fix: don't end a steady-current run on a ramp-up overshoot

tests/test_wallmonitor.py

@@ -419,6 +419,27 @@ async def test_thermal_fits_the_current_exponent_from_a_current_spread(db):
     assert all(fit["rise_c"] * (48.0 / fit["current_a"]) ** 2 > 40.0 for fit in low)
 
 
+def test_steady_prefix_restarts_after_a_ramp_up_overshoot():
+    # 2026-09-14, first live calibration probe: the car overshot toward 48 A
+    # for two samples while the 32 A cap was taking effect, then held 32.5 A
+    # for forty minutes. The prefix ended at the overshoot, four samples in,
+    # and the whole window was lost.
+    def samples(currents):
+        return [
+            {"ts": 1000.0 + 2.0 * i, "contactor_closed": 1, "vehicle_current_a": amps, "handle_temp_c": 32.0}
+            for i, amps in enumerate(currents)
+        ]
+
+    ramp = [6.1, 14.6, 23.9, 31.1, 32.6, 32.7, 32.9, 37.6, 43.3] + [32.5] * 600
+    prefix = thermal._steady_current_prefix(samples(ramp))
+    assert len(prefix) == 600 and prefix[0]["vehicle_current_a"] == 32.5
+    # A current change once the run has settled still ends it (a derate, a
+    # cap): the leniency is for the first minute only.
+    settled = [32.5] * 400 + [44.0] * 100
+    prefix = thermal._steady_current_prefix(samples(settled))
+    assert len(prefix) == 400 and prefix[-1]["vehicle_current_a"] == 32.5
+
+
 def test_thermal_sustainable_current_follows_the_fitted_exponent():
     square = thermal.ThermalParams(rise_ref_c=36.0, current_exp=2.0)
     gentle = thermal.ThermalParams(rise_ref_c=36.0, current_exp=1.5)

wallmonitor/thermal.py

@@ -222,6 +222,14 @@ def ambient_from_idle_handle(handle_c: float, model: IdleOffset = BUILTIN_IDLE_O
 MIN_SPAN_TAU = 1.8
 PREFIX_SPAN_TAU = 2.5
 PREFIX_SPAN_MIN_S = 1800.0
+# A steady run that ends within its first minute did not end; the ramp-up
+# was still wobbling. Vehicles overshoot on the way to a cap: one session
+# read 32.6, 32.7, 32.9, 37.6, 43.3 and then 32.5 A for forty minutes — the
+# two overshoot samples being the car heading for 48 A in the seconds
+# before the amp controller's probe cap took effect — and ending the run at
+# the overshoot kept a four-sample window and lost the real one. Every
+# probe lands mid-ramp, so every probe would have been lost the same way.
+PREFIX_SETTLE_S = 60.0
 
 # The steady-prefix band (10% of the reference current) is wide enough to
 # hide a substantial current reduction: 48.6 A trimmed to 44.7 A never
@@ -383,7 +391,10 @@ def _steady_current_prefix(samples: list[dict], max_span_s: float = PREFIX_SPAN_
     its samples at the reduced current, and a whole-session median would put
     the initial full-rate ramp — the part with the thermal signal — outside
     the band. Leading samples still ramping up to the plateau are skipped
-    rather than treated as the end of the run.
+    rather than treated as the end of the run, and a run that leaves the
+    band within PREFIX_SETTLE_S of starting is discarded and restarted
+    rather than ended: that is the ramp overshooting, not a steady run
+    ending.
     """
     charging = [
         sample
@@ -404,9 +415,10 @@ def _steady_current_prefix(samples: list[dict], max_span_s: float = PREFIX_SPAN_
         if prefix and sample["ts"] - prefix[-1]["ts"] > SEGMENT_SPLIT_GAP_S:
             break  # a charging gap: the next samples belong to a later segment
         if abs(sample["vehicle_current_a"] - i_ref) > band:
-            if prefix:
+            if prefix and sample["ts"] - prefix[0]["ts"] >= PREFIX_SETTLE_S:
                 break  # the steady run ended (derate or charge stop)
-            continue  # still ramping up to the plateau
+            prefix = []  # still ramping up, or the ramp overshot: start over
+            continue
         prefix.append(sample)
         if sample["ts"] - prefix[0]["ts"] > max_span_s:  # the ramp lives in the first few tau
             break