ZERO EMISSION VEHICLES AUSTRALIA
4-12 CELL LITHIUM BATTERY MANAGEMENT SYSTEM
long as necessary. Temperature limits may be adjusted in settings, to warn if the temperature
exceeds safe range, restrict charging if the battery is too cold, or shut everything down if the
battery gets too hot.
Sleep Mode
The BMS12i has a Sleep terminal which may be used to put the device into a lower power
standby mode by connecting a wire from it to a Ground terminal. It is most commonly
used in mobile applications, for when a vehicle is neither being driven or being charged.
(Stationary applications typically run 24/7 so the Sleep terminal is often unused.)
Sleep mode will shut down the CAN bus and turn off both relay outputs. The device continues
to monitor cell voltages and shunt current, and will turn itself off completely if any cells get
extremely low.
Automatic Pack balancing
In battery packs built from many cells, optimum performance is attained when all cells are at
the same State of Charge, also known as pack balance. The BMS12i uses a system known as
“shunt balancing”, which switches on resistors across any cells which are above a threshold
voltage to reduce their charge and bring them into line with other cells.
By default the BMS12i uses a dynamic balance threshold, where any cells more than 0.01V
above the average receive balancing. The threshold may also be set manually – if so it is
best to set it to the average maximum charge voltage, so that only cells above average at the
end of a charge receive balancing. Manually setting a shunt threshold below the maximum
charge voltage is not recommended since it may result in all balancers running concurrently,
which actually negates the effect and generates extra heat. Balancing can also be disabled.
The shunt balancers are quite small and can take a long time to correct large imbalances. If
possible it is recommended to manually balance your cells prior to initial pack assembly (e.g
by charging each cell individually, or wiring them all together in parallel to equalise with each
other). However the shunts will get an unbalanced pack incrementally closer to balanced each
charge, and once balanced are able to maintain balance with minimal effort.
CAN Bus Wiring
The BMS12i has four screw terminals for connecting CAN bus
wiring: 6V Out, CAN H, CAN L and Ground. The monitor CAN
plug uses a Molex Eurostyle screw terminal. These connectors
require some force to plug in, so make sure the plug is fully
inserted for a reliable connection. Wiring for the Eurostyle
connector is shown right.
Wire gauge around 20-24AWG is recommended for appropriate current rating and
mechanical strength. For best noise immunity in high EMI environments, shielded twisted
pair cable is recommended, with two conductor pairs – one pair for CAN signals, and one pair
for bus power. The shield can be connected to the Ground terminal at the BMS. We use and
recommend Belden 8723 wire or equivalent.
For optimum performance, CAN buses should be wired as a single daisy chain of devices
(without branching), and terminated at both ends of the bus with a 120Ω resistor across the
CAN H and CAN L lines. The monitor includes termination resistor built-in. Note that the
CAN bus shares its ground reference with battery negative (it is not an isolated interface).
The 6V Out terminal is limited to 200mA current and is primarily intended for powering CAN
devices such as a monitor and/or CAN current sensor, but may also be used to power other
small 6V devices such as solid state relays.
BMS12i Monitor
The BMS12i Monitor is an optional colour touchscreen which may be used to remotely
interact with the BMS12i. The Monitor displays various pages of information:
BMS Status: Running
Voltage
Current
Power
Temp SoC
23˚C 90%
48.5V
12.3A
0.6kW
The BMS Status page is the standard display when
the system is running, showing pack voltage, current,
power, temperature, battery state of charge. This will
only be shown if a current sensor is present.
Tapping the panel on the left or right side will toggle
between the different display pages.
BMS Summary: 12 cells
Avg Volts Temperature
Min Volts Max Volts
3.32V 25˚C
3.31V 3.33V
The BMS Summary page shows the average, minimum
and maximum cell voltages, and the temperature
sensor, plus a bar graph overview of all cells across
the bottom.
If no current sensor is present, this will be the default
page, and Pack Voltage will be shown instead of
Average Voltage.
BMS Details: 12 cells
3.324 3.332 3.327 3.315
3.331 3.313 3.325 3.322
3.318 3.326 3.339 3.322
The BMS Details page shows all individual cell
voltages, plus a bar graph below. Dotted lines on the
graph show HV and LV thresholds plus hysteresis.
Graph bars will be green when a cell is in range, blue
for under-voltage, red for over-voltage, and orange if a
cell is currently being shunt balanced.
5
6
Shield
Ground
CAN L
CAN H
6VDC
