Orion Jr. BMS Manual
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Current Sensor
The above image shows current sensor wiring.
The Orion Jr. BMS uses an inline shunt resistor to measure current. When current runs through
the shunt resistor, a small voltage drop is produced. The BMS measures this small voltage drop to
calculate the current flow.
Because the shunt is in-line and is electrically connected to the battery pack, the shunt resistor
may ONLY be electrically wired between the negative terminal of the negative-most cell and the
load negative. While it is permissible for cable to be between the negative
terminal and the shunt
resistor, no fuses, safety disconnects, contactors, or other disconnecting means may be between the
negative terminal and the shunt current sensor as this may cause full stack potential to be applied to
the current sensor monitoring circuit, damaging the BMS. Always unplug the BMS before making any
wiring changes to any battery wiring. All current, including the charger, must flow through the shunt
current sensor to be measured (as pictured above).
While the direction of the current sensor shunt itself does not matter, the connection of the
current sensor + and current sensor - wires does matter. The current sensor - wire should be on the
side of the current sensor connected to the negative terminal of the battery and the current sensor +
side should be connected to the load side of the sensor. The above diagram shows the standard
orientation of the current sensors, but if the wiring is backwards, the current sensor direction can be
inverted in the Orion Jr. profile using the utility software. In the initial setup of the unit, the direction of
current should be verified by using the BMS utility to ensure proper configuration. Test the wiring of the
voltage taps and current sensor wires before connecting to the BMS.
The Orion Jr. units are designed to be used with 50mV shunts. 75mV shunts can also be used,
but
the current measurement is limited by the equivalent 50mV rating (⅔ of the 75mV rating.)
For
example, a 200A 75mV shunt is the equivalent of a 150A 50mV shunt. Either shunt may be used, but
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the maximum amperage in either case is 150A. The table below lists currently supported shunt sizes.
Support for additional sizes may be available on request.
Max Current 50mV Shunt
75mV Shunt Equivalent
20A
20A / 50mV
30A / 75mV
50A
50A / 50mV
75A / 75mV
100A
100A / 50mV
150A / 75mV
150A
150A / 50mV
200A / 75mV
200A
200A / 50mV
300A / 75mV
333A
N/A
500A / 75mV
400A
400A / 50mV
600A / 75mV
500A
500A / 50mV
750A / 75mV
600A*
600A / 50mV
900A / 75mV
1000A*
1000A / 50mV 1500A / 75mV
* Current sensors sized above 500 A are supported, but will have a reduced accuracy and resolution.
Orion Jr. BMS Manual
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Wiring for high impedance cables and busbars
Voltage measurements are taken by the Orion Jr. with respect to the next lowest cell or the negative
wire in each cell group. For example, when the Orion Jr
. measures cell 1’s voltage, it measures the
voltage between tap 1- and 1. Likewise, for cell 2, the voltage is measured between tap 1 and tap 2 to
determine cell 2’s voltage.
While battery cables and busbars may be very large and have a minimal resistance, all cables have
some electrical resistance.
The cell taps by necessity will “see” the additional resistance from busbars,
battery interconnects, and cables unless they fall between cell groups (12 cells). The diagram below
shows the first 3 cells wired in a group.
Because of the way the cells are connected, the differences in resistance from one interconnect to
another will be reflected in the instantaneous voltage measurements and would show up to the Orion
Jr. as extra resistance for that particular cell. In the example below, all of the cells have a resistance of
3 milli-ohm, but due to the busbar resistances, the BMS sees the extra 2 mOhm resistance for a total of
5 mOhm on cell 2. Even though cell #2 is still healthy, it appears to be a weak cell without any
compensation. This is where busbar compensation comes in.
For relatively low resistance, this extra resistance can be compensated out by the BMS using “busbar
compensation” (see the
software manual for information on setting up busbar compensation). For high
resistance busbars / cables (or higher amperage applications), it is possible for the voltage drop (or
voltage increase if the battery is being charged) to be large enough that it can cause the voltage at the
tap to exceed 5V or drop below 0V, which are the maximum and minimum voltages for the Orion Jr.
BMS. If the voltage can swing outside those maximum voltages, the Orion Jr. must be wired such that
the cable falls between a cell group break (between cells 12 and 13) and be wired such that voltage
drop induced by the busbar cannot be seen by the Orion Jr.