diff --git a/custom_components/givenergy_local/binary_sensor.py b/custom_components/givenergy_local/binary_sensor.py
new file mode 100644
index 0000000..bea64ca
--- /dev/null
+++ b/custom_components/givenergy_local/binary_sensor.py
@@ -0,0 +1,188 @@
+"""Binary sensor platform — a single plant-level battery out-of-spec alert (#78)."""
+
+from __future__ import annotations
+
+from collections.abc import Iterator
+from dataclasses import dataclass, field
+from datetime import datetime
+
+from homeassistant.components.binary_sensor import (
+    BinarySensorDeviceClass,
+    BinarySensorEntity,
+)
+from homeassistant.config_entries import ConfigEntry
+from homeassistant.core import HomeAssistant
+from homeassistant.helpers.entity import DeviceInfo, EntityCategory
+from homeassistant.helpers.entity_platform import AddEntitiesCallback
+from homeassistant.helpers.update_coordinator import CoordinatorEntity
+
+from .const import DOMAIN
+from .coordinator import GivEnergyUpdateCoordinator
+
+# LFP soft operating band. A cell that has genuinely drifted outside this for a
+# sustained period warrants attention; the band is deliberately wider than the
+# nominal working range so normal operation never crosses it.
+CELL_MIN_V = 3.0
+CELL_MAX_V = 3.5
+# Unused cell slots in smaller packs read ~0 V. Anything below this floor is an
+# unpopulated slot or a dropped read, not a real over-discharged cell, so it is
+# excluded from the low-voltage check (the debounce handles dropped reads anyway).
+CELL_PRESENT_FLOOR_V = 1.0
+
+# Default cell-group temperature alert band (°C). Conservative LFP envelope; a
+# tunable knob can follow if real-world data shows it needs widening.
+TEMP_MIN_C = 0.0
+TEMP_MAX_C = 50.0
+
+# Hybrid debounce: a value must be out of spec for at least this long in
+# wall-clock AND across at least this many distinct polls before the alert
+# trips. Both bounds comfortably exceed the ~2-minute persistence that dongle
+# bad-read garbage can exhibit (modbus#78), so a sustained fake read can't trip
+# it, while the dual form survives both fast and slow pollers.
+DEBOUNCE_SECONDS = 300
+DEBOUNCE_MIN_POLLS = 3
+
+
+@dataclass
+class _Offender:
+    """Tracks one metric that is currently out of spec across consecutive polls."""
+
+    battery: str
+    metric: str
+    value: float
+    first_seen: datetime
+    poll_count: int = 1
+
+
+@dataclass
+class _Reading:
+    battery: str
+    metric: str
+    value: float
+    detail: dict[str, object] = field(default_factory=dict)
+
+
+async def async_setup_entry(
+    hass: HomeAssistant,
+    entry: ConfigEntry,
+    async_add_entities: AddEntitiesCallback,
+) -> None:
+    coordinator: GivEnergyUpdateCoordinator = hass.data[DOMAIN][entry.entry_id]
+    # Pointless on a battery-less (PV-only) install.
+    if not coordinator.data.batteries:
+        return
+    async_add_entities([GivEnergyBatteryOutOfSpecBinarySensor(coordinator)])
+
+
+def _iter_readings(batteries: list) -> Iterator[_Reading]:
+    """Yield every in-spec-checkable cell-voltage and group-temperature reading."""
+    for index, battery in enumerate(batteries):
+        serial = battery.serial_number or f"battery_{index}"
+        for cell in range(1, 17):
+            value = getattr(battery, f"v_cell_{cell:02d}", None)
+            if value is None or value < CELL_PRESENT_FLOOR_V:
+                continue  # unpopulated slot or dropped read
+            yield _Reading(serial, f"cell_{cell:02d}_voltage", float(value), {"cell": cell})
+        for lo, hi in ((1, 4), (5, 8), (9, 12), (13, 16)):
+            value = getattr(battery, f"t_cells_{lo:02d}_{hi:02d}", None)
+            if value is None:
+                continue
+            yield _Reading(
+                serial,
+                f"cells_{lo:02d}_{hi:02d}_temperature",
+                float(value),
+                {"cell_group": f"{lo}-{hi}"},
+            )
+
+
+def _out_of_spec(reading: _Reading) -> bool:
+    if reading.metric.endswith("_voltage"):
+        return not (CELL_MIN_V <= reading.value <= CELL_MAX_V)
+    return not (TEMP_MIN_C <= reading.value <= TEMP_MAX_C)
+
+
+class GivEnergyBatteryOutOfSpecBinarySensor(
+    CoordinatorEntity[GivEnergyUpdateCoordinator], BinarySensorEntity
+):
+    """One plant-level alert: on when any monitored battery value has been out of
+    spec for a sustained period, debounced against transient bad reads."""
+
+    _attr_has_entity_name = True
+    _attr_name = "Battery Out Of Spec"
+    _attr_device_class = BinarySensorDeviceClass.PROBLEM
+    _attr_entity_category = EntityCategory.DIAGNOSTIC
+
+    def __init__(self, coordinator: GivEnergyUpdateCoordinator) -> None:
+        super().__init__(coordinator)
+        serial = coordinator.data.inverter_serial_number
+        self._attr_unique_id = f"{serial}_battery_out_of_spec"
+        self._attr_device_info = DeviceInfo(identifiers={(DOMAIN, serial)})
+        self._offenders: dict[str, _Offender] = {}
+        self._last_processed_refresh: datetime | None = None
+        self._evaluate()
+
+    def _handle_coordinator_update(self) -> None:
+        self._evaluate()
+        super()._handle_coordinator_update()
+
+    def _evaluate(self) -> None:
+        """Advance the per-metric debounce trackers off the latest poll.
+
+        Runs once per distinct successful refresh so each poll counts exactly
+        once towards the sustained-poll requirement.
+        """
+        refresh = self.coordinator.last_successful_refresh
+        if refresh is None or refresh == self._last_processed_refresh:
+            return
+        self._last_processed_refresh = refresh
+
+        current: dict[str, _Reading] = {
+            f"{r.battery}:{r.metric}": r
+            for r in _iter_readings(self.coordinator.data.batteries)
+            if _out_of_spec(r)
+        }
+        # Drop anything that has returned to spec.
+        for key in list(self._offenders):
+            if key not in current:
+                del self._offenders[key]
+        # Record/advance current offenders.
+        for key, reading in current.items():
+            existing = self._offenders.get(key)
+            if existing is None:
+                self._offenders[key] = _Offender(
+                    battery=reading.battery,
+                    metric=reading.metric,
+                    value=reading.value,
+                    first_seen=refresh,
+                )
+            else:
+                existing.value = reading.value
+                existing.poll_count += 1
+
+    @property
+    def is_on(self) -> bool:
+        refresh = self.coordinator.last_successful_refresh
+        if refresh is None:
+            return False
+        return any(
+            offender.poll_count >= DEBOUNCE_MIN_POLLS
+            and (refresh - offender.first_seen).total_seconds() >= DEBOUNCE_SECONDS
+            for offender in self._offenders.values()
+        )
+
+    @property
+    def extra_state_attributes(self) -> dict[str, object]:
+        refresh = self.coordinator.last_successful_refresh
+        offenders = [
+            {
+                "battery": o.battery,
+                "metric": o.metric,
+                "value": o.value,
+                "polls_out_of_spec": o.poll_count,
+                "seconds_out_of_spec": (
+                    int((refresh - o.first_seen).total_seconds()) if refresh else 0
+                ),
+            }
+            for o in self._offenders.values()
+        ]
+        return {"offenders": offenders}
diff --git a/custom_components/givenergy_local/const.py b/custom_components/givenergy_local/const.py
index de35cc8..1f3d82c 100644
--- a/custom_components/givenergy_local/const.py
+++ b/custom_components/givenergy_local/const.py
@@ -12,7 +12,7 @@
 
 DEFAULT_PASSIVE = False
 
-PLATFORMS = ["sensor", "switch", "number", "select", "time"]
+PLATFORMS = ["binary_sensor", "sensor", "switch", "number", "select", "time"]
 
 SERVICE_REBOOT_INVERTER = "reboot_inverter"
 SERVICE_CALIBRATE_BATTERY_SOC = "calibrate_battery_soc"
diff --git a/tests/test_binary_sensor.py b/tests/test_binary_sensor.py
new file mode 100644
index 0000000..0a85824
--- /dev/null
+++ b/tests/test_binary_sensor.py
@@ -0,0 +1,161 @@
+"""Tests for the battery out-of-spec binary sensor (issue #78)."""
+
+from datetime import UTC, datetime, timedelta
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+from homeassistant.helpers import entity_registry as er
+
+from custom_components.givenergy_local.binary_sensor import (
+    DEBOUNCE_MIN_POLLS,
+    DEBOUNCE_SECONDS,
+    GivEnergyBatteryOutOfSpecBinarySensor,
+)
+from custom_components.givenergy_local.const import DOMAIN
+
+BASE = datetime(2026, 1, 1, tzinfo=UTC)
+
+
+def _battery(serial="BT1", cells=None, temps=None):
+    cells = cells if cells is not None else [3.30] * 16
+    temps = temps if temps is not None else [22.0] * 4
+    bat = SimpleNamespace(serial_number=serial)
+    for i, volt in enumerate(cells, start=1):
+        setattr(bat, f"v_cell_{i:02d}", volt)
+    for (lo, hi), temp in zip(((1, 4), (5, 8), (9, 12), (13, 16)), temps):
+        setattr(bat, f"t_cells_{lo:02d}_{hi:02d}", temp)
+    return bat
+
+
+def _entity(batteries):
+    coordinator = MagicMock()
+    coordinator.data.batteries = batteries
+    coordinator.data.inverter_serial_number = "SA1234G123"
+    coordinator.last_successful_refresh = None
+    return GivEnergyBatteryOutOfSpecBinarySensor(coordinator), coordinator
+
+
+def _poll(entity, coordinator, at: datetime, batteries=None) -> None:
+    if batteries is not None:
+        coordinator.data.batteries = batteries
+    coordinator.last_successful_refresh = at
+    entity._evaluate()
+
+
+# ---------------------------------------------------------------------------
+# Debounce state machine (unit-level)
+# ---------------------------------------------------------------------------
+
+
+def test_in_spec_never_trips():
+    entity, coordinator = _entity([_battery()])
+    for n in range(6):
+        _poll(entity, coordinator, BASE + timedelta(seconds=n * DEBOUNCE_SECONDS))
+    assert entity.is_on is False
+
+
+def test_sustained_low_voltage_trips_after_time_and_polls():
+    bad = [_battery(cells=[2.5] + [3.30] * 15)]
+    entity, coordinator = _entity(bad)
+
+    _poll(entity, coordinator, BASE)
+    assert entity.is_on is False  # 1 poll, 0 s
+
+    _poll(entity, coordinator, BASE + timedelta(seconds=150))
+    assert entity.is_on is False  # 2 polls, 150 s — below both thresholds
+
+    _poll(entity, coordinator, BASE + timedelta(seconds=DEBOUNCE_SECONDS))
+    # 3 polls AND >= 300 s elapsed
+    assert entity.is_on is True
+
+
+def test_time_met_but_too_few_polls_does_not_trip():
+    bad = [_battery(cells=[2.5] + [3.30] * 15)]
+    entity, coordinator = _entity(bad)
+    # Two polls far apart: duration satisfied, poll count is not.
+    _poll(entity, coordinator, BASE)
+    _poll(entity, coordinator, BASE + timedelta(seconds=DEBOUNCE_SECONDS * 2))
+    assert DEBOUNCE_MIN_POLLS > 2
+    assert entity.is_on is False
+
+
+def test_transient_excursion_clears_and_resets():
+    good = [_battery()]
+    bad = [_battery(cells=[2.5] + [3.30] * 15)]
+    entity, coordinator = _entity(good)
+
+    # Out of spec for two polls (mimicking the ~2-min dongle garbage), then back.
+    _poll(entity, coordinator, BASE, batteries=bad)
+    _poll(entity, coordinator, BASE + timedelta(seconds=150), batteries=bad)
+    _poll(entity, coordinator, BASE + timedelta(seconds=300), batteries=good)
+    assert entity.is_on is False
+    assert entity._offenders == {}
+
+    # A fresh excursion starts its counter from scratch.
+    _poll(entity, coordinator, BASE + timedelta(seconds=450), batteries=bad)
+    assert entity._offenders["BT1:cell_01_voltage"].poll_count == 1
+
+
+def test_unpopulated_cell_reading_zero_is_ignored():
+    # Cell 16 unpopulated (~0 V) must not be read as an over-discharged cell.
+    batteries = [_battery(cells=[3.30] * 15 + [0.0])]
+    entity, coordinator = _entity(batteries)
+    for n in range(5):
+        _poll(entity, coordinator, BASE + timedelta(seconds=n * DEBOUNCE_SECONDS))
+    assert entity.is_on is False
+
+
+def test_sustained_overtemperature_trips():
+    batteries = [_battery(temps=[60.0, 22.0, 22.0, 22.0])]
+    entity, coordinator = _entity(batteries)
+    for n in range(DEBOUNCE_MIN_POLLS):
+        _poll(entity, coordinator, BASE + timedelta(seconds=n * 150))
+    assert entity.is_on is True
+
+
+def test_none_readings_are_skipped():
+    bat = _battery()
+    bat.v_cell_05 = None  # a dropped read on one cell
+    bat.t_cells_05_08 = None
+    entity, coordinator = _entity([bat])
+    for n in range(5):
+        _poll(entity, coordinator, BASE + timedelta(seconds=n * DEBOUNCE_SECONDS))
+    assert entity.is_on is False
+
+
+def test_attributes_enumerate_current_offenders():
+    bad = [_battery(serial="BTX", cells=[2.5] + [3.30] * 15)]
+    entity, coordinator = _entity(bad)
+    _poll(entity, coordinator, BASE)
+    _poll(entity, coordinator, BASE + timedelta(seconds=120))
+    offenders = entity.extra_state_attributes["offenders"]
+    assert len(offenders) == 1
+    (offender,) = offenders
+    assert offender["battery"] == "BTX"
+    assert offender["metric"] == "cell_01_voltage"
+    assert offender["polls_out_of_spec"] == 2
+    assert offender["seconds_out_of_spec"] == 120
+
+
+# ---------------------------------------------------------------------------
+# Creation gating (integration-level)
+# ---------------------------------------------------------------------------
+
+
+def _entity_id(hass, unique_id: str) -> str | None:
+    return er.async_get(hass).async_get_entity_id("binary_sensor", DOMAIN, unique_id)
+
+
+async def test_entity_created_and_off_for_in_spec_plant(hass, setup_integration):
+    entity_id = _entity_id(hass, "SA1234G123_battery_out_of_spec")
+    assert entity_id is not None
+    assert hass.states.get(entity_id).state == "off"
+
+
+async def test_not_created_for_battery_less_plant(hass, mock_client, mock_plant, mock_config_entry):
+    mock_plant.batteries = []
+    mock_plant.number_batteries = 0
+    mock_config_entry.add_to_hass(hass)
+    await hass.config_entries.async_setup(mock_config_entry.entry_id)
+    await hass.async_block_till_done()
+    assert _entity_id(hass, "SA1234G123_battery_out_of_spec") is None