Rebuild the “connected car” that the dead OEM cloud + TBox used to provide, self-hosted:

• Remote preconditioning — heat/cool the cabin before you get in (the headline feature).
• Remote telemetry — SoC, range, charge state, doors/locks — to your own app / Home Assistant.
• Better in-car UX — real turn-by-turn navigation and real speed-limit warnings on the

driver cluster; climate controllable in parallel with normal head-unit use.

• Give the head unit internet again (the OEM data SIM is dead).

Remote preconditioning means a “start climate now” command must reach the car any time it is parked and off. So something with a cellular link has to stay reachable 24/7 — a permanent power draw that is a requirement, not something to design away.

• The 12 V battery is small (days of parasitic draw) and must never be deep-discharged — the

car needs it to wake and close its HV contactors.

• The HV traction pack is huge (~63 kWh). So treat the 12 V as a buffer and the HV pack as the real energy source: monitor 12 V, and when it sags, wake the HV system so the DC-DC converter recharges the 12 V. This sustains an always-on load essentially indefinitely,

bounded by an HV state-of-charge floor. (This is how the OEM TBox survives long parking.)

• Key synergy: the reverse-engineered “wake HV” primitive is the same for

preconditioning and for 12 V recharge — one result enables both.

• Tier 1 — always-on low-power node (e.g. ESP32/STM32 + LTE-M/NB-IoT modem, on the CAN

bus, with a wake-capable transceiver). The TBox replacement: permanently reachable at

  single-digit mA, measures 12 V directly, monitors pack SoC over CAN, wakes the HV for
  preconditioning or a 12 V top-up, and wakes Tier 2 on demand.
* **Tier 2 — heavy peripherals, powered/woken only when needed:**
  * A **companion Android tablet** (own SIM + internet): computes navigation, acts as a Wi-Fi
    hotspot to give the head unit internet, and issues climate commands.
  * The **head unit** (these notes): executes climate/AC commands and renders turn-by-turn +
    speed-limit warnings on the cluster panel.

• 12 V low-voltage cutoff — stop the load / alert before stranding the car's wake logic.
• HV-SoC floor — stop recharging so the traction pack is never flattened.