Cell Structure & Chemistry
What are the three main components inside a typical lithium-ion cell?
The cathode, anode and electrolyte.
What does SoC stand for?
State of Charge.
What is the purpose of a battery diagnostic report?
To record and communicate the battery's condition, measurements, faults and diagnostic findings.
Name one common lithium-ion battery fault.
Cell imbalance, overheating, insulation fault, high resistance or physical damage.
What is thermal runaway?
An uncontrolled self-heating process in which a battery cell's temperature rises rapidly and can lead to fire or other dangerous reactions.
What is the function of the electrolyte in a lithium-ion cell?
It allows lithium ions to move between the electrodes during charging and discharging.
What is battery capacity, and what unit is it normally measured in?
The amount of electrical energy the battery can store; it is commonly expressed in kWh for a vehicle battery.
A diagnostic report shows SoC = 80%. What does this mean?
The battery is approximately 80% charged relative to its usable capacity.
What is cell imbalance?
A condition in which individual cells or cell groups have significantly different voltages or states of charge.
Give one possible cause of thermal runaway.
Internal short circuit, severe overheating, physical damage, overcharging or another condition that causes uncontrolled heat generation.
Explain the difference between a cell, module and battery pack.
A cell is the basic electrochemical unit. Several cells are combined into a module, and several modules and associated systems form the battery pack.
Explain SoH and how it differs from SoC.
SoH indicates the battery's condition or remaining performance compared with when it was new. SoC indicates how much charge is currently available.
A battery report shows: SoC 65%, SoH 88%, cell voltage range 18 mV. What does this information tell you?
The battery is 65% charged, its estimated health is 88%, and the relatively small cell-voltage difference suggests the cells are reasonably well balanced.
What is internal resistance, and why is it important in battery diagnosis?
It is the resistance within the battery that opposes current flow. Increased internal resistance can cause voltage drop, heat generation and reduced performance.
What symptoms could indicate a serious battery thermal problem?
Rapid temperature increase, warning messages, abnormal smells, smoke, unusual sounds or visible damage.
What is the role of the separator inside a lithium-ion cell?
It keeps the electrodes physically apart to help prevent an internal short circuit while allowing lithium ions to pass through.
What does C-rate describe?
The charging or discharging current relative to the battery's capacity. A higher C-rate means the battery is being charged or discharged more quickly relative to its capacity.
A diagnostic report shows: SoH 72%, high internal resistance and one cell group with significantly lower voltage than the others. What could these findings indicate?
The battery may have significant degradation and a cell or cell-group problem. Further testing is required to determine the root cause.
A battery shows repeated overheating, increased internal resistance and reduced available capacity. Name two possible failure mechanisms.
Cell degradation, electrolyte degradation, increased internal resistance, loss of active material or damage caused by repeated high-temperature operation.
What should a technician do if thermal runaway is suspected?
Follow the manufacturer's emergency procedure, keep people away from the vehicle, isolate the area and contact the appropriate emergency/safety personnel. The technician should not attempt an unsafe intervention on a potentially unstable high-voltage battery.
Explain how lithium ions move during charging and discharging in a typical lithium-ion cell.
During charging, lithium ions move through the electrolyte from the cathode toward the anode, where they are stored. During discharge, the ions move back toward the cathode while electrons flow through the external circuit to provide electrical power.
A battery has a capacity of 75 kWh and is discharged at a constant 1C rate. Approximately how much power is being delivered?
Approximately 75 kW
Interpret this diagnostic report: SoC 40% / SoH 78% / Cell group 7 voltage significantly lower / Internal resistance above specification / Battery temperature normal. What is your conclusion?
The battery shows significant degradation and a possible cell-group fault. The low cell-group voltage and high internal resistance require further investigation. Normal temperature suggests that overheating is not the immediate issue.
A customer reports reduced range and intermittent battery warnings. The diagnostic report shows one cell group with abnormal voltage deviation. What are two possible causes, and what should the technician investigate?
Possible causes include cell degradation, an internal connection problem or a sensing/wiring fault. The technician should inspect diagnostic data, cell voltages, connections and relevant sensing circuits according to the manufacturer's procedure.
Write a short battery condition report for this vehicle: SoH 82% / SoC 55% / no thermal faults / minor cell-voltage deviation / range slightly reduced.
“Battery state of health is currently estimated at 82%. The battery is operating at 55% state of charge and no thermal faults were detected. A minor cell-voltage deviation was identified and should be monitored. The reduced range may be consistent with the current battery condition. Further testing is recommended if the deviation increases or additional warnings occur.”