All modern power inverters and motor controllers have a large capacitor bank at their DC input terminals to reduce the ripple current and its associated EMI interference. Input capacitors help to provide smooth power conversion from DC to an AC sine wave and back to DC when charging the battery. When initially connecting a battery to a capacitive input, there is an inrush of current as the input capacitance is charged up to the battery voltage. With large batteries (with a low source resistance) and powerful loads (with large capacitors across the input), the inrush current can easily peak 1000A or more. A pre-charge circuit limits that inrush current, without limiting the operating current.
Applications and Benefits
Pre-charge circuits are often used in electric vehicles (EVs) such as battery management systems, onboard chargers, and in industrial applications such as power supplies and power distribution units. In EVs, controllers with high capacitive loads regulate motors. High voltage (HV) positive and negative contactors are used in this system to act as an emergency disconnect when the motor regulator fails. Without a pre-charge circuit, welding can occur within the contactor as it closes and there could be a brief arc resulting in pitting.
A design guide to DC precharge circuits.
When initially connecting a battery to a load with capacitive input, there is an inrush of current as the load capacitance is charged up to the battery voltage. With large batteries (with a low source resistance) and powerful loads (with large capacitors across the input), the inrush current can easily peak 1000 A.
A precharge circuit limits that inrush current, without limiting the operating current.
A precharge circuit between a battery and its load is required if any of the following are issues:
The load has input capacitors will be damaged by the inrush current
The main fuse will blow if asked to carry the inrush current
The contactors, if present, will be damaged by the inrush current
The battery cells are not rated for the inrush current
