High input capacitance systems such as inverters, dc-dc converters, etc. can be exposed to large inrush currents during the initial power up procedure. If appropriate measures are not employed, these currents can overly stress or even damage the system components. The pre-charge unit eliminates high in-rush currents by charging the input capacitor before the main contactor switches on, prolonging lifespan of the contactor and other components dramatically.
To set the pre-charge delay properly, system voltage should be measured at the end of pre-charge timer. Disconnect the Contactor + wire from the Pre-charge unit to disable contactor engaging. Now turn on the BMS and measure system voltage. After the pre-charge timer the system voltage should rise to at least 80% of the battery voltage. If the voltage stays below this value increase the Precharge delay. Some of the inverters/controllers on the system side may start to work with lower voltage and their power consumption prevent system voltage to rise.