Circuit/System Description

Internal to the A4 Hybrid Electric Vehicle Battery Pack, the A28 Hybrid Electric Vehicle Battery Disconnect Relay assembly contains all ten high voltage contactors and two single-use High Voltage cut-off actuators. The high voltage contactors allow the high voltage DC batteries to be connected to the vehicle or contain the high voltage DC within the A4 Hybrid Electric Vehicle Battery Pack assembly during normal operation. Additionally, they allow for series configuring the two battery layers for 800V DC charging.

A level 3 off-vehicle direct current (DC) fast charge drive motor battery charger allows charging using DC current supplied directly to the battery pack, bypassing the on-vehicle battery charging. Level 3 charging includes a locking device on the receptacle to prevent removal while large amounts of current is flowing.

The A4 Hybrid Electric Vehicle Battery Pack is assembled with two layers of cell module assemblies, essentially operating like two 400V DC battery packs in one assembly. The upper level is pack 1 and the lower level is pack 2. Under normal operating conditions these two layers operate electrically in parallel with each other and charge at 400V. If conditions allow, the high voltage contactors can be configured such that each layer is wired in series to facilitate faster level 3 charging at 800V.

While both packs are being charged in series at 800V, the vehicle high voltage components operate from only one pack at a time ensuring they only operate at 400V. To maintain balance between the two packs, the contactors cycle which battery pack operates the vehicle high voltage components.

The potential exists that either on-vehicle faults or level 3 off-vehicle direct current (DC) fast charge drive motor battery charger faults could expose the vehicle high voltage components to voltages that exceed their maximum rating of 450V.

DTCs U3687 and U3688 monitor the vehicle high voltage components for exposure to voltages in excess of 450V, counting and storing the exposure time (in seconds). Additionally these DTCs will abort level 3 DC fast charge series charging when over voltage is detected. When a vehicle HV components lifetime over voltage exposure limit as been reached, DTC U3689 is set and 800V charging is prohibited until the effected HV component has been replaced, the exposure time is reset and the lock-out condition has been cleared with the scan tool.