REC ACTIVE BMS 4S User manual

Rozna ulica 20, 6230 Postojna, Slovenia
e-mail: [e-mail]; www.rec-bms.com
1
ACTIVE BATTERY MANAGEMENT SYSTEM
REC ACTIVE BMS
Features:
-robust and small design
-4 cells
-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 20 mA @ ±500 A)
-2 galvanically isolated user defined multi-purpose digital output
-2 programmable relay (normally open or normally closed)
-galvanically isolated RS-485 communication protocol
-CAN communication (on request)
-error LED + buzzer indicator
-PC user interface for changing the settings and data-logging (optional accessory)
-hibernate switch
-one IP65 protected connector for all connections
-one-year warranty

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General Description of the BMS Unit:
Battery management system (BMS) is a device that 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 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 whole pack or from whole pack to 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
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 the new battery pack.
The BMS default parameters are listed in Table 1.
Default Parameters:
Table 1: Default BMS parameter settings.
parameter
value
unit
chemistry
3 (LiFePO4)
n.a.
capacity
100
Ah
balance start voltage
3.5
V
balance end voltage
3.65
V
maximum diverted current per cell
up to 2.5 (5 pp)
A
cell over voltage switch-off
3.85
V
cell over voltage switch-off hysteresis per cell
0.05
V
charger end of charge switch-off pack
3.65
V
cell under voltage protection switch-off
2.8
V
under voltage protection switch-off hysteresis per cell
0.05
V
pack under voltage protection switch-off timer
4
s
cells max difference
0.25
V
BMS maximum pack voltage
16.8
V
BMS charge hysteresis per cell
0.2
V
BMS SOC charge hysteresis
5
%
BMS over temperature switch-off
55
°C
BMS over temperature switch-off hysteresis
5
°C
cell over temperature switch-off
55
°C
under temperature charging disable
-15
°C
voltage to current coefficient
0.01953125
A/bit
max DC current relay @ 60 V DC
0.7
A
max AC current relay @ 230 V AC
2
A
max DC current @ optocoupler
15
mA
max DC voltage@ optocoupler
62.5
V
BMS unit disable power supply
< 1
mW
BMS unit stand-by power supply
< 60
mW
BMS unit cell balance fuse rating (SMD)
3
A
internal relay fuse
2 slow
A
dimensions (w × l × h)
105 x 135 x 44
mm
IP protection
IP65

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System Overview:
Figure 1: System overview.
BMS Unit Connections:
Figure 2: BMS unit front panel function overview.

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Table 2: BMS unit male socket connections.
connection
description
1
Charge Relay
Normally closed
2
Charge Relay
Fused input
3
Discharge Relay
Normally closed
4
Discharge Relay
Normally open
5
Discharge Relay
Fused input
6
Hibernate switch signal
-
7
Hibernate switch ground
-
8
Cell 4 positive +
Analog signal
9
Cell 3 positive +
Analog signal
10
Cell 2 positive +
Analog signal
11
Cell 1 positive +
Analog signal
12
Cell 1 ground -
Analog signal
13
Charge Relay
Normally open
14
Optocoupler charge collector
-
15
Optocoupler charge emitter (darlington + reverse
protection diode + polyfuse)
-
16
Optocoupler discharge collector
-
17
Optocoupler discharge emitter (darlington +
reverse protection diode + polyfuse)
-
18
CAN Vcc
4 V output from the
19
CANL
-
20
RS485 Vcc
-
21
RS485 A
-
22
RS485 ground
-
23
RS485 B
-
24
Shunt-
System ground
25
Shunt+
Cell 1 ground
26
-
-
27
Dallas 18B20 temp. sensor
GND + shield
28
Dallas 18B20 temp. sensor
+ 5 V
29
Dallas 18B20 temp. sensor
1-wire digital signal
30
CANH
-
31
CAN ground
GND potential of the battery pack
32
-
-
33
Address pin 3
Normally 0, connect to pin 35 to
change to 1
34
Address pin 2
Normally 0, connect to pin 35 to
change to 1
35
Address pin ground
Fused ground for Address pins

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Setting the RS-485 Address:
Address of the BMS unit is selected via Address pins. Factory address is 2 . Formula for changing address is:
   



















  



















 
! 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 assembly please go to website:
http://www.te.com/catalog/pn/en/776164-1?RQPN=776164-1
…and read 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 voltages and polarities with voltmeter of each connection!
Figure 3: Battery pack connection plug –front side.

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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.
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 3: RS-485 DB9 connector pin designator.
pin
designator
1
-
2
AGND
3
B
4
A
5
-
6
+5V to AGND
7
-
8
-
9
-
Galvanically isolated RS-485 (EN 61558-1, EN 61558-2) serves for logging and changing BMS parameters. Dedicated
PC BMS Control Software or another RS-485 device may be used for the communication. Default RS-485 address is
2.
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
ETX - end transmission <0xAA> (always)

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Dataflow:
Bit rate: 56k
Data bits: 8
Stop bits: 1
Parity: None
Mode: Asynchronous
Table 4: RS-485 instruction set.
INSTRUCTION
DESCRIPTION
BMS ANSWER
'*','I','D','N','?'
Identification
Answer “ACTIVE”
'L','C','D','1','?'
Main data
Returns 7 float values
LCD1 [0] = min cell voltage,
LCD1 [1] = max cell voltage,
LCD1 [2] = current,
LCD1 [3] = max temperature,
LCD1 [4] = pack voltage,
LCD1 [5] = SOC (state of charge) interval 0-1->
1=100% and
LCD1 [6] = SOH (state of health) interval 0-1->
1=100%
'C','E','L','L','?'
Cell voltages
BMS first responds with how many BMS units are
connected, then it sends the values of the cells in
float format
'P','T','E','M','?'
Cell temperatures
BMS first responds with how many BMS units are
connected then it sends the values of the
temperature sensors in float format
'R','I','N','T','?'
Cells internal DC resistance
BMS first responds with how many BMS units are
connected then it sends the values in float format
'B','T','E','M','?'
BMS temperature
BMS first responds with value 1, then it sends the
values of the BMS temperature sensor in float
format
'E','R','R','O','?'
Error
Responds with 4 bytes as follows
ERRO [0] = 0 –no error, 1 –error
ERRO [1] = BMS unit
ERRO [2] = error number (1-13) in
ERRO [3] = number of the cell, temp. sensor where
the error occurred
'B','V','O','L', '?'/
'B','V','O','L', ' ','x.xx'
Cell END balancing
Returns float voltage [V]
'C','M','A','X','?'/
'C','M','A','X',' ','x.xx'
Max allowed cell voltage
Returns float voltage [V]
'M','A','X','H', '?'/
'M','A','X','H', ' ','x.xx'
Max allowed cell voltage
hysteresis
Returns float voltage [V]
'C','M','I','N', '?'/
'C','M','I','N', ' ','x.xx'
Min allowed cell voltage
Returns float voltage [V]
'M','I','N','H', '?'/
'M','I','N','H', ' ','x.xx'
Min allowed cell voltage
hysteresis
Returns float voltage [V]
'T','M','A','X', '?'/
'T','M','A','X', ' ','x.xx'
Maximum allowed cell
temperature
Returns float temperature [°C]
'T','M','I','N', '?'/
'T','M','I','N', ' ','x.xx'
Minimum allowed
temperature for charging
Returns float temperature [°C]
'B','M','I','N', '?'/
'B','M','I','N', ' ','x.xx'
Balancing START voltage
Returns float voltage [V]

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'C','H','A','R', '?'/
'C','H','A','R', ' ','x.xx'
End of charging voltage per
cell
Returns float voltage [V]
'C','H','I','S', '?'/
'C','H','I','S', ' ','x.xx'
End of charging voltage
hysteresis per cell
Returns float voltage [V]
'I','O','F','F','?'/
'I','O','F','F',' ','x.xx'
Current measurement zero
offset
Returns float current [A]
'T','B','A','L','?'/
'T','B','A','L',' ','x.xx'
Max allowed BMS
temperature
Returns float temperature [°C]
'B','M','T','H','?'/
'B','M','T','H',' ','x.xx'
Max allowed BMS
temperature hysteresis
Returns float temperature [°C]
'V','M','A','X','?'/
'V','M','A','X',' ','xxx'
Number of exceeded value of
CMAX
Returns integer value
'V','M','I','N','?'/
'V','M','I','N',' ','xxx'
Number of exceeded value of
CMIN
Returns integer value
'C','Y','C','L','?'/
'C','Y','C','L',' ','xxx'
Number of battery pack
cycles
Returns integer value
'C','A','P','A','?'/
'C','A','P','A',' ','x.xx'
Battery pack capacity
Returns float capacity [Ah]
'I','O','J','A','?'/
'I','O','J','A',' ','x.xx'
Voltage to current coefficient
Returns float value
'R','A','Z','L','?'/
'R','A','Z','L',' ','x.xx'
Package cell difference
Returns float voltage [V]
'C','H','E','M', '?'/
'C','H','E','M', ' ','xxx'
Li-ion chemistry
Returns unsigned char value
'S','O','C','H','?'/
'S','O','C','H',' ','x.xx'
Charger SOC hysteresis
Returns float value 0 - 0.99
'S','O','C','S','?'/
'S','O','C','S',' ','x.xx'
SOC manual re-set
Returns float value 0 –1.0
Parameter accepted and changed value is responded with 'SET' answer.
Example: proper byte message for 'LCD1?' instruction for BMS address 1 is:
<0x55><0x02><0x00><0x05><0x4C><0x43><0x44><0x31><0x3F><0x53><0x90><0xAA>
RS-485 message are executed when the microprocessor is not in interrupt routine so a timeout of 350 ms should
be set for the answer to arrive. If the timeout occurs the message should be sent again. If an array of data is sent
little endian is used for 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.

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CAN Communication Protocol (installed and programmed on request):
Figure 5: CAN female DB9 connector front view.
Table 4: CAN DB9 connector pin designator.
pin
designator
1
TERMINATION
2
CANL + TERMINATION
3
GND
4
5
-
6
+ 5V
7
CANH
8
-
9
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. After the test
procedure red error LED turns off and the BMS unit starts working in normal mode.
BMS Unit LED Indication:
Power LED (green) is turned on in 1 s intervals, if the BMS is powered. Error LED (red) is turned on in case of system
error and blinks number of error with 50% duty cycle. Between every number blinking, a small timeout is
introduced.
Cell Voltage Measurement:
Cell voltages are measured every second. The cell measurement algorithm performs several measurements to
digitally filter the influence of 50, 60, 100 and 120 Hz sinus signal. 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.

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Balancing START Voltage:
If errors 2, 4, 5, 8, 10, 12 are not present, highest cell voltage rises above Balancing 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:
If errors 2, 4, 5, 8, 10, 12 are not present, the cells above balancing 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 sensors. Up to eight sensors can
be used in parallel. BMS should be turned off before adding additional sensors. If the temperature sensors wiring
is placed near the power lines shielded cables should be used.
BMS Current Measurement:
A low-side precision shunt resistor for current measurement is used. A 4-wire Kelvin connection is used to measure
the 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 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. Shunt connection is shown in Fig. 6.
Figure 6: Shunt resistor connection.

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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 as 'I','O','J','A',' ','xxxxx' 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:
     V
 A currentx dropx
where the Vdropx represents the voltage drop on different shunt resistor at current Icurrentx.
ADC has a pre-set gain of 8. With a maximum input voltage difference of 0.256 V.
Battery Pack SOC Determination:
SOC is determined by integrating the charge in to or out of the battery pack. Different Li-ion chemistries may be
selected:
Table 6: Li-ion chemistry designators.
Number
Type
1
Li-Po Kokam High power
2
Li-Po Kokam High capacity
3
Winston/Thunder-Sky/GWL LiFePO4
4
A123
5
Li-ion LiMn2O4
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. Actual cell 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 minimum SOC of 35 % or less was reached in the cycle.
Battery Pack’s Charging Algorithm:
When all the cells reach End of Charge voltage SOC is reset to 100 %, charging hysteresis and SOC charging hysteresis
are set to prevent contactor ringing. If maximum cell voltage is reached, charging contactor is disconnected. Some
of the BMS Errors also disconnect charging sources from the battery. Digital output optocoupler charge follows the
same principle.
Battery Pack’s Discharging Algorithm:
When the lowest cell drops below cell under voltage protection switch-off the discharge contactor is switched off
after 4 s time intervals. Some of the BMS Errors also disconnect charging sources from the battery. Digital output
optocoupler discharge follows the same principle.

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Digital Outputs:
Digital outputs are implemented with galvanical isolation. Darlington optocouplers with diode reverse protection
are used. When closed, 0.9 V voltage drop over the digital output should be taken into account. Optocoupler
outputs can drive small signal relays or LED diodes. Fig. 7 shows two different connection schematics. Both outputs
can be used to drive LED diodes for charge/discharge relay indication. Both digital outputs can be modified as non-
galvanically isolated outputs with PWM or even digital inputs.
Figure 7: BMS digital outputs schematics.
Current limit resistor R can be calculated as:
      

VFVLED represents LED forward voltage drop (typ. 1.9 –2.3 V) while ILED represents LED current (2-5 mA).

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Contactor Connection:
Charging/discharging contactors are driven by charge/discharge relays in the BMS. If there is high input capacity (>
2,000 uF) at the charging sources/discharging loads, pre-charge should be used to avoid high current spikes when
the contactor is turned on. High current spikes degrade the contactor, cells and input capacitors in the electronic
device. Fig. 8 shows contactor connection with or without the pre-charge circuit.
Figure 8: Contactor connection schematics.
Parallel Cells Connection:
Capacity can be increased by connecting multiple cells in parallel and then connect these sub-packs in series. Fig. 9
shows 3P4S connection with 3 cells in parallel and 4 pack like this in series.
Figure 9: 3P4S battery pack connection.

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System Error Indication:
System errors are indicated with red error LED by the number of ON blinks, followed by a longer OFF state.
Table 7: BMS error states.
Number of
ON blinks
ERROR
BMS
OWNER
1
Single or multiple cell
voltage is too high
(cell over voltage
switch-off).
BMS will try to balance down the
problematic cell/cells to safe voltage
level (10 s error hysteresis + single cell
voltage hysteresis is applied).
Charging is disabled, discharging is
enabled. Charge optocoupler is disabled,
discharge optocoupler is enabled.
Wait until the BMS does its job.
2
Single or multiple cell
voltage is too low
(cell under voltage
protection switch-
off).
BMS will try to charge the battery
(10 s error hysteresis + single cell voltage
hysteresis is applied).
Charging is enabled, discharging is
disabled.Charge optocoupler is enabled,
discharge optocoupler is disabled.
Plug in the charger.
3
Cell voltages differs
more than set.
BMS will try to balance the cells
(5 s error hysteresis + 20 mV voltage
difference hysteresis).
Charging is enabled, discharging is
enabled. Charge optocoupler is enabled,
discharge optocoupler is enabled.
Wait until the BMS does its job. If
the BMS is not able to balance
the difference in a few hours,
contact the service.
4
Cell temperature is
too high (over
temperature switch-
off).
Cells temperature or cell inter-
connecting cable temperature in the
battery pack is/are too high. (10 s error
hysteresis 2°C hysteresis).
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Wait until the pack cools down.
5
BMS temperature is
too high –internal
error (BMS over
temperature switch-
off).
Due to extensive cell balancing the BMS
temperature rose over upper limit (5 s
error hysteresis + 5 °C temperature
hysteresis).
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Wait until the BMS cools down.
6
Number of cells,
address is not set
properly.

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7
The temperature is
too low for charging
(under temperature
charging disable).
If cells are charged at temperatures
lower than operating temperature range,
cells are aging much faster than they
normally would, so charging is disabled
(2 °C temperature hysteresis).
Charging is disabled, discharging is
enabled. Charge optocoupler is disabled,
discharge optocoupler is enabled.
Wait until the battery’s
temperature rises to usable
range.
8
Temperature sensor
error.
Temperature sensor is un-plugged or not
working properly (2 s error hysteresis).
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Turn-off BMS unit and try to re-
plug the temp. sensor. If the BMS
still signals error 8, contact the
service. The temperature sensors
should be replaced.
9
Communication
error.
RS-485 Master-Slave communication
only.
10
Cell in short circuit or
BMS measurement
error.
Single or multiple cell voltage is close to
zero or out of range, indicating a blown
fuse, short circuit or measuring failure
(20 s error hysteresis + 10 mV voltage
difference hysteresis).
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Turn-off the BMS and check the
cells connection to the BMS and
fuses. Restart the BMS.
If the same error starts to signal
again contact the service.
11
Main relay is in short
circuit.
If the main relay should be opened and
current is not zero or positive, the BMS
signals error 11. When the error is
detected, the BMS tries to un-shorten
the main relay by turning it ON and OFF
for three times.
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Restart the BMS unit. If the same
error starts to signal again
contact the service.
12
Error measuring
current.
Current sensor is disconnected or not
working properly.
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Turn-off the BMS and check the
sensor connections, re-plug the
current sensor connector. Turn
BMS back ON. If the BMS still
signals error 12, contact the
service.
13
Wrong cell chemistry
selected.
In some application the chemistry pre-
set is compulsory (5 s error hysteresis).
Charging is disabled, discharging is
disabled. Charge optocoupler is disabled,
discharge optocoupler is disabled.
Use PC Control Software to set
proper cell chemistry.

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BMS Unit Dimensions:
Figure 10: 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. Sufficient contact surface
for cooling the balancing resistors should be provided (aluminum recommended). The PCB has four 3.2 mm
mounting holes.