Battery & REDS

Overview

The REDS (Recovery Event Driver System) is a safety-critical power hub. It provides an isolated environment for 6-cell battery charging, precise rail monitoring via buffered dividers, and high-side switching for propulsion vent valves.

REDS System Architecture Figure 4: High-level block diagram of charging isolation and recovery logic.

Stakeholder Relevance

  • Avionics: Critical facet of power on electronics; ensures logic survival during high-current events.
  • Propulsion: Drive capacity for PeterPaul Series 20 (NO/NC) vent valves.
  • Operations: Pre-launch camera power and battery health monitoring.

Power System Reference

Net Name Source Nominal Voltage Max Current Notes & Hardware Specs
GSE+ Ground pad 24V 5A* External input used at pad.
BATT_MAIN+ 6S LiPo 22.2V - 25.2V — Primary vehicle battery bus.
GND Ground 0V — Reference for ECU logic and valve returns.
ISO_5V Cincon EC4SAW 5V 600mA Isolated supply per charging block; floats relative to cell GND.
ISO_GND Cincon EC4SAW — — Isolated return; MUST NOT be shorted to System GND.
3V3_REDS RECOM RPM-3.0 3.3V 3A Independent logic rail; survives primary ECU bus failure.
3V3_MAIN ECU Buck Reg 3.3V — Primary logic rail for non-recovery ECU functions.
GSE_VSNS MCP6001 0–3.3V — Buffered ADC signal scaling GSE+ rail (1V:7.67V).
BATT_VSNS MCP6001 0–3.3V — Buffered ADC signal scaling BATT_MAIN+ (1V:7.67V).
5V0_REDS IP6525T 5V 3A Aux rail for Insta360; sourced from GSE+ via D3.
STAT_OUT[0-5] PS2811-1 3.3V Logic — Isolated charge status (Active-Low) from BQ25170.

Design Details

1. Isolated Charging Architecture

To prevent ground loops and cell-balance shorts during charging, the system utilizes 6 independent charging blocks.

Schematic Snippet: BQ25170 Charging Block Figure 5: Isolated charging stage showing Cincon DC-DC and BQ25170.

Schematic Snippet: Optocoupler Interface Figure 6: Isolated status feedback via PS2811-1 Optocouplers.

  • Power Isolation: Each cell uses a Cincon EC4SAW (U10) to decouple the GSE power from the specific battery cell potential.
  • Signal Isolation: STAT_OUT signals use PS2811-1 optocouplers (U12) to cross the isolation barrier to the 3.3V logic rail.
  • Design Deviation (ISET): Target I_SET was 375 Ω. Current assembly uses 500 Ω (±1%, <200 ppm/ºC) due to availability, resulting in a slightly lower, safer charge rate for the BQ25170.

2. 3.3V_REDS Logic Rail

The 3V3_REDS rail is independent of the main ECU 3.3V bus to ensure recovery capability during bus failures.

Schematic Snippet: U50 Regulator Figure 7: U50 RECOM regulator stage with input Pi-filter.

  • Regulator: U50 (RECOM RPM-3.0) 24V to 3.3V.
  • Input Pi-Filter: L3 (2.2 μH) and C102-104.
  • Thermal Note: Inductor L3 is rated for 3A. While the RECOM module is robust, L3 is the primary thermal bottleneck if draw exceeds 3A.

3. Rail Voltage Sensing (MAIN & REDS)

The ECU employs two identical dual-stage sensing circuits to monitor the health of the 24V GSE and Battery rails for both the primary ECU (MAIN) and REDS.

Schematic Snippet: V_Sense MAIN Figure 8: V_Sense MAIN buffers (U53/U54) monitoring primary power rails.

  • Circuit Identity: The REDS sensing stage (U5/U44) is topologically identical to the MAIN stage (U53/U54) shown above.
  • Precision Divider: A 100 kΩ / 15 kΩ network provides a 1V : 7.67V scaling ratio.
  • High-Impedance Buffering: MCP6001 Op-amps are configured as voltage followers. This ensures that the sensing network does not load the battery and prevents ADC input impedance from introducing measurement error.
  • Component Requirement: Resistors must be ±1% tolerance or better to ensure measurement accuracy across the flight temperature envelope.

4. Valve Drive Circuitry

The board features two high-side MOSFET driver channels (Q4/Q5 and Q8/Q9) designed to drive solenoids.

Schematic Snippet: MOSFET Drivers Figure 9: Typical NMOS-to-PMOS level shifting for 24V valve control (Channel 1 shown).

  • Topology: NMOS-to-PMOS level shifting (3.3V to 24V).
  • Flyback Protection: Integrated 100V 8A Schottky diodes (D1, D7) clamp inductive spikes from solenoids.
  • Safety Logic: NMOS pulldowns ensure valves remain in their "Normal" state if logic pins float.

5. Insta360 Camera

A dedicated high-current 5V rail is provided for external peripherals, primarily the Insta360 flight camera.

Schematic Snippet: 5V 3A Insta360 Figure 10: IP6525T Step-down converter for USB-C power delivery.

  • Regulator: U6 (IP6525T) provides a stable 5V output at up to 3A.
  • Power Source: This rail is tied to GSE+ via a 30V 5A Schottky diode (D3).
  • Ops Note: Ops must verify the camera switches to internal battery smoothly during disconnect to ensure recording does not stop at T-0.
  • Filtering: A 22 μH inductor (L1) and a bulk 220 μF capacitor (C108) minimize switching ripple to prevent video artifacts.
  • Indicator: LED D5 (Green) indicates the 5V Aux rail is active.

Hardware–Software Interface

Optocoupler Logic Calculation

The pull-up resistor (R36) for the STAT_OUT signal is 1.5 kΩ. * CTR Calculation: * V_{CC} - V_{OUT} = 3.3V - 0.8V = 2.5V * 2.5V / 2mA = 1250 Ω ≈ 1.5 kΩ * Diagnostic: If status signals fail to toggle, verify the optocoupler is driving at least 2mA.

Errata / Revision Notes

  • Diode Corrected: Fixed orientation of 30V 2A diode (reversed in v1.0).
  • ADC Simplification: Deleted ADC from schematic for charge status; routed charging status to main ECU MC.