State-of-health monitoring of 18650 4S packs with a single-point impedance diagnostic

• A single-point impedance method tracks the state-of-health (SOH) of Li-ion batteries.
• The impedance response at 316 Hz is reflective of the SOH for a commercial 18650 cell.
• The SOH of single 18650 cells and 4S packs are monitored during overcharge.
• The overcharge abuse of a single cell is detected within a 4S battery pack.
• The single-point impedance diagnostic works on packs without single-cell monitoring.

The state-of-health (SOH) of Li-ion batteries and battery packs must be monitored effectively for abuse to prevent failure and accidents. In a previous publication, we described a single-point impedance diagnostic method for detecting damage in single prismatic lithium polymer rechargeable cells subjected to overcharge abuse. We now determine whether the single-point impedance diagnostic method is applicable to 4S battery packs. At 316 Hz, commercial 18650 LiCoO2 cells are determined to have the least change in impedance response when cycled between 3.0 and 4.2 V, for states-of-charge (SOC) of 0–100%. The impedance response of single cells at 316 Hz changes dramatically during overcharge (SOC = 125%), presumably due to change in their solid electrolyte interface (SEI) layers at the electrodes. When a single cell is purposely subjected to such overcharge abuse and then integrated into a 4S pack with 3 other healthy cells, the impedance response of the 4S pack at 316 Hz also changes, despite variances in the impedance response of each of the 3 healthy cells. The results suggest that this single-point impedance method could serve as a diagnostic in an all-inclusive battery management system to identify overcharge abuse of single cells without individual cell monitoring.