2.6 Power Path

This page explains how the BMS switches the charge and the discharge path through its outputs, which wiring layout to choose and how to assign the output and input functions. The precharge of the load is described in 2.7 Precharge.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810.

2.6.1 How It Works

Fig.2.6.1: Power Path With Main Contactor and Precharge Circuit

Battery pack + − Current sensor Main Main Contactor Output Main Contactor Auxiliary PCHG R precharge resistor Pre Charge Contactor Output Load inverter, motor drive input capacitance C The precharge current flows through the current sensor, so the BMS can check it (PCHG Time and Current Limit). Simplified: fuses and other protection devices are not shown. Blue labels are LiMon™ input and output functions.

  • The BMS controls a charge path and a discharge path. Each output is assigned a function on the Peripheral tab:
    • Charge Contactor Output follows the charge path.
    • Discharge Contactor Output follows the discharge path.
    • Main Contactor Output is ON only while both the charge and the discharge path are ON.
    • Pre Charge Contactor Output is ON during the precharge (2.7 Precharge).
  • A protection switches off the path it endangers (3.1 Protection and Recovery); the outputs follow their paths.

Main contactor auxiliary contact: assign Main Contactor Auxiliary to an input wired to the auxiliary contact of the main contactor. If the input disagrees with the commanded contactor state several times in a row, the BMS sets Main Contactor Malfunction (error bit 30, CRITICAL).

2.6.2 Wiring Layouts

Choose the layout that fits how the battery is charged and used, and assign the output and input functions to match.

Fig.2.6.2: Power Path Layouts

A · One main contactor Battery Main Main Contactor Output PCHG + R Pre Charge Contactor Output Load Charger One contactor for charge and discharge. Any protection opens it: a full cell also cuts the load off. Precharge protects the load at switch-on. B · Separate charge and discharge contactors Battery Charge Charge Contactor Output Charger Discharge Discharge Contactor Output Load PCHG + R Pre Charge Contactor Output Charger and load on separate ports. A full cell opens only the charge contactor; the load keeps running. An empty cell opens only the discharge contactor; the battery can still charge. C · Separate contactors with a charge interlock Battery Charge Charge socket CHG Power Path Interlock sense contact Discharge Load PCHG + R Pre Charge Contactor Output The socket's sense contact goes to a BMS input with CHG Power Path Interlock. Plugged in: charge path ON after 5 s, discharge path OFF. Unplugged: charge path OFF. Outputs as in B. Simplified: fuses, the current sensor and the negative rail are not shown. Blue labels are LiMon™ output and input functions.

Table.2.6.1: Output and Input Functions per Layout

Layout Output functions Input functions Use it when
A · One main contactor Main Contactor Output, Pre Charge Contactor Output Main Contactor Auxiliary (optional) The charger and the load share one port and the system may stop as a whole on any protection.
B · Separate contactors Charge Contactor Output, Discharge Contactor Output, Pre Charge Contactor Output — The charger has its own port and the load must keep running when only charging is stopped, or the other way round.
C · Charge interlock as B CHG Power Path Interlock The system must not drive or discharge while the charge cable is plugged in, for example a vehicle.
  • Layout A: the main contactor is ON only while both paths are ON, so every protection opens it. A full cell at the end of the charge also switches the load off.
  • Layout B: each contactor follows its own path. A full cell opens only the charge contactor; an empty cell opens only the discharge contactor and the battery can still be charged.
  • Layout C: while the interlock input is inactive (cable unplugged), the charge path stays OFF. When the input has been active for 5 s (cable plugged in), the charge path switches ON, unless a charge protection holds it OFF, and the discharge path is held OFF.

2.6.3 Settings

Table.2.6.2: Power Path Settings (LiMon™ Peripheral tab)

Setting What it does How to choose Unit Default
OUT n Assignment Output function per output. Main, Pre Charge, Charge or Discharge Contactor Output, to match the layout. list No Function
INPUT n Assignment Input function per input. Main Contactor Auxiliary, if the main contactor has an auxiliary contact; CHG Power Path Interlock for layout C. list No Function
Charge Power Path Control Automatic Control, Path Always ON or Path Always OFF. Automatic Control in service. list Automatic Control
Discharge Power Path Control Automatic Control, Path Always ON or Path Always OFF. Automatic Control in service. list Automatic Control

The LiMon™ reference describes every field of the screen: 7.5 Peripheral Settings.

Path Always ON overrides the protections

With Path Always ON the path stays ON whatever the cell voltages, temperatures or currents are. Use it only on a test bench under supervision, and set Automatic Control again before the battery goes into service.

2.6.4 Checking the Result

  • With no protection active, the contactors of both paths are closed; with layout C the charge contactor closes only while the charge cable is plugged in.
  • A charge protection (for example a full cell) opens only what follows the charge path; a discharge protection only what follows the discharge path. With layout A the main contactor opens in both cases.
  • No Main Contactor Malfunction in the events.

2.6.5 Common Mistakes

  • Main Contactor Auxiliary on an input that is not wired: the input stays inactive while the main contactor is ON, and the BMS sets Main Contactor Malfunction.
  • Layout A for a system that must keep its load running: a full cell at the end of the charge also switches off the load. Use layout B.
  • Charger and load on the same bus with separate charge and discharge contactors: the charge path is not precharged, so the charge contactor can close onto the empty load capacitance (2.7 Precharge).
  • Path Always ON left set after a test.