goodwe-addon/goodwe_controller/DECISIONS/SAFETY-78-drain-at-two.md

SAFETY-78 - a maintenance drain at two inverters

Status: DECIDED 2026-09-29. Fixes MAINT-05 §6 (OPT-11 P1 F1, UX-13 P3 check 5, OPT-11 P3 F4). Tests: test_safety78.py.

The defect. At N>=2 the drain pins its pack (a unit SET at P_MAINTENANCE) and the site target is split around the pin. Whenever the site target was below the pin, the other pack had to charge. The allocator refuses opposite directions (SAFETY-25 §0a, SAFETY-31), so the site went to 0 W and logged ARBITER: REFUSED every cycle. That covers a house below the drain rate, a grid-charge trade, and a house above it too: after a refusal the loop restarts from 0 W and its 400 W slew step is below the pin again.

The fix. While a pack drains at N>=2, main.py files a site limit drain, lo = what the pin really places (allocator.pin_w(): the drain rate inside that pack's own hard limits), at P_MAINTENANCE. So the site target is never below the pin and the other pack only adds power in the same direction. Opposite splits stay forbidden.

• The order is the arbiter's own: a safety SET (100) outranks the limit and the pin, so everything parks; the peak floor (50) and the drain floor are both applied, so the higher wins; a trade or the loop below the drain is lifted to it.
• Below the drain rate the drained pack exports the rest, as the drain does at N=1 (maintenance.py: "forced discharge, exports the surplus"). Holding it to the house load would make N=2 drain differently from N=1; that is an owner question, not this fix.
• A draining pack that can place nothing (a refusal, a stale link) adds no floor, and the wait is logged once per episode.
• With battery modes on, no pack is a grid charger while another drains (modetrade.decide(draining=), why cycle), and the other battery's card says why (other_cycle). Not while the drain waits: no floor is filed then, so the other battery may charge from solar.
• Not filed at N=1, where the drain is the site SET itself: byte-identical (tools/safety78_identity.py, which hashes the claims too).

Not changed (measured in the PR #154 review, follow-ups).

• At N=1 a drain (60) outranked the peak floor (50): a 5000 W house imported 3000 W against a 2500 W limit. Fixed by SAFETY-83.
• At N=2 a maintenance CHARGE in a cheap window has the same stall: the charging pack's hi = -floor against the loop asking the other to give is refused on 141 of 261 cycles. The mirror fix is a site hi, which can meet the peak floor's lo (0 W), so it is not this card. Fixed by SAFETY-84.