6. Battery Junction Box

This section presents an example of a typical battery junction box. The design must be adapted to the application and specific care must be taken by sizing the components to ensure the required safety level.

6.1. Introduction

6.1.1. System Specifications

The Battery Junction Box (BJB) described in this document is designed for mobile and stationary battery systems. The target system has a voltage range between 315V and 567V, while being capable to source and sink a continuous current of up to 320A. The stationary battery consists of up to 14 series connected battery modules (i.e., 28 daisy chained LTC6804). Each module is a 15s2p configuration of 2.3V 20Ah lithium-ion NMC/LTO prismatic battery cells.

6.1.2. System Overview

The BJB contains the Battery Management System (BMS) and all safety relevant components. Fig. 6.3 shows a BJB integrated into a 19’’ enclosure with a 4U height.


Fig. 6.3 BJB top view

Fig. 6.4 shows the block diagram of the BJB. The battery is connected to the BJB at the left side (Batt_Positive and Batt_Negative). The source/sink (e.g., load, charger, inverter) is connected to the right side (Out_Positive and Out_Negative).

The main contactors disconnect the battery from the output terminals. These are normally off and switched on in BMS ON-Mode. The contactors are opened if the BMS detects hazardous conditions (e.g., abnormal battery cell temperature) or in BMS OFF-Mode.


A precharge contactor and resistor are used to limit the inrush current into the inverter DC-link capacitor when closing the main contactors at startup.

To break the current flowing in case a short circuit condition occurs between the two high voltage battery system poles, a fuse is placed in the positive current path. A Cooper Bussmann FWP-Series (700Vdc) or better FWJ-Series (800Vdc) fuse was selected for this purpose.


A special high-voltage DC fuse must be used to break high short circuit currents in high voltage battery systems. This fuse must be very carefully chosen by electrically skilled engineers to ensure proper protection in the specific test environment using specific testing equipment and test conditions. In case the fuse is not appropriately chosen, its protection effect will not be provided.


Fig. 6.4 BJB block diagram

A Manual Service Disconnect (MSD) is placed in the positive current path to ensure a manual disconnection while the system is serviced. It is mounted on the front of the BJB for easy access. The MSD used is available with an integrated fuse or as shunted version without fuse. In case the fused version is chosen, the Cooper Bussmann fuse can be omitted, but the integrated fuse version is limited to 200A.

The Battery Management System (BMS) is the main control unit of the whole battery system. It collects data from the battery modules (e.g., battery cell voltages, cell temperatures) and from the current sensor, and uses these data for battery state calculations (e.g., SOC, SOH, SOF). In addition, the BMS controls the power contactors and communicates with a superior management unit through the CAN bus.

6.2. Battery Junction Box Part List

6.2.1. Commercial Off-The-Shelf (COTS) Parts

Table 6.3 shows the components used in the BJB. All components were selected to fit the system specifications listed in System Specifications. If another battery configuration than specified here is used, the voltage and current ratings of these components have to be checked and adapted.

Table 6.3 BJB part list

Part Description


Part Suggested


Power Contactors


GX16BEB (600A)

HVC Technologies

Precharge Contactor




Precharge Resistor


LPS 300 Series (300W)


DC Fuse*

Cooper Bussmann

FWJ-Series or FWP-Series


Current Sensor




Emergency Stop Button


M22-PV/K11 + M22 K01


Insulation Monitoring





Fraunhofer IISB

foxBMS Master Unit with foxBMS Slave Units

Fraunhofer IISB

Manual Service Disconnect


AMP + Manual Service Disconnect

Power & Signal Group

12V Power Supply




Power Switch

Kraus & Naimer

G20S D322-600 E

Kraus & Naimer

* rated currents and voltages are depending from the used battery cells

6.2.2. Custom Parts

In addition to the components listed in table 6.3, sundries not listed here are needed (e.g., terminal blocks). Table 6.4 shows the wires used inside the BJB. Basically only 0.5mm2 wires are used. Wires are used for signals and low voltage only (e.g., 12V or 24V power supply). The continuous insulation rating must exceed the maximum voltage of the whole battery pack.

Table 6.4 Litz wires used inside the BJB


Cross Section




12V Supply Positive



12V Supply Ground



Insulation Monitoring



Insulation Monitoring



CAN High






Interlock Line



Battery Monitoring Backup Interface



Battery Monitoring Backup Interface



Battery Monitoring Main Interface



Battery Monitoring Main Interface

For the high power DC connections, copper bus bars are used, since high current pulses can occur. The copper bus bars were fabricated with a cross section of 150mm2 (5mm x 30mm) for the maximum continuous current specified in System Specifications.

6.3. Main and Precharge Contactors Wiring

The main and precharge contactors are delivered with bare wire ends. The corresponding crimps and plugs must be used to connect them to the foxBMS Master Unit. The direction of the current flow through the power contactors must be carefully considered, since the ability of the power contactor to interrupt the current is dependant on the direction of the current flowing through them.

6.4. Insulation Monitor Wiring

The Bender insulation monitor is delivered with a socket on its PCB. The corresponding crimps and plugs must be used used to connect it to the foxBMS Master Unit.

6.5. Current Sensor Wiring

The current sensor is supplied without its crimps and housings (e.g., by JST). The current sensor has to be wired to the CAN bus and to a power supply. In addition, voltage sense wires may be connected to measure up to 3 voltages in the battery system (e.g., battery voltage, voltage over the DC fuse, voltage over the contactors, voltage on the load side). In the given BJB example, the voltage sensing inputs are monitoring the following:

  • Voltage measurement 1: between fuse and main contactor

  • Voltage measurement 2: between fuse and service disconnect (MSD)

  • Voltage measurement 3: between battery positive and precharge contactor


When the precharge contactor is closed after the main negative contactor has been closed, the measured voltages are used to ensure a correct precharge procedure.

6.6. Summary of the Assembly Procedure

For developing and building the BJB, the following procedure may be used:

  1. Defining the specification of the battery (e.g., maximum current, maximum voltage)

  2. Defining the placement of the input (from the battery) and the output (to the user) connectors and the manual service disconnect

  3. Placement of the main parts of the current path (e.g., main contactors)

  4. Designing of the copper bus bars (alternatively wire with appropriate cross section may be used)

  5. Placement of the BJB electronic parts (e.g., BMS)

  6. Wiring of the electronic parts