Deriving the Validation Protocol for Isolator Switches Used in Commercial Vehicles

All automotive components undergo stringent testing protocol during the design validation phase. Nevertheless, there are certain components in the field which are seldom captured during design validation. One of these components is the battery isolator switch. This project aims at optimizing a validation methodology for this component based on field usage and conditions. The isolator switch is the main control switch which connects and disconnects the electrical loads from the battery. This switch is used in the electrical circuit of the vehicle to prevent unwanted draining of battery when it is not needed and when the vehicle is in switched off. An electrical version of this switch uses electromagnetic coils to short the contacts. The failure mode being investigated is a high current load causing the input and output terminal to be welded. The instrumented isolator switch was fitted in a customer vehicle to acquire various parameters like coil voltage, coil current, battery voltage, contact voltage and contact current values during engine cranking with different state of charge (SOC) in the battery. Battery voltage and coil voltage values were found to be most critical factor affecting the isolator switch performance, over a certain state of charge of the battery. There was a huge dip in battery voltage during cranking the engine. To reproduce the same, the entire setup was assembled with a suitable wiring harness with the engine to conduct the lab level simulation. Durability test was carried out at certain SOC to the corresponding coil voltage and contact current values observed in vehicle. Analysis of these tests showed that coil resistance and coil material used could lead to failure of the isolator switch. After the design modification, revised design isolator switch samples were validated and achieved an improved life over the earlier design.