Buying Cells against Buying Packs and Where Responsibility Sits
Buying cells requires owning BMS development, weld quality, thermal isolation, and pack safety files; buying packs trades unit margin for transferred liability.
Internal Ac Resistance
Meaning
The resistance of an electrochemical cell measured by applying an alternating current at a specific frequency, typically one kilohertz. This internal ac resistance is a primary parameter for monitoring the electronic contacts and electrolyte conductivity. In industrial battery sourcing, this metric is used for fast sorting of cells on production lines.
The measurement ignores slow chemical polarization and focuses on the high-frequency ohmic response of the cell. It applies only to dynamic sinusoidal test conditions and does not represent the resistance during continuous steady-state discharge.
Measurement Technique
Measuring this value is performed by injecting a small alternating current and detecting the resulting voltage phase shift. This measurement is executed quickly, taking less than a second per cell. Sourcing technicians use handheld meters or automated testers to check thousands of cells daily.
The test is non-destructive and does not alter the cell’s state of charge. This speed makes the measurement ideal for high-throughput incoming inspections. The resulting value is composed mostly of contact resistance between layers and current collector resistance.
This test is done at room temperature to ensure consistency across different batches.
Quality Sort
Manufacturers rely on this metric to group cells with nearly identical characteristics into pack modules. The internal ac resistance must remain within a narrow band to prevent thermal imbalances in the finished pack. If a cell has a high resistance value, it is rejected during the sorting process.
This variation indicates poor tab welding or incomplete electrolyte wetting of the separator. Sourcing managers specify maximum allowable deviation limits in their procurement contracts. This practice protects the buyer from receiving poorly manufactured cells that degrade prematurely.
Aging Evaluation
Monitoring this resistance over time provides clear data on the degradation of the cell’s internal structure. This internal ac resistance rises as the electrolyte decomposes or when the active materials detach from the foil. The increase in resistance causes a higher voltage drop during load transients.
This drop reduces the power output of the pack in high-current applications. Engineers track this parameter during cycle-life testing to detect the onset of dry-out. The measurement helps identify when the cell’s contact points are separating due to mechanical stress.