate corrosion, water loss, and paste degradation. A battery in Phoenix may last 3 years; the same battery in Minneapolis may last 5-6 years.

Warranty Claims

When a customer requests a warranty replacement:

1. Verify the date code — confirm the battery is within the warranty period
2. Test the battery — most warranty programs require a failed load test or conductance test result
3. Determine pro-rated vs full replacement — many warranties offer full replacement for the first period (e.g., 2 years) and pro-rated credit for the remainder
4. Document the failure — record test results, date code, and customer information
5. Check for external causes — if the failure is caused by a parasitic draw, loose hold-down, or charging system problem, the warranty may not apply

Pro-rated example: A battery with a 5-year warranty fails at 3 years. The customer receives credit for the remaining 2 years (40% of the replacement cost), paying 60% out of pocket.

3D Battery Component: The Cell 6 label (Failure Modes) summarizes the major failure mechanisms covered in this module. Use it as a quick reference for the relationship between cause and effect.

Key Takeaways

1. Sulfation is caused by chronic undercharging and is the most common failure mode — hard sulfate crystals permanently reduce plate surface area and battery capacity
2. Plate shedding causes active material to fall from the grids, building sediment that can short a cell internally
3. Cell imbalance means the weakest of six cells limits the entire battery — one bad cell drops the battery from 12.6V to 10.5V
4. Parasitic draw above 50 mA is excessive and will drain a battery over days to weeks
5. The fuse-pulling method isolates which circuit is responsible for excessive key-off drain
6. Common draw sources include modules not sleeping, aftermarket accessories, and faulty switches
7. Battery lifespan is typically 3-6 years depending on climate, with heat being the primary life-shortening factor
8. Date codes use a letter-number format (A-L for month, digit for year) to identify manufacturing date

Vocabulary

Term | Definition
Cell imbalance | Condition where individual cells within a battery age at different rates, with the weakest cell limiting overall performance
Dark current | Another term for parasitic draw — the small current flowing from the battery when the vehicle is off
Date code | Letter-number stamp on a battery indicating the month and year of manufacture
Desulfation | Attempted reversal of sulfation using specialized chargers or prolonged low-amperage charging
Fuse-pulling method | Diagnostic technique for isolating parasitic draw by removing fuses one at a time and monitoring current drop
Parasitic draw | Electrical current consumed by vehicle systems when the ignition is off and the key is removed
Plate shedding | Physical separation of active material (paste) from the plate grid, causing it to fall as sediment
Pro-rated warranty | Warranty that provides partial credit based on remaining time in the coverage period
Sediment | Accumulated plate material that collects at the bottom of a battery cell after shedding
Sulfation | Formation of hard, crystalline lead sulfate on battery plates due to chronic undercharging or prolonged discharge