3.5. Slave 18-Cell v1.1.3 and above

3.5.1. Overview

Important

The following description only applies for the 18-cell BMS-Slave Board hardware versions 1.1.3 and above.

Hint

All connector pinouts described below follow the Convention for Connector Numbering.

3.5.1.1. Block Diagram

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Fig. 3.8 foxBMS BMS-Slave 18-Cell Block Diagram

3.5.1.2. Schematic and Board Layout

More information about the board schematic and layout files can be found in section Design Resources.

3.5.1.3. Mechanical Dimensions

The size of the foxBMS Slave PCB is 160x100mm. A *.step file and a 3D-PDF of the PCB can be found in section Design Resources.

3.5.2. Functions

The following general descriptions apply to both, the primary and the secondary unit on the foxBMS BMS-Slave. If there are any differences in hardware between the primary and the secondary unit they will be marked as such.

3.5.2.1. Cell Voltage Measurement

The cell voltage sense lines are input on the connector X200. The pinout is described in table 3.25.

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Table 3.25 Cell voltage sense connector
Pin Signal Direction Description
1 VBAT- Input Battery module negative terminal
2 CELL_0+ Input Cell 0 positive terminal
3 CELL_2+ Input Cell 2 positive terminal
4 CELL_4+ Input Cell 4 positive terminal
5 CELL_6+ Input Cell 6 positive terminal
6 CELL_8+ Input Cell 8 positive terminal
7 CELL_10+ Input Cell 10 positive terminal
8 CELL_12+ Input Cell 12 positive terminal
9 CELL_14+ Input Cell 14 positive terminal
10 CELL_16+ Input Cell 16 positive terminal
11 VBAT+ Input Battery module positive terminal
12 NC
13 CELL_0- Input Cell 0 negative terminal
14 CELL_1+ Input Cell 1 positive terminal
15 CELL_3+ Input Cell 3 positive terminal
16 CELL_5+ Input Cell 5 positive terminal
17 CELL_7+ Input Cell 7 positive terminal
18 CELL_9+ Input Cell 9 positive terminal
19 CELL_11+ Input Cell 11 positive terminal
20 CELL_13+ Input Cell 13 positive terminal
21 CELL_15+ Input Cell 15 positive terminal
22 CELL_17+ Input Cell 17 positive terminal
23 NC
24 NC

Each of these lines is fused with a fast acting 250 mA surface-mount fuse (F301 - F319) on the board except of the VBAT+ and VBAT- lines which are fused with a value of 500 mA (F300 and F320). This essentially is important for an evaluation environment. The VBAT+ and VBAT- connection is used for the internal power supply of the slave board. If the battery module does not contain these separate wires to the positive and negative module terminal the solder jumpers SJ300 and SJ301 have to be shorted. In this case the slave will be supplied through the sense lines CELL_0- and CELL_11+. Running the slave in this configuration could result in cell measurement errors due to voltage drop on the sense wires.

The cell input lines are filtered by a grounded or differential capacitor filter (both possibilities provided on the PCB). More information on the corner frequency of this filtering can be found in the schematic. The grounded capacitor filter should be used in environments affected with high noise as it offers a high level of battery voltage ripple rejection. The differential capacitor filter can be used when noise is less occurrent or the design is subjected to cost optimization.

3.5.2.2. Passive Cell Balancing

The passive balancing circuit is realized by a parallel connection of two 130 Ohm discharge-resistors that can be connected to each single cell in parallel. The MOSFET switches (T1500 - T1517) that control the connection to the cells are controlled by the primary LTC6813-1 monitoring IC. The LTC6813-1 on the secondary unit does not support balancing. The resistor value of 2x 130 Ohm results in a balancing current of about 55mA at a cell voltage of 3.6V. This current results in a power dissipation of about 0.2W per balancing channel (at 3.6V).

3.5.2.3. Global Cell Balancing Feedback

In order to check the proper function of the balancing process or to detect a malfunction in the balancing control circuit, a global balancing feedback signal is connected to the LTC6813-1. This allows the system to check whether any balancing action is currently taking place at any time. The feedback signal is connected to the GPIO3 of the LTC6813-1. The signal remains in a logic zero state until any balancing action on any cell starts.

3.5.2.4. Temperature Sensor Measurement

The cell temperature sensors are connected to the connectors X201 (primary) and X202 (secondary). The pinout is identical for the primary and secondary unit and is described in table 3.26.

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Table 3.26 Temperature sensor connector
Pin Signal Direction Description
1 T-SENSOR_0 Input NTC Sensor 0 terminal 1
2 T-SENSOR_1 Input NTC Sensor 1 terminal 1
3 T-SENSOR_2 Input NTC Sensor 2 terminal 1
4 T-SENSOR_3 Input NTC Sensor 3 terminal 1
5 T-SENSOR_4 Input NTC Sensor 4 terminal 1
6 T-SENSOR_5 Input NTC Sensor 5 terminal 1
7 T-SENSOR_6 Input NTC Sensor 6 terminal 1
8 T-SENSOR_7 Input NTC Sensor 7 terminal 1
9 FUSED_VBAT- Input NTC Sensor 0 terminal 2
10 FUSED_VBAT- Input NTC Sensor 1 terminal 2
11 FUSED_VBAT- Input NTC Sensor 2 terminal 2
12 FUSED_VBAT- Input NTC Sensor 3 terminal 2
13 FUSED_VBAT- Input NTC Sensor 4 terminal 2
14 FUSED_VBAT- Input NTC Sensor 5 terminal 2
15 FUSED_VBAT- Input NTC Sensor 6 terminal 2
16 FUSED_VBAT- Input NTC Sensor 7 terminal 2

Standard 10kΩ NTC resistors (e.g. Farnell-Nr. 1299926) are recommended for use. When using other values than these, the series resistors (R100-R107) on the board may have to be adjusted. Please note that the accuracy of the internal voltage reference VREF2 decreases heavily with a load of over 3mA. Using 8x 10kΩ NTC resistors with the corresponding 10kΩ series resistors results in a current of 1.2mA (at 20°C) which is drawn from VREF2.

Each 8 temperature sensors are connected to an analog multiplexer. The analog multiplexer can be controlled via I²C by the LTC6813-1 (7-bit address: 0x4C). In order to ensure fast settling times after switching the multiplexer input, the output signal of the multiplexer is buffered by an operational amplifier. Finally the analog voltage of the selected sensor is measured on the GPIO1 pin of the LTC6813-1.

3.5.2.5. On-board EEPROM

The primary unit as well as the secondary unit of the foxBMS BMS-Slave board is equipped with an EEPROM IC (IC801). The EEPROM for example can be used for storing data such as calibration values. Similar to the analog multiplexers, the EEPROM device is connected to the I²C bus of the LTC6813-1 (7-bit address: 0x50).

3.5.2.6. On-board Ambient Temperature Sensor

For an additional monitoring of the ambient temperature an on-board temperature sensor is used. This temperature sensor can be read by the LTC6813-1 via the I²C bus (7-bit address: 0x48). It is possible to program an alert temperature. Once the measured temperature reaches this alert temperature the alert pin of the IC is set to a logic low level. Currently this signal is not used on the BMS-Slave board, but it is accessible on the connector X404.

3.5.2.7. Additional Inputs and Outputs

Several additional analog and digital inputs and outputs are provided on the BMS-Slave board via pin headers. Each 16 analog inputs are provided on connector X400 (primary) and X401 (secondary). The pinout for the connectors for the primary and secondary unit is identical and is described in table 3.27.

Table 3.27 Connector for analog inputs
Pin Signal Direction Description
1 ANALOG-IN_0 Input Analog input 0
2 ANALOG-IN_1 Input Analog input 1
3 ANALOG-IN_2 Input Analog input 2
4 ANALOG-IN_3 Input Analog input 3
5 ANALOG-IN_4 Input Analog input 4
6 ANALOG-IN_5 Input Analog input 5
7 ANALOG-IN_6 Input Analog input 6
8 ANALOG-IN_7 Input Analog input 7
9 ANALOG-IN_8 Input Analog input 8
10 ANALOG-IN_9 Input Analog input 9
11 ANALOG-IN_10 Input Analog input 10
12 ANALOG-IN_11 Input Analog input 11
13 ANALOG-IN_12 Input Analog input 12
14 ANALOG-IN_13 Input Analog input 13
15 ANALOG-IN_14 Input Analog input 14
16 ANALOG-IN_15 Input Analog input 15
17 +3.0V_VREF2 Output LTC6813-1 3.0V voltage reference
18 FUSED_VBAT- Output GND

Each 8 analog inputs are connected to an analog multiplexer. The analog multiplexers can be controlled via I²C by the LTC6813-1 (7-bit addresses: 0x4D and 0x4E). In order to ensure fast settling times after switching the multiplexer input, the output signals of the multiplexers are buffered by operational amplifiers. Finally the analog voltage of the selected sensor can be measured on the GPIO2 pin of the LTC6813-1.

Each 8 digital inputs/outputs are provided on the connectors X402 (primary) and X403 (secondary). The pinout for the connectors for the primary and secondary unit is identical and is described in table 3.28.

Table 3.28 Connector for digital IOs
Pin Signal Direction Description
1 DIGITAL-IO_0 Input/Output Digital input/output 0
2 DIGITAL-IO_1 Input/Output Digital input/output 1
3 DIGITAL-IO_2 Input/Output Digital input/output 2
4 DIGITAL-IO_3 Input/Output Digital input/output 3
5 DIGITAL-IO_4 Input/Output Digital input/output 4
6 DIGITAL-IO_5 Input/Output Digital input/output 5
7 DIGITAL-IO_6 Input/Output Digital input/output 6
8 +5.0V_VREG Output LTC6813-1 5.0V regulated voltage
9 FUSED_VBAT- Output GND

Each 8 digital inputs/outputs are connected to an I²C controlled port expander (7-bit address: 0x20). The direction of the inputs/outputs as well as the logic levels on the pins can be selected by register settings. Each of the 8 digital inputs/outputs has a discrete pull up resistor that for example can be used for directly connecting a tactile switch.

3.5.2.8. isoSPI Daisy Chain Connection

The data transmission between the slaves and between the slaves and the basic board takes place using the isoSPI interface. The isoSPI signals are input/output on the connectors X500/X501 (primary) and X502/X503 (secondary). The isoSPI ports are bidirectional, that means they can be used in forward and reverse direction. The isoSPI connections are isolated galvanically using pulse transformers (TR1400). The voltage amplitude of the differential signal can be adjusted by setting resistors (see section Daisy Chain Communication Current).

The pinout of the isoSPI connectors is described in table 3.29 and table 3.30.

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Table 3.29 isoSPI Daisy Chain Input Connectors
Connector Pin Daisy Chain
1 IN+ (Primary/Secondary LTC6813-1)
2 IN- (Primary/Secondary LTC6813-1)
Table 3.30 isoSPI Daisy Chain Output Connectors
Connector Pin Daisy Chain
1 OUT+ (Primary/Secondary LTC6813-1)
2 OUT- (Primary/Secondary LTC6813-1)

3.5.2.9. Hardware Settings / Options

3.5.2.9.1. Software Timer

The internal software timer of the LTC6813-1 can be enabled/disabled by a dedicated external pin (SWTEN, pin 36 of the LTC6813-1). In order to support all features, the foxBMS BMS-Slave board offers a possibility to switch the software timer. The software timer is enabled in the standard configuration, which means pin 36 is pulled to VREG via a zero-ohm resistor (R1407). The timer can be disabled by removing the resistor R1407 and placing a zero-ohm resistor to R1406.

3.5.2.9.2. Daisy Chain Communication Current

The daisy chain communication current can be set by the resistors R1400 and R1402. The default value is 820Ω for R1402 and 1.2kΩ for R1400. This values result in a bias current of approx. 1mA and a differential signal amplitude of 1.18V. Theses values are suitable for high noise environments with cable lengths of over 50m. For more information please have a look at the LTC6813-1 datasheet.

3.5.2.9.3. Status LED

The status LEDs LD1400 show the current mode of each, the primary and secondary LTC6813-1. The LED is on in STANDBY, REFUP or MEASURE mode, whereas the LED is off in SLEEP mode. The LED can be disabled by removing the resistor R1403 next to the LED.

3.5.2.9.4. GPIO Extension Connector

The internal GPIO lines 1 to 5 of the primary or secondary LTC6813-1 can be connected to the GPIO extension pin header X404 via optional zero-ohm resistors. In the standard configuration these resistors are not placed. Of course it is possible to place each both resistors for a parallel connection of the internal signals to the GPIO extension connector. For more information see the corresponding page of the schematics. The placement of the resistors and the resulting connection is shown in table 3.31.

Table 3.31 GPIO extension connector
GPIO connect to pin header connect to internal function
1 R1408 R1409 (default)
2 R1410 R1411 (default)
3 R1412 R1413 (default)
4 R1414 R1415 (default)
5 R1416 R1417 (default)

The pinout of the extension connector X404 is described in table 3.32.

Table 3.32 Extension connector
Pin Signal Direction Description
1 +3.0V_VREF2_0 Output Primary LTC6813-1 3.0V reference voltage 2
2 +3.0V_VREF2_1 Output Secondary LTC6813-1 3.0V reference voltage 2
3 +5.0V_VREG_0 Output Primary LTC6813-1 5.0V regulated voltage
4 +5.0V_VREG_1 Output Secondary LTC6813-1 5.0V regulated voltage
5 PRIMARY-GPIO1-OPT Input/Output Primary LTC6813-1 GPIO1
6 SECONDARY-GPIO1-OPT Input/Output Secondary LTC6813-1 GPIO1
7 PRIMARY-GPIO2-OPT Input/Output Primary LTC6813-1 GPIO2
8 SECONDARY-GPIO2-OPT Input/Output Secondary LTC6813-1 GPIO2
9 PRIMARY-GPIO3-OPT Input/Output Primary LTC6813-1 GPIO3
10 SECONDARY-GPIO3-OPT Input/Output Secondary LTC6813-1 GPIO3
11 PRIMARY-GPIO4-OPT Input/Output Primary LTC6813-1 GPIO4
12 SECONDARY-GPIO4-OPT Input/Output Secondary LTC6813-1 GPIO4
13 PRIMARY-GPIO5-OPT Input/Output Primary LTC6813-1 GPIO5
14 SECONDARY-GPIO5-OPT Input/Output Secondary LTC6813-1 GPIO5
15 PRIMARY-WDT Output Primary LTC6813-1 watchdog output
16 SECONDARY-WDT Output Secondary LTC6813-1 watchdog output
17 PRIMARY-TEMP-ALERT Output Primary board temp. sensor alarm output
18 SECONDARY-TEMP-ALERT Output Secondary board temp. sensor alarm output
19 FUSED_VBAT- Output GND
20 FUSED_VBAT- Output GND

The GPIO lines 6 to 9 are wired to the connector X405 permanently. There is no internal function for this GPIO lines. The pinout of the extension connector X405 is described in table 3.33.

Table 3.33 Additional GPIO extension connector
Pin Signal Direction Description
1 PRIMARY-GPIO6 Input/Output Primary LTC6813-1 GPIO6
2 SECONDARY-GPIO6 Input/Output Secondary LTC6813-1 GPIO6
3 PRIMARY-GPIO7 Input/Output Primary LTC6813-1 GPIO7
4 SECONDARY-GPIO7 Input/Output Secondary LTC6813-1 GPIO7
5 PRIMARY-GPIO8 Input/Output Primary LTC6813-1 GPIO8
6 SECONDARY-GPIO8 Input/Output Secondary LTC6813-1 GPIO8
7 PRIMARY-GPIO9 Input/Output Primary LTC6813-1 GPIO9
8 SECONDARY-GPIO9 Input/Output Secondary LTC6813-1 GPIO9

3.5.2.10. External Isolated DC-Supply

It is possible to supply the BMS-Slave Board by an external DC power supply with a voltage range of 8V to 24V. The DC input is protected against reverse voltage and over-current (with a 1.25A fuse). The external DC supply has to be connected on connector X1001 or X1002 (both connectors are in parallel for daisy chaining the supply). The pinout of the connectors X1001 and X1002 is shown in table 3.34.

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Table 3.34 External DC supply connector
Pin Signal Direction Description
1 DC+ Input positive supply terminal
2 DC- Input negative supply terminal