REC ACTIVE BMS 4S
11
www.rec-bms.com
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:
𝑘
𝐿𝑆𝐵
= 0.01171875 ∙
0.05
V
300
A
∙
𝐼
currentx
𝑉
dropx
where the
V
dropx
represents the voltage drop on different shunt resistor at current
I
currentx
.
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 LiMn
2
O
4
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.
