====== Project vision & architecture ======

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).

===== The always-on constraint =====

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.

===== Powering always-on without killing the battery =====

  * 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.

===== Two-tier hardware architecture =====

  * **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.

===== Safety floors =====

  * **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.