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newSchoolWorks.com › VoTechWorks › Transportation › Automotive Technology › EV/Hybrid Safety and High-Voltage Systems Service › Lesson 10
TRN-AUTO19-08 · Lesson 10 · 22 clock hours · lab

Battery State-of-Health Testing and Battery Service Safety

Objective. Students will perform an OEM-specified battery state-of-health evaluation using scan-tool data and available diagnostic procedures, interpret results against OEM acceptance criteria, and follow all safe-handling procedures for a traction battery module removal and reinstallation exercise.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Battery State-of-Health: What It Means and Why It Matters

A traction battery's state of health (SOH) is a measure of how much of its original energy storage and power delivery capacity remains compared to when it was new. SOH is expressed as a percentage: a new pack is nominally 100 %, and capacity degrades over charge cycles, temperature extremes, and time. OEMs publish minimum SOH thresholds—commonly in the range of 70–80 % depending on the application—below which the battery is flagged for further evaluation or replacement. Technicians must compare scan-tool readings directly against the OEM service information for the specific model year and pack chemistry being tested; never apply one OEM's threshold to another manufacturer's vehicle.

Key Diagnostic Parameters

Cell-group voltage spread is the difference between the highest and lowest cell-group (or module) voltage reported by the battery management system (BMS) during testing. A small spread—typically just a few millivolts at rest or under light load—indicates cells that are well matched. A large spread signals cell imbalance, a weak cell group, or internal damage. OEM service data specify the maximum allowable spread; exceeding it triggers further diagnosis.

Internal resistance indicates how much the battery opposes current flow. As a battery ages, internal resistance rises, reducing peak power output. Some OEM scan tools report internal resistance per module or per cell group in milliohms. Compare measured values to OEM limits; values above the limit indicate degraded cells.

State of charge (SOC) calibration verifies that the BMS's reported SOC matches a known reference. If the BMS consistently misreports SOC, range estimates and charge decisions are compromised. Recalibration or BMS reprogramming may be required per OEM procedure.

Scan-Tool Data Capture Procedure

1. Connect an OEM-compatible scan tool to the vehicle's OBD-II port with the vehicle in the ready state or in the specific mode called out in OEM service information.

2. Navigate to the battery or high-voltage system menu and select the battery health or diagnostic function.

3. Record SOH percentage, cell-group voltage spread (minimum and maximum cell-group voltages), internal resistance where available, and SOC value.

4. Enter all values on the provided data sheet and compare each parameter to the OEM acceptance limits printed on the sheet. Flag any out-of-limit values.

5. Document any stored DTCs related to the battery system before clearing; DTCs provide context for abnormal SOH readings.

High-Voltage Disable and Lockout/Tagout

Before any physical battery work, the high-voltage system must be fully disabled following the OEM-specified procedure. Steps generally include:

Full HV PPE Requirements

For traction battery module work, wear:

PPE must be inspected before each use; gloves are air-tested for pinholes and checked for cuts or swelling. Damaged PPE is removed from service immediately.

Module Removal and Reinstallation

With HV disabled, LOTO applied, and PPE donned, module removal follows OEM torque specifications exactly. Use a calibrated torque wrench for every fastener. Under-torquing bus-bar bolts creates resistance and heat; over-torquing can crack ceramic cell housings or strip threads. Use only insulated hand tools rated to at least 1000 V. Keep modules oriented as specified (many lithium chemistries must stay upright). Never stack modules; store on a non-conductive surface.

During reinstallation, clean mating surfaces, install any OEM-specified thermal interface material, and torque in the sequence shown in service information. After assembly, follow OEM verification steps before re-enabling HV.

Damaged Battery: Disposal and Transport Safety

A mechanically damaged traction battery showing electrolyte venting is a thermal-runaway risk. Isolate the vehicle outdoors or in a well-ventilated area away from structures. Do not attempt to charge or drive it. Consult current DOT (Department of Transportation) hazardous materials regulations for lithium battery transport classification and packaging requirements, and EPA regulations for hazardous waste handling. Contact a licensed battery recycler or the vehicle OEM's emergency response line. Document the damage, vent odor, temperature, and any fire suppression actions taken. Use a large-volume water application for active lithium-ion fires only as directed by the fire authority having jurisdiction; some OEMs provide specific guidance in their emergency response guides.

Key terms9

State of Health (SOH)A percentage indicating how much of a traction battery's original energy capacity and performance capability remains compared to its as-new specification.
Cell-Group Voltage SpreadThe difference between the highest and lowest cell-group voltage readings reported by the BMS; a large spread indicates cell imbalance or degradation.
Internal ResistanceThe opposition to current flow within a battery cell or module, measured in milliohms; rising internal resistance reduces peak power output and indicates aging.
State of Charge (SOC)The current charge level of the traction battery expressed as a percentage of its usable capacity, analogous to a fuel gauge reading.
Manual Service Disconnect (MSD)A removable plug or switch installed by the OEM that physically interrupts the high-voltage circuit, enabling safe service without specialized disabling equipment.
Lockout/Tagout (LOTO)A safety procedure using physical locks and warning tags to prevent unexpected re-energization of equipment during service, required by OSHA standards.
Battery Management System (BMS)The onboard electronic system that monitors and manages traction battery cell voltages, temperatures, SOC, and SOH, and communicates data to the scan tool.
Class 0 Insulating GlovesRubber insulating gloves rated by ASTM standard for use up to 1000 V AC; the minimum rating commonly required for hybrid and BEV traction-battery service.
Thermal RunawayA self-sustaining, accelerating chemical reaction inside a lithium battery cell that generates heat faster than it can dissipate, potentially leading to fire or explosion.

Worked examples4

1. The scan tool shows the highest cell-group voltage as 3.94 V and the lowest as 3.81 V in a 96-cell-group pack. The OEM limit for maximum allowable cell-group voltage spread is 0.10 V. Is this pack within spec?
  1. Step 1 – Calculate the spread: subtract the lowest from the highest cell-group voltage. 3.94 V − 3.81 V = 0.13 V.
  2. Step 2 – Compare to the OEM limit: 0.13 V > 0.10 V limit.
  3. Step 3 – Record the result on the data sheet as OUT OF LIMIT and flag for further diagnosis.
Answer: The voltage spread is 0.13 V, which exceeds the OEM limit of 0.10 V. The battery fails the cell-group voltage spread criterion and requires further diagnostic evaluation.
2. A scan tool reports battery SOH as 74 %. The OEM acceptance threshold for this model is a minimum SOH of 70 %. Does the battery pass the SOH criterion?
  1. Step 1 – Identify the measured value: SOH = 74 %.
  2. Step 2 – Identify the OEM minimum acceptance limit: 70 %.
  3. Step 3 – Compare: 74 % ≥ 70 %, so the battery meets the minimum threshold.
  4. Step 4 – Record on the data sheet as WITHIN LIMIT. Note that being close to the limit warrants customer advisement.
Answer: 74 % SOH meets the OEM minimum of 70 %; the battery passes the SOH criterion for this test, though the result should be noted as approaching the limit.
3. A module bus-bar bolt requires a torque of 8 N·m. You are converting to pound-feet for a dial-type torque wrench that reads only in lb-ft. What is 8 N·m in lb-ft (use the conversion: 1 N·m = 0.7376 lb-ft)?
  1. Step 1 – Write the conversion: 1 N·m = 0.7376 lb-ft.
  2. Step 2 – Multiply: 8 N·m × 0.7376 lb-ft/N·m = 5.9008 lb-ft.
  3. Step 3 – Round to a practical precision: approximately 5.9 lb-ft.
  4. Step 4 – Set the torque wrench to 5.9 lb-ft and torque the fastener; verify with a second pass.
Answer: 8 N·m is approximately 5.9 lb-ft. Set the torque wrench to 5.9 lb-ft.
4. Before touching HV components, a technician measures 48 V DC on the HV bus after removing the MSD and waiting the specified discharge time. The OEM states that work may proceed when HV bus voltage is below 60 V DC. May the technician proceed?
  1. Step 1 – Identify measured voltage: 48 V DC.
  2. Step 2 – Identify OEM safe-to-touch threshold: below 60 V DC.
  3. Step 3 – Compare: 48 V DC < 60 V DC; the bus is below the OEM threshold.
  4. Step 4 – However, 48 V DC can still present a shock hazard. Maintain full HV PPE throughout and proceed per OEM service procedure.
Answer: 48 V DC is below the OEM 60 V DC threshold, so the technician may proceed per OEM procedure—but must keep full HV PPE on because 48 V DC is still a potential shock source.

The math in this lesson2

Learners convert 8 N·m to lb-ft for a bus-bar bolt.
N-Q.1N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: SOH data capture: students connect a scan tool to a training BEV or hybrid, navigate to the battery health menu, capture cell-group voltage spread, SOH percentage, internal resistance data (where available via OEM tool), and SOC calibration values, then compare results to OEM acceptance limits on a data sheet. · Module handling exercise: with HV fully disabled, LOTO applied, and full HV PPE worn, students remove and reinstall a single accessible battery module from a training pack (or training mockup approved for student disassembly), following OEM torque specifications with a calibrated torque wrench and using insulating tools throughout. · Disposal and transport safety briefing and activity: students review applicable DOT and EPA guidance for damaged or end-of-life HV batteries, then complete a written scenario response describing the correct procedure for a vehicle arriving with a mechanically damaged traction battery showing signs of electrolyte venting.
Materials: BEV or hybrid training vehicle, OEM-compatible scan tool with battery health function, calibrated torque wrench, insulated hand tools (1000 V rated), LOTO set, full HV PPE, DOT/EPA battery transport reference materials, data sheet, scenario handout
Check: Instructor-observed module R&R skills check plus completed data sheet; all torque specifications met (verified by instructor), all PPE steps observed correctly, data sheet values match scan tool within rounding. Pass: zero torque deviations, zero PPE omissions, data sheet complete.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASE EV L3 – Hybrid/EV Battery Systems Diagnosis
1. A technician captures cell-group voltage data on a BEV and finds the maximum cell-group voltage is 4.01 V and the minimum is 3.78 V. What does a spread of 0.23 V most likely indicate?
ASE EV L3 – Safety and High-Voltage Procedures
2. Which action must be completed BEFORE a technician physically removes the manual service disconnect on a hybrid traction battery?
ASE EV L3 – Safety and High-Voltage Procedures
3. After removing the MSD and waiting the OEM-specified discharge time, what is the correct next step before touching orange-cable HV components?
ASE EV L3 – Battery Safety, Handling, and Disposal
4. A vehicle arrives with a traction battery showing signs of mechanical damage and active electrolyte venting. What is the FIRST correct action?
ASE EV L3 – Battery Service and Repair
5. During a module reinstallation, a technician torques a bus-bar bolt to a value lower than the OEM specification. What is the most likely consequence?
ASE EV L3 – Safety and High-Voltage Procedures
6. Which insulating glove class is commonly required as the minimum for hybrid and BEV traction-battery service at voltages up to 1000 V AC?
ASE EV L3 – Battery Systems Diagnosis
7. A scan tool reports a traction battery SOH of 65 % on a model whose OEM minimum acceptance limit is 70 %. What should the technician record on the data sheet?
ASE EV L3 – Battery Safety, Handling, and Disposal
8. When transporting a damaged high-voltage traction battery off-site, which regulatory body's rules govern the hazardous materials classification and packaging requirements?
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