You can also see resistors R1 and R2 with switches (PC1 and 2) for doing a final balancing of the packs before going back into parallel mode. That prevents a really high in-rush current.@azbill Eureka, you found it! Pack switching - You made me finally print out and read GM's patent application (see attachment below) for the two-pack 400/800 VDC pack switching process. The "dip" appears to happen when the charging process takes a short break so the EV controller can switch the 400 VDC AC compressor and IPE modules to the other pack. Since these 400 VDC devices can only be powered by one of the two 400 VDC packs when both packs are connected in series, the powering pack gets less charging amps and its SOC falls behind the non-powering pack, creating an imbalance. Periodically switching these loads back and forth from pack to pack helps to alleviate substantive imbalances. Relevant excerpt from the patent app text below (I inserted some translating data in bold):
0037] During charging at the second voltage level V2 [800 volts DC], there may be a need at times to power an electrical load aboard the mobile platform 20, e.g., to thermally condition to the battery packs 12A or 12B, a cabin of the mobile platform 20, etc.[power the 400 VDC AC compressor, IPE DC/DC converter, etc] Battery pack 12A provides such power when operating in mode (1 ). When operating in mode (2), this function is instead performed by battery pack 12B. The flexibility of using either battery pack 12A or 12B to energize the electrical load while simultaneously charging reduces the chances of a charge imbalance between the battery packs 12A and 12B. Such an imbalance might otherwise occur if one of the battery packs 12A or 12B were used for this purpose during charging to the exclusion of the other battery pack 12B or 12A.
FYI, when looking at the diagram below, the switches in-play for making the 400 VDC electric load change from pack 12A to pack 12B are Sets SA1/SA2 and SB1/SB2
View attachment 3381