These settings deal with options specific to the precharge control logic present on Orion BMS product lines. Precharge is used to bleed off inrush current that would otherwise be present when a high capacitance system (such as a motor controller or inverter) is initially connected to a battery pack. Inrush current can cause damaging transients and may weld contactors or blow stack level fuses, all of which can cause safety hazards.
Precharge works by briefly engaging a high power resistor across the main contactor before it is closed, allowing the inrush to be bled off safely by the resistor prior to engaging. Once the precharge sequence begins, it goes through several defined stages.
Typically a circuit will have 2 contactors, called HVPOS (for high voltage positive) and HVNEG (for high voltage negative) which are placed on the corresponding poles of the battery pack. The precharge resistor is usually connected across HVPOS by means of a relay to allow it to be taken out of circuit once its role is completed. Only one contactor is technically required (HVPOS in this example), however it does leave a vulnerability if that contactor welds as the BMS will then be unable to open the pack. The software supports both 1 and 2 HV contactors.
Please see the documentation regarding precharge available on the Orion BMS website under Application Notes for details on how precharge works.
NOTE: Either one of the multi purpose outputs or the multi purpose enable pin MUST be assigned the function of "Precharge Relay Output' in order for precharge to operate.
Auto-start precharge on power-up: By default, the precharge circuit will not start by itself without direct authorization via CANBUS or from a configured multi purpose input. Selecting this option will cause the BMS to begin the precharge sequence as soon as it powers up and confirms the pack is permitted to be used.
Feedback Input #1 and Feedback Input #2: The BMS has two configurable feedback inputs to allow it to determine if the corresponding contactor is active (CLOSED). These feedback inputs require the use of auxiliary contact enabled contactors. If auxiliary contact contactors are not being used, these feedback inputs should be disabled.
HVPOS and HVNEG Output Designation: HVPOS and HVNEG must have an assigned physical output on the BMS to tell it which pole is being controlled by which pin on the BMS.
HVNEG Output Mirrored to HVPOS: In some circumstances, it may be desirable to have 2 inline contactors in series on the same pole of the battery, versus having 1 on the negative and 1 on the positive. This option allows for treating both contactors as if they were one output (engaging and disengaging them at the same time).
*Precharge Relay Duration: This value defines the number of seconds that the BMS will hold the precharge resistor relay ON (closed) to bleed off inrush current. This value must be calculated in conjunction with the resistor value and size and is beyond the scope of this documentation to determine.
Max Current After Precharge Completes: When the Precharge Relay Duration timer expires, the BMS will look for the pack current to be equal to or less than this value to ensure that the precharge process was a success. If the current is above this value, it will set a fault and terminate precharge as it is assumed that the resistor was ineffective at fully bleeding off the inrush. The resistor sizing may need adjusting, or the duration may need to be extended (both of these are beyond the scope of this document to advise upon).
Enable CANBUS DC Application Bus Voltage With Maximum Delta: Because the BMS does not have a direct DC voltage monitor on the application side of the contactor(s), it is unable to know definitively that the precharge sequence was effective at equalizing the voltage between the pack and application. If there is a device on the CANBUS, such as a motor controller, inverter or other generalized ECU that does have the ability to measure the application side bus voltage, the BMS can accept or receive this value over CANBUS and use it to determine if it was successful by comparing the pack voltage against the provided DC bus voltage. This parameter above allows for the operator to specify the maximum permissible delta between the DC bus voltage and the pack voltage in order for the precharge sequence to successfully complete.
Only open contactors if critical fault is set: By default, the BMS will open the main contactors whenever the corresponding output pin would otherwise switch off. For example, if HVPOS is assigned to Discharge Enable, the contactors would then open whenever the Discharge Enable logic opens. Enabling this option allows the outputs to disregard the specific logic behind the corresponding output and remain active so long as no critical faults (such as internal failsafes, out of range cells, out of range temperatures, etc) are set.
Inhibit Outputs While Precharge is Active: These fields allow for specific outputs to be held off until precharge is complete. This can be useful for situations where specific equipment cannot be permitted to turn on or activate before the DC bus voltage is equalized (example, the Charger Safety relay might be held off to prevent the charger from turning on during the precharge process which could otherwise force additional current through the precharge resistor and trigger a fault). Additionally, the generalized Charge Current Limit (CCL) and Discharge Current Limit (DCL) can also be inhibited (held at 0A) in the same way to enforce digital controls over discharge & charge.