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The Voltage Management System
Each BMI battery pack incorporates at least one VMS module to monitor each 3.3V cell (or parallel cluster) in 4 cell groups. The VMS modules are easily daisy chained together and are individualy addressed by unique DIP switch settings on each board. Over and under voltage monitoring consists of triggering an on-board buzzer, external signal outputs and internal data recording.
The table below shows the voltages at which the different indicators trigger, at this stage of my testing they are very approximate.
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< Lower (Discharging) Voltages
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Higher (Charging) Voltages >
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Low-Volt Event Stored On-board
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Red LED, TTL Output & Fast Buzzer
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Slow Buzzer
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Yellow LED (LED position indicates which cell)
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Slow Buzzer
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Red LED, TTL Output & Fast Buzzer
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High-Volt Event Stored On-board
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2.2
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2.3
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2.5
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2.6
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4.0
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4.2
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4.3
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The VMS module also does some cell balancing by shunting current through small on-board resistors, 100mA @ 3.3V (1/3W) according to BMI. The VMS also provides an external serial interface for data gathering via a PC.
The 4 indicator LEDs can be seen through a window on top of the battery and indicate the total voltage of the 4 cells that the VMS is monitoring;
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LEDs On
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4 Cell Voltage
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Average Cell Voltage
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OOOO
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> 13.2
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> 3.3
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OOO
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12.6 - 13.2
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3.15 - 3.30
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OO
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12.0 - 12.6
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3.0 - 3.15
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O
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11.4 - 12.0
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2.85 - 3.0
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-
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< 11.4
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< 2.85
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Here you can see the I/O board connected to one VMS module. One VMS module monitors 4 cells and up to 255 modules can be connected together in series, internally and by external cables between battery packs. The Temperature Sensor is taped to one cell and triggers a High Temp event at 50-60C.
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This is the reverse side of the VMS board showing;
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Power Switch (turn off when connecting cells).
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8 bit addressing DIP switches.
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Six wire connector to the 4 cells, two wires go either side of the fusible link between cells 2 and 3.
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4 more balancing shunt resistors.
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