REC ACTIVE BMS 4S
2
www.rec-bms.com
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 LiFePO
4
. 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 (LiFePO
4
)
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
