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Rivian 864-Cell Battery Module: BMS Interface_

Last updated by i5ys0p on October 17, 2026

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## Overview
The Rivian 864-cell battery module is physically divided into an upper and lower layer, separated by a cooling plate. Electrically, the module is a 12S72P configuration.

Crucially, the cells are physically split down the center into left and right halves. The aluminum busbars run vertically, meaning the left and right sides of each layer are independent 36-parallel strings that are tied together at the main terminals. The factory BMS ribbon cables run down the center, providing independent voltage taps for both the left and right cell groups.

To build a custom distributed BMS (slave board) that mounts directly to this module, the PCB must provide three mating headers to accept the factory cables.

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## 1. Upper Voltage Harness: Cells 1-6 (White Connector)
This ribbon cable carries the voltage sensing taps for the bottom half of the series string (0V to 23.7V). Because the left and right cell groups are physically separate until the main terminals, this connector provides independent monitoring for both halves.

*   **Factory Receptacle:** TE Connectivity `2303088-3` (NanoMQS 32-Position, Code B, White)
*   **Mating PCB Header (Vertical):** TE Connectivity `2177748-3`
*   **Mating PCB Header (Right-Angle):** TE Connectivity `2177419-3`

### Upper Layer Pinout
*Note: Pins 1-14 are unpopulated.*

| Pin | Cell Group | Function | Nominal Voltage |
| :--- | :--- | :--- | :--- |
| **15** | Left Half | Cell 1 Positive | +3.9V |
| **16** | Left Half | Cell 3 Positive | +11.8V |
| **17** | Left Half | Cell 5 Positive | +19.8V |
| **18, 19** | Left Half | Cell 6 Positive (Midpoint) | +23.7V |
| **20** | Left Half | Cell 4 Positive | +15.8V |
| **21** | Left Half | Cell 2 Positive | +7.9V |
| **22, 23** | Left Half | Main Negative | 0V |
| **24** | Right Half | Cell 1 Positive | +3.9V |
| **25** | Right Half | Cell 3 Positive | +11.8V |
| **26** | Right Half | Cell 5 Positive | +19.8V |
| **27, 28** | Right Half | Cell 6 Positive (Midpoint) | +23.7V |
| **29** | Right Half | Cell 4 Positive | +15.8V |
| **30** | Right Half | Cell 2 Positive | +7.9V |
| **31, 32** | Right Half | Main Negative | 0V |

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## 2. Lower Voltage Harness: Cells 7-12 (Black Connector)
This ribbon cable physically emerges from the lower cell layer and monitors the upper half of the series string (23.7V to 47.6V).

Unlike the perfectly symmetrical upper harness, the lower harness is highly asymmetrical. Teardowns reveal the use of staggered "tuning fork" busbars that bridge the left and right cell groups. Consequently, the right side of the ribbon provides a continuous voltage tap for every cell in the upper string, while the left side skips several even-numbered cell taps and leaves pins 20-23 floating (No Connection).

*   **Factory Receptacle:** TE Connectivity `2303088-2` (NanoMQS 32-Position, Code A, Black)
*   **Mating PCB Header (Vertical):** TE Connectivity `1-2177748-3`
*   **Mating PCB Header (Right-Angle):** TE Connectivity `5-2177419-3`

### Lower Layer Pinout
*Note: Pins 1-14 are unpopulated. Pins 20-23 are unpopulated/floating.*

| Pin | Location | Function | Nominal Voltage (Ref to Main 0V) |
| :--- | :--- | :--- | :--- |
| **15, 16** | Left Half | Cell 6 Positive (Midpoint Ref) | +23.7V |
| **17** | Left Half | Cell 7 Positive | +27.7V |
| **18** | Left Half | Cell 9 Positive | +35.6V |
| **19** | Left Half | Cell 11 Positive | +43.6V |
| **20, 21, 22, 23** | Left Half | **No Connection (Floating)** | N/A |
| **24, 25** | Right Half | Cell 12 / Main Positive | +47.6V |
| **26** | Right Half | Cell 10 Positive | +39.6V |
| **27** | Right Half | Cell 8 Positive | +31.7V |
| **28, 29** | Right Half | Cell 6 Positive (Midpoint Ref) | +23.7V |
| **30** | Right Half | Cell 7 Positive | +27.7V |
| **31** | Right Half | Cell 9 Positive | +35.6V |
| **32** | Right Half | Cell 11 Positive | +43.6V |

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## 3. Module Temperature Sensors
The module contains two standard 2-wire NTC thermistors (one for the upper layer with blue wires, one for the lower layer with black wires). Both thermistors share a single unsealed connector.

*   **Factory Receptacle:** Molex `34791-0080` (Mini50 Unsealed 8-Circuit, Polarization A, Black)
*   **Mating PCB Header (Vertical THT):** Molex `34792-0080`
*   **Mating PCB Header (Right-Angle THT):** Molex `34793-0080`
*   **Mating PCB Header (Right-Angle SMT):** Molex `34912-8080`

### Sensor Wiring
Because these are passive analog thermistors, polarity does not matter when routing them to the BMS reference voltage and ground. 
*   **Pins 1 & 2:** Upper Layer Sensor (Blue Wires)
*   **Pins 3 & 4:** Unpopulated
*   **Pins 5 & 6:** Lower Layer Sensor (Black Wires)
*   **Pins 7 & 8:** Unpopulated

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