REC WS500 Active Battery Management System Instruction Manual

REC WS500 Active Battery Management System Instruction Manual

Novi trg 9, 6230 Postojna, Slovenia
mail: info@rec-bms.com; www.rec-bms.com

All information in this document is subject to copyright and other intellectual property rights of REC d.o.o. and its licensors. The material may not be modified, reproduced or copied, in whole or in part without the prior written permission of REC d.o.o. or its licensors. REC d.o.o., 2024.

Features:

– robust and small design
– 4 cells connections (10.5 to 16.8 V)
– up to 8 temperature sensors
– single cell voltage measurement (0.1 – 5.0 V, resolution 1 mV)
– single cell – under/over voltage protection
– single cell internal resistance measurement
– SOC and SOH calculation
– over temperature protection
– under temperature charging protection
– active cell balancing up to 2.5 A DC per cell
– shunt current measurement (resolution 19.5 mA @ ± 500 A)
– galvanically isolated user defined multi-purpose digital output
– programmable relay (normally open or normally closed)
– galvanically isolated RS-485 communication protocol
– CAN communication (Victron compatible)
– error LED + buzzer indicator
– 11 ERROR log on the device FIFO
– PC user interface for changing the settings and data-logging (optional accessory)
– hibernate switch
– one IP65 protected connector for all connections
– ISO16315, ISO10133, EN61558-1, EN61558-2 and EN50498 compliant
– supports Wakespeed WS500 Advanced Alternator Regulator
– supports Victron GX firmware 2.90
– new SW 2.92 : forced charge CAN message, low SOC reset @ low current only, CFVC [0,1]

General Description of the BMS Unit:

The Battery management system (BMS) monitors and controls each cell in the battery pack by measuring its parameters. The capacity of the battery pack differs from one cell to another and this increases with number of charging/discharging cycles. The Li-poly batteries are fully charged at typical cell voltage 4.16 – 4.20 V or 3.5 – 3.7 V for LiFePO4. Due to the different capacity this voltage is not reached at the same time for all cells in the pack. The lower the cell’s capacity the sooner this voltage is reached. When charging series connected cells with a single charger, voltage on some cells might be higher than the maximum allowed voltage. Overcharging the cell additionally lowers its capacity and number of charging cycles. The BMS equalizes cells’ voltage by diverting some of the charging current from higher voltage cells to the whole pack or from the whole pack to a lower voltage cells – active balancing. The device’s temperature is measured to protect the circuit from over-heating due to unexpected failure. Battery pack’s temperature is monitored by Dallas DS18B20 digital temperature sensor/s. Current is measured by a low-side shunt resistor. Battery pack current, temperature and cell’s voltage determine state of charge (SOC). State of health (SOH) is determined by comparing cell’s current parameters with the parameters of a new battery pack. The BMS default HW parameters are listed in Table 1.

Hardware Parameters:

**Table 1: ABMS hardware parameters.

* defined by internal RTC back-up 2032 battery
IP64 on request
**

ABMS Power Supply :

**Table 2: BMS power supply.

* Normal mode (only main relay and charge optocoupler are active)

Default Software Parameters:

Table 3: Default BMS parameter settings*

*all parameters’ values may be changed with PC user interface or REC WiFi module.
** X=1: relay 2 – discharge turns off OV and UV, X=2: relay 2 – discharge turns off only UV!

System Overview:

Figure 1: System overview.

BMS Unit Connections:

Figure 2: BMS unit front panel function overview.

Table 4: BMS unit male socket connections.

Setting the RS-485 Address:

Address of the BMS unit is selected via the Address pins. Factory address is 2. Formula for changing the address is:

ActiveBMS ??????? = 2² ∗ ??????? ??? 3 + 2¹ ∗ ??????? ??? 2 + 2⁰

! If multiple BMS units are used distinguished addresses should be set to avoid data collision on the RS-485 communication bus!

BMS Unit Connector:

Before starting the assembly please go to website: http://www.te.com/catalog/pn/en/776164-1?RQPN=776164-1 …and read the connector assembly datasheet:
AMPSEAL Automotive Plug Connector and Header Assembly in Application Specification and AMPSEAL Automotive Plug Assemblies 776268… in Instruction Sheet (U. S.).

BMS Unit Connector, Cells Part:

Connect each cell to the BMS unit cell connector plug. Use silicon wires with cross section of 0.5 – 1.4 mm² (20-16 AWG). ! Before inserting the connector check the voltage and polarity of each connection!

Figure 3 : Battery pack connection plug – wiring side.

BMS Unit Power Supply:

BMS unit is always supplied from the 4-th cell connection.

BMS Unit Connection Instructions:

Connect all necessary connections to the BMS connector first, check the polarities and then plug the female connector into the BMS. When the system components are plugged in, the enable switch can be turned ON and the unit starts the test procedure. Connection instruction video link: http://www.rec- bms.com/ABMS.html

When disconnecting the unit from the battery pack, the procedure should be followed in reverse order.

RS-485 Communication Protocol:

Figure 4 : RS-485 DB9 connector front view.

Table 5 : RS-485 DB9 connector pin designator.

Galvanically isolated RS-485 (EN 61558-1, EN 61558-2) serves for logging and changing BMS parameters. Dedicated PC BMS Control Software, REC Wi-Fi module or another RS-485 device may be used for the communication. Default RS-485 address is 2.
Unlock password: Serial without the first minus e.g. 1A-XXXX.

Messages are comprised as follows:
STX, DA, SA, N, INSTRUCTION- 4 bytes, 16-bit CRC, ETX

• STX start transmission <0x55> (always)
• DA – destination address <0x01> to <0x10> (set as 6)
• SA – sender address <0x00> (always 0)
• N – number of sent bytes
• INSTRUCTION 4 bytes for example.: ‘L’,’C’,’D’,’1′,’?’, – (combined from 4 ASCII characters, followed by ‘?’, if we would like to receive the current parameter value or ‘ ’,’xx.xx’ value in case we want to set a new value
• 16-bit CRC – big endian, for the whole message except STX in ETX – https://www.lammertbies.nl/comm/info/crc-calculation.html
• ETX – end transmission <0xAA> (always)

Dataflow:

• Bit rate: 56k
• Data bits: 8
• Stop bits: 1
• Parity: None
• Mode: Asynchronous
• Little endian format when an array is sent

Table 6 : RS-485 instruction set.

*BMS->Time/Date pop-up window can be used to set both values at once.

Parameter accepted and changed value is responded with ‘SET’ answer.
Example: proper byte message for ‘LCD1?’ instruction for BMS address 2 is:

RS-485 message is executed when the microprocessor is not in interrupt routine so a timeout of 350ms should be set for the answer to arrive. If the timeout occurs the message should be sent again. Little endian format is used for all sent float or integer values. In case of single data is sent ASCII characters are used e.g. -1.2351e2
Custom made instructions can be added to the list to log or set the parameters that control the BMS algorithm or its outputs.

Video instruction link for settings change: REC changing settings – YouTube
Video instruction link for firmware update:
-with PC user interface: REC firmware update procedure – YouTube
-with REC WiFi module: REC WiFi and BMS update procedure

CAN Communication:

Figure 5 : CAN female DB9 connector front view.

Table 7 : CAN DB9 connector pin designator.

No termination is used inside ABMS connector. Terminate pins 2 and 7 with 120 Ohm resistor to prevent ABMS to reset. REC CAN cables are already terminated using 120 Ohm resistors between CANH and CANL inside the DB9 connector. Additional RJ45 connector with 120 Ohms across CANL and CANH should be used for the end device on the CAN bus for end termination.
11-bit TX identifiers: 0x351, 0x355, 0x356, 0x35A, 0x35E, 0x35F, 0x360, 0x372, 0x373, 0x374, 0x375, 0x376, 0x377, 0x379, 0x380 and 0x381.
11-bit RX heart-beat 0x305 message from GX unit is neglected.
CAN messages are sent every 200 ms.

When the CAN frequency is changed via RS-485 instruction CANF, BMS has to be reset to enable the new setting.

Figure 6 : Wakespeed CAN RJ45 pinout

GX device settings:

Update GX device to 2.90 or higher.
Connect BMS and GX device with CAN cable. Use BMS-Can port with Cerbo GX or VE-Can port with other GX devices.
Open Settings menu and select Services. Set CAN-bus profile to CAN-bus BMS (500kbit/s) for Cerbo GX or VE.Can & CAN-bus BMS (250kbit/s) for other GX devices.
Then in Settings menu select System setup. Change:
– Battery monitor to REC BMS in CAN-bus.
In menu DVCC:
– DVCC→ON
– SVS→ON
– STS → ON

GX device settings are automatically saved when you change it. Make sure you set the same CAN-bus frequency at the BMS and at the GX device. You can change the BMS CAN-bus frequency with command “CANF” with a WiFi module or BMS control PC software.

IMPORTANT: Always keep CAN communication connected to BMS when resetting inverters and chargers.

Wakespeed System Configuration:

Download and install latest Wakespeed firmware from: http://wakespeed.com/technical.html

On your Wakespeed device set proper DIP switches and connect BMS to Wakespeed WS500 through CAN bus.

Voltage/temperature hysteresis:

Most of the BMS setting thresholds also have a dedicated hysteresis parameter. This way the BMS prevents ringing due to the oscillation of the controlled parameter above and under the set threshold. If the threshold limits the top value of the parameter like Maximum cell voltage CMAX or temperature TMAX, the value of hysteresis should be negative to prevent the ringing. If the threshold limits the bottom value of the parameter like Minimum cell voltage CMIN or temperature TMIN the value of hysteresis should be positive to prevent the ringing. For a simplicity, all the BMS settings are set without the sign and the BMS firmware takes care for proper sign value.

BMS Unit Start Procedure:

When the BMS unit is turned ON it jumps into a boot-loader and checks if the user tries to upload a new firmware. Then it commences the test procedure by checking the balancing fuses and temperature sensors. Power LED (green) blinks while balancing is tested. After the test procedure red error LED turns off and the BMS unit starts working in normal mode.
If an error is detected a sound alarm/blinking red LED signal will notify the user. Each error is coded to a number.
An overview of all possible system errors is presented in the System Error Indication section.

BMS Unit LED Indication:

Power LED (green) signals the state of the battery pack. Low SOC is signaled by a single ON blink. Normal mode is signaled by 2 consecutive ON blinks while the balancing mode is indicated by 3 consecutive ON blinks before the longer pause. When the battery pack is fully charged and SOC/End of Charge Hysteresis are set, POWER LED is turned 100% on.

Error LED (red) is turned on in case of system error and signals the error number with 50 % duty cycle. Between repeated error number 1 s timeout is introduced.

Cell Voltage Measurement:

Cell voltages are measured every second. The cell measurement performs 4 ms cell measurement by Sigma Delta ADC. Each cell voltage is measured after the balancing fuse, in case the fuse blows, BMS signals error 10 to notify the user.

BMS Cell Balancing:

Cells are balanced actively with very high efficiency in opposite to passive balancing, where all energy is lost in heat. Another benefit of active balancing is charging of dangerously low cell, if other cells are above dangerous level, consequently longer pack usage is possible.

Balancing START Voltage (BMIN):

If errors 2, 4, 5, 8, 10, 12 are not present and the highest cell voltage rises above Balance START voltage and current is > 0.2 A (charging stage) the BMS initiates balancing algorithm. A weighted cell voltage average is determined including cells DC internal resistance. Balancing algorithm calculates the voltage above which the cells are balanced. The lowest cell voltage is taken into account determining balancing voltage.

Balancing END Voltage (BVOL):

If errors 2, 4, 5, 8, 10, 12 are not present any cell is above balance END voltage are balanced regardless the battery pack current.

Cell Internal DC Resistance Measurement:

Cell internal DC resistance is measured as a ratio of a voltage change and current change in two sequential measurement cycles. If the absolute current change is above 20 A, cells internal resistance is calculated. Moving average is used to filter out voltage spikes errors.

Battery Pack Temperature Measurement:

Battery pack temperatures are measured by Dallas DS18B20 digital temperature sensor/s. Up to two sensors can be used in parallel connected directly to the wiring. Up to 8 sensors may be used with a junction box. BMS should be turned off and main connector disconnected before adding sensors. If the temperature sensors wiring is placed near the power lines shielded cables should be used.

BMS Current Measurement:

Low-side only precision shunt resistor for current measurement should be used. A 4-wire Kelvin connection is used to measure voltage drop on the resistor. As short as possible shielded cable should be used to connect the power shunt and BMS. The battery pack current is measured every second. A high precision Sigma-Delta ADC is used to filter out the current spikes. The first current measurement is timed at the beginning of the cell measurement procedure for a proper internal DC resistance calculation. Two more 300 ms measurements are performed through the whole ABMS measurement interval Shunt connection is shown in Fig. 6. If the BMS measures charging/discharging current that is higher than the double value of the rated shunt for more than 2 consecutive cycles error 12 is triggered and the MAIN relay is turned off. This serves for shunt, contactor and fuse protection in case of short circuit. Fuse, manual DC switch or other disconnection device s should be all placed to battery positive/system positive since the CAN GND may supply the inverter.

Figure 7 : Shunt resistor connection.

Voltage-to-current Coefficient:

Different size and resistance shunts can be used, since the voltage-to-current coefficient can be changed in the BMS Control software or REC WiFi module as IOJA x.xxxx Current is calculated by the voltage drop at the shunt resistor. 1 LSB of the 18-bit ADC represents different current values according to the shunt resistance. The LSB coefficient can be calculated as:

where the represents the voltage drop on shunt resistor at current .

Table 8 : Voltage-to-current coefficients for typical shunt resistors.

Battery Pack SOC Determination:

SOC is determined by integrating the charge in or out of the battery pack. Different Li-ion chemistries may be selected:

Table 9 : Li-ion chemistry designators.

Temperature and power correction coefficient are taken into consideration at the SOC calculation. Li- Po chemistry algorithms have an additional voltage to SOC regulation loop inside the algorithm. BMS calculates battery self- discharge upon selected chemistry, SOC and temperature. State of health (SOH) is calculated as number of cycles compared to battery end of life cycles and compensated with SOH and temperature. Operational capacity is recalculated by the number of the charging cycles as pointed out in the manufacturer’s datasheet.

When BMS is connected to the battery pack for the first time, SOC is set to 50 %. SOC is reset to 100 % at the end of charging. Charging cycle is added if the coulomb counter had reached the Battery Pack’s Capacity CAPA.

Battery Pack’s Charging Algorithm:

The communication between the REC ABMS and the Victron GX device is established through the CAN bus. All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to the GX device in each measurement cycle.

The charging current is controlled by the Maximum charging current parameter sent to the GX device. It’s calculated as Charge Coefficient CHAC x Battery capacity CAPA. The parameter has an upper limit which is defined as Maximum Charging Current per Device MAXC x Number of Inverter/Charger Devices STRN. Lowest value is selected .

Table 10 : Maximum charging current calculation.

Charge Coefficient CHAC x Battery Capacity CAPA = 0.6 1/h x 100Ah = 60 A
Maximum Charging current per device MAXC x Number of Inverter/Charger devices STRN = 75 A x 2 = 150 A

Maximum charging current is set to 60 A due to lower value of the Charge Coefficient CHAC x Battery Capacity CAPA.

When the highest cell reaches the End of charging CHAR voltage setting, charging current starts to ramp down to 1.1 A x Number of Inverter/Charger Devices STRN until the last cell rises near the End of charging CHAR (CC/CV) – UBDI (End of charge unbalance difference). At that point the Maximum charging voltage allowed is set to Number of cells x (End of Charge Voltage per cell CHAR – Maximum Cell Float Voltage Coefficient CFVC x End of charging hysteresis per cell). End of charge SOC hysteresis SOCH and End of charge cell voltage hysteresis CHIS is set to prevent unwanted switching. SOC is calibrated to 100 % and Power LED lights ON 100 %. Charge optocoupler is turned off.
Maximum allowed charging current is set to 50% to allow supplying DC loads from charging devices like MPPTs. Charging current is limited to 30 % of the maximum charging current, but more than 5 A near both ends of temperature (Max cell temperature TMAX and Min temperature for charging TMIN) and when the battery is empty (Max discharging current is set to zero). In case the Cell Float Voltage Coefficient CFVC is set to zero, max charging current is set to zero and max voltage to Number of cells x (End of Charge Voltage per cell CHAR).

Charging is stopped in case of systems errors (see System Errors indication chapter).
SOC is calibrated to 96 % when the maximum open circuit cell voltage rises above 0.502 x (Balance start voltage BMIN + End of charging voltage CHAR), minimum open circuit voltage above Balance start voltage BMIN and system current is positive.

ABMS forces a full charge if the pack was not fully charged for more than three weeks.

In case BMS is not able to control the MPPT/Non-Victron charging sources directly (MPPT should be set to charge when the remote is in short), a small signal relay can be used to amplify the signal. MPPT should be programmed with its own charging curve set as End of charging voltage CHAR x number of cells. Digital output may be programmed with another task on request e.g. heater, under-voltage alarm, …

Figure 8 : Charging diagram

Maximum Cell Float Voltage Coefficient (CFVC):

Maximum Cell Float Voltage Coefficient CFVC has been introduced into the charging algorithm to enable cell float voltage change after the full charge. It may be set from 0.0 to 1.0 of the End of Charging Hysteresis CHIS. When End of Charging Hysteresis CHIS and End of Charging SOC hysteresis SOCH have been met, full charge is enabled again. 50 % of maximum charging current is allowed to supply DC loads from MPPTs directly without discharging the battery pack below End of Charging Hysteresis CHIS and End of Charging SOC hysteresis SOCH if CFVC is set > 0.0.

Battery Pack’s Discharging Algorithm:

Calculated maximum discharging current is sent to the GX device by CAN communication in each measurement cycle. When the BMS starts/recovers from the error or from Discharging SOC hysteresis, maximum allowed discharging current is set. It is calculated as Discharge Coefficient DCHC x Battery Capacity CAPA. If this value is higher than Maximum Discharging Current per device MAXD x Number of Inverter/Charger Devices STRN, maximum discharging current is decreased to this value.

Table 11 : Maximum discharging current calculation.

Discharge Coefficient DCHC x Battery Capacity CAPA = 1.5 1/h x 100Ah = 150 A
Maximum Discharging Current per device MAXC x Number of Inverter/Charger devices STRN = 100 A x 2 = 200 A
Maximum discharging current is set to 150 A.

When the lowest cell open circuit voltage is discharged bellow the set threshold CLOW maximum discharging current starts to decrease down to 0.02 C (2 % of Capacity CAPA in A). After decreasing down, maximum allowed discharging current is set to 0 A. If the battery discharge current is lower than 0.05 C or positive, SOC is reset to 3 % and Discharging SOC hysteresis is set to 5 %. If the cell discharges below Minimum Cell voltage CMIN, BMS signals Error 2 and internal relay switches off. SOC is reset to 1 % in case of discharge current lower than 0.05C or positive.
If the Charger/inverter is connected to the grid maximum allowed discharge current is drawn from the grid. Otherwise, 100 % load current is drawn from the battery until maximum allowed discharging current is set to 0 A. Discharging current is also limited near both ends of temperature (Max cell temperature TMAX and Min temperature for charging TMIN) to 30%, but more than 5 A. If the minimum cell discharges under the Cell-under voltage protection switch-off CMIN x 0.98 for more than 60 s BMS goes to deep sleep mode to protect the cells from over-discharging. OFF-ON switch sequence wakes the BMS from this state. CLOW cell voltage setting should be set to the voltage that corresponds to 3 % of the usable capacity.

Figure 9 : Discharging diagram

ABMS Outputs Settings:

System devices can be controlled by digital outputs Charge optocoupler, optocoupler 2 and relay 1. Charge optocoupler is already pre-programmed as charge sources control, while optocoupler 2 and relay 1 can be user programmed.
Relay 1 is factory programmed as top disconnect (charging sources). Optocoupler 2 is factory programmed as bottom disconnect (inverter/discharging disconnect).
Errors 4, 5, 8, 10, 11, 12 and 13 disconnect relay 1 and optocoupler 2.
Programming is possible with a REC WiFi module under tab Tasks. Please read the WiFi manual, under Task tab for additional information. Both outputs can be programmed depending on temperature, cell voltage, pack voltage, current flow and SOC.

Digital Outputs:

Digital outputs are implemented with galvanic isolation. Optocouplers with diode reverse protection are used. When closed, a 0.7 V voltage drop over the digital output should be taken into account. Optocoupler outputs can drive a small signal relay or LED diodes. Fig. 9 shows two different connection schematics. Both outputs can be used to drive LED diodes for charge/discharge relay indication.

Figure 10: BMS digital outputs schematics.

Current limit resistor R can be calculated as:

represents LED forward voltage drop (typ. 1.9 – 2.3 V) while represents LED current (2-5 mA).

Pre-charge Connection:

Battery pack main contactor is driven by ABMS internal main relay. In case of system with high input capacity (> 2,000 μF) a pre-charge system should be used to avoid high in-rush current spikes when the main contactor is turned on. High currents like this degrade the contactor, cells and input capacitors in the electronic devices. Fig. 10 shows contactor connection with the pre- charge unit. System voltage should be measured after the pre-charge without the connected contactor. System voltage should rise to > 70 % of the battery pack voltage. Otherwise longer pre-charge time should be used to ensure normal in-rush current.
Pre-charge module can work with normal or bi-stable contactor. Before connecting it to the system, please read the Pre-charge module manual and set the proper DIP switch positions inside the module.

Figure 11 : Contactor – Pre-charge module connection schematics with battery voltage contactor drive.

Parallel Cells Connection:

Capacity can be increased by connecting multiple cells in parallel and then connect these sub-packs in series. Fig. 9 shows 2P4S connection with 2 cells in parallel and 4 pack like this in series. For proper current distribution 2 connection bars should be used between each 2P to 2P sub-pack.

Figure 12: 2P4S battery pack connection.

System Error Indication:

System errors are indicated with red error LED by the number of ON blinks, followed by a longer OFF state. Each and every error number trigger algorithm has a normal delay time of 3 measuring cycles with sensed/measured error – 3 x 1.25 s before the error is triggered.

Errors 2 and 10 are set to trigger at the first measured error when the BMS is turned ON. If the two errors are not present normal delay timer is set.

Table 12 : BMS error states.

BMS Unit Dimensions:

Figure 13 : BMS dimensions.

M4 bolts are preferred to use for mounting. BMS unit can be also supplied without the enclosure, if an application is weight or space limited. The dimensions of the BMS (including connector) without the enclosure are 109 mm x 100 mm x 38 mm. The PCB has four 3.2 mm mounting holes.

References

• REC BMS – REC Battery Management System
• REC BMS – REC Battery Management System
• On-line CRC calculation and free library - Lammert Bies

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