diff --git a/goodwe_controller/test_safety103.py b/goodwe_controller/test_safety103.py
index f4729d3..1893445 100644
--- a/goodwe_controller/test_safety103.py
+++ b/goodwe_controller/test_safety103.py
@@ -118,7 +118,7 @@ def measure(rows, n, house):
     seg = [r["target"] for r in rows if r["t"] >= te]
     return dict(
         end=te, swing=sw,
-        first=max(win[0]["cmd"][1:]),
+        first=max(win[0]["cmd"][1:]) if win else math.nan,
         step=max(abs(b - a) for a, b in zip(seg, seg[1:])),
         give=max(max(r["cmd"][1:]) for r in rows if r["t"] >= te),
         gmin=min(r["grid"] for r in win), gmax=max(r["grid"] for r in win),
@@ -319,13 +319,13 @@ _c.inverters = [ia, ib]
 check("N=1: nothing is tracked", _c._read_span == {})
 
 
-def tail(now, grid, target, held=None):
+def tail(now, grid, target, held=None, live=AB):
     """`_drain_grid_w` with A settling: the error it hands the loop (grid
     minus aim) and the house point it holds."""
     _c._now_mono, _c.grid, _c.target = now, grid, target
     _c._drain_tail, _c._settle = {A}, {A: 0.0}
     _c._drain_house = held
-    g = _c._drain_grid_w(AB)
+    g = _c._drain_grid_w(live)
     return round(g - AIM, 6), _c._drain_house
 
 
@@ -374,6 +374,10 @@ check("a range from an older reading than the one in hand (a cycle the "
       "loop did not run) is not used: A at its reading, 1241 W",
       tail(15.0, 2000.0, 1140.0, held=1140.0) == (2011.0, 3151.0))
 _c._read_span[A] = (0.0, 1241.0, 13.0)
+check("⭐ a battery out of the split is not counted (the meter has its "
+      "power): B dropped, a grid of +2000 W puts the house at least at "
+      "2000 + 10 + 0 = 2010 W, not 1910 W",
+      tail(15.0, 2000.0, 1140.0, held=1140.0, live={A}) == (870.0, 2010.0))
 _c._drain_tail, _c._settle, _c._drain_house = {A}, {}, 1140.0
 _c.grid = -330.0
 check("⭐ the tail's end lets the hold go: the meter again, and the next "