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What you’ll learn:
What is a pre-charge circuit and what role does it play?
The fundamental elements of a pre-charge circuit.
Some basic best practices and where to find more information.
A common challenge in systems operating at higher voltage occurs during initial power-up with so-called “inrush” current, particularly when a significant capacitive load is part of the circuitry (though resistive and inductive issues are also important to consider and understand).
Inrush current has the potential to damage or at least stress components if not managed in some way. This basic situation is common in modern electric vehicles (EVs) that often operate at 400 V, with some now operating at 800 V.
EVs are particularly affected by this problem since they’re often turned off and then on many times each day, repeating the cycle of inrush current. Energy conservation can also dictate depowering when an immediate demand for power isn’t present, requiring yet more reapplications of power.
Pre-Charging Ups the Safety Level in EVs
The standard approach to managing this problem is pre-charging. Its goal is to protect electrical and electronic components from damage and ensuring long, trouble-free service for the vehicle and its systems. By keeping systems functioning as intended, pre-charging has the additional benefit of reducing the likelihood of safety hazards associated with electrical power such as fire or shock.
For example, even arcing due to a fault condition can be reduced by pre-charging. Furthermore, pre-charging can ensure a better diagnostic environment, making it possible to detect problems before they become catastrophic and cause more serious damage. Even protective devices such as circuit breakers can be protected from tripping simply due to the application of power to the system by implementing pre-charging.
Unlike in systems without pre-charging, during a pre-charging process, system voltage rises relatively slowly and in a controlled manner. As voltage rises to reach a steady state, pre-charging is no longer needed. It can be taken out of the circuit, normally through some automatic method. This is usually designed to occur when the main circuit reaches 90% to 95% of the designed operating voltage.
While there are many variations in approach, EV motor controllers often include a capacitor as well as a high-current relay called a contactor that can serve as an emergency disconnect as well as disable the system when not in use. Contactors are vulnerable to arcing and pitting, like old electromechanical “points” in early internal combustion engines, when going through a duty cycle that involves high-voltage battery power.
Pre-charging addresses such a challenge by applying current to the capacitor in the circuit.