2. System Voltage And Current Monitoring

Several different voltages inside a battery system need to be measured. The exact number of voltages is dependent on the system topology and the configuration of the battery system.

Fig. 2.3 shows the voltages that need to measured in a multi-string system, here for example for a 3-string system.

The left side of the image shows the Battery Pack. The Battery Junction Box is depicted in the center of the figure with an indicated connected application shown on the right side the of the picture.

Fig. 2.3 Voltages and currents of interest in a multi-string system (click to enlarge)

Table 2.6 Color description

Symbol	Meaning
red	Power path
dotted-red	Second power path (optional)
pink	Communication (e.g., CAN)
green	Contactor control and feedback
purple	Insulation measurement line
blue	Daisy-chain

2.1. Multi-String System

The following section describes important parameters within a multi-string battery pack.

2.1.1. Measurements Inside the Battery Pack

Fig. 2.4 Voltages and currents to be measured in a multi-string system inside the battery pack (click to enlarge)

The strings are depicted in ascending order from right to left starting with string 1 (S1) until left-most string m (Sm). Each string consists of n modules, where every module has its own BMS-Slave. Each string features a current sensor, a string fuse and one or two string contactors. Thus, the following voltages need to be measured in each string:

• String voltage, short V_Sm, where m indicates the number of the string.

• Fused string voltage, short V_FSm, where m indicates the number of the string. It is measured between the lowest module and after the string fuse.

• Pack voltage, short V_Pm, where m indicates the number of the string. It is measured between the negative pole of the lowest module and behind the positive string contactor. This voltage is identical for all connected strings and enables measurement validation.

The current sensor additional measures each string current.

Comprehension of measured voltages and current is found in Table 2.7.

Table 2.7 List of measured voltages and current in the pack

Symbol	Reference Name	Description
VSm	StringVoltageM	Voltage of string m
VFSm	FusedStringVoltageM	Voltage after string fuse of string m
VPm	PackVoltageM	Pack voltage
ISm	StringCurrentM	Current in string m

2.2. Single-String System

A single-string system reduces the amount of required voltage and current measurements. Fig. 2.5 shows the single-string topology.

Fig. 2.5 Voltages and current to be measured in a single-string system (click to enlarge)

The list of measurements is therefore reduced to Table 2.8 and no further measurements inside the string are required.

Table 2.8 List of measurements in single-string system

Symbol	Reference Name	Description
VP	PackVoltage	Pack voltage
VFP	FusedPackVoltage	voltage after main fuse
VSV	SystemVoltage	System voltage
VSV2	SystemVoltage2	System voltage: second power path
IP	PackCurrent	Pack current

2.3. Measurements Inside the Battery Junction Box

Fig. 2.6 Voltages to be measured in a multi-string system in the BJB (click to enlarge)

• The pack voltage is measured before the main fuse.

• The fused pack voltage is measured after the main fuse.

• The system voltage is measured after the contactors.

• optional: If the system uses a second power path, the second power path voltage is measured after the contactors of the second power path.

Comprehension of measured voltages and current in the battery junction box:

Table 2.9 List of measurements in multi-string system

Symbol	Reference Name	Description
VP	PackVoltage	Pack voltage
VFP	FusedPackVoltage	voltage after main fuse
VSV	SystemVoltage	System voltage
VSV2	SystemVoltage2	System voltage: second power path
IP	PackCurrent	Pack current