EV & high-voltage electronics
Before an EV can drive, a careful sequence connects the high-voltage battery. Here's what each part does and how failures show up.
24 October 2026 · 5 min read
Related from the bench
Contactors are heavy-duty relays that connect the battery to the vehicle. They stay open when the car is off, isolating the high-voltage pack.
The inverter has large capacitors. Connecting them straight to the battery would cause a huge inrush current that welds contacts. A precharge circuit first charges them slowly through a resistor, then the main contactor closes. A failed precharge resistor or relay usually produces a 'won't go ready' fault.
The high-voltage interlock loop is a low-voltage circuit threaded through high-voltage connectors and covers. Unplug a connector or open a cover and the loop breaks, and the system shuts down high voltage. Corroded or loose HVIL connections cause intermittent shutdowns.
These systems are safety-critical and high voltage. Diagnosis and repair belong with trained technicians using proper isolation procedures.
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