goodwe-addon/goodwe_controller/DECISIONS/SAFETY-25-loop-soc-floor.md

SAFETY-25 — does the grid-following loop get a reserve floor?

Status: OPEN. This is a memo, not a decision. It sets out the options, what each costs, what each protects, and a recommendation. The choice belongs to the product owner, because it decides whether this house's battery is allowed to discharge to its BMS cut-out on a sustained-load evening, and that moves real energy.

Filed with SAFETY-25. The other half of that ticket — making peak_soc_floor's scope honest in the config, the docs and on the status page — shipped in the same PR and did not wait on this, because it is required whichever way the answer goes.

This directory exists so a recorded decision has somewhere to live that is not a comment in a source file nobody reads and not a Trello card that scrolls away. SAFETY-27 asks for a second one (the naming of the floors); it belongs here too.

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1. The question

Does the grid-following (self-consumption) loop get an SoC floor of its own, or is the BMS cut-out deliberately the only backstop under demand-driven discharge?

Either answer is defensible. What is not defensible is the state we have been in for four tickets: silence, plus a config key called peak_soc_floor that reads like the answer is "yes".

2. What is actually exposed — replayed, not argued

The exposure was written down three times (SAFETY-18's finding, SAFETY-24's module note and PR body, and SAFETY-24's honestly-uncaught 13th mutation) and never measured. SAFETY-25 measured it. The replay is test_peakguard.py part 10; it drives the shipped control.compute(), peakguard.decide(), peakguard.claim() and arbiter.resolve() over the committed capture scenarios/p1-net-2026-08-20.jsonl.gz, with the battery and the state of charge modelled (the captures carry meter power only — see §7).

2a. A sustained-load evening takes the pack to the BMS cut-out

19:00–22:00 UTC on 2026-08-20 — three hours of house load after the sun has gone, 830 W mean, minimum 701 W, maximum 1223 W, not one watt of export in the whole window. The pack is the 13.8 kWh reference pack (300 Ah × 51.2 V × DoD 90), started at 21 % — one point above the shipped peak_soc_floor.

pack at the start of the evening	21.0 %
pack three hours later	10.0 % — bms_soc_floor
energy the loop delivered	1525 Wh
control cycles with the pack below peak_soc_floor	549 of 587
…of those, cycles still delivering discharge	343
cycles on which the peak guard filed any floor	0

2b. It is a behaviour, not a config value

The same replay, same pack, same evening, with peak_soc_floor set to 5, 11, 20 and 60 — the shipped value and both ends of the schema range:

peak_soc_floor =  5  ->  ends at 9.9715 %, 1524.58 Wh out
peak_soc_floor = 11  ->  ends at 9.9715 %, 1524.58 Wh out
peak_soc_floor = 20  ->  ends at 9.9715 %, 1524.58 Wh out
peak_soc_floor = 60  ->  ends at 9.9715 %, 1524.58 Wh out

Identical to four decimal places. Raising the option does nothing, because the guard it bounds is not the thing doing the discharging.

2c. The two states coexist in the same cycle

On the same day's high-load block (15:00–17:00 UTC), with the pack started one point below the floor, the guard reports PEAK GUARD FLOOR-LIMITED — "commanding no forced discharge" — on 49 cycles, and on all 49 the arbiter simultaneously grants the loop a discharge of up to 2000 W. The status page tells the operator the truth about the guard while the pack empties.

2d. Stated fairly, because overstating it is how a real finding gets ignored

This is not a runaway. Self-consumption discharge is demand-driven and self-limiting: it stops when the house stops drawing, and every watt of it is serving load that would otherwise have been imported. It is not SAFETY-18's frozen-meter rail, and nothing here is spending the pack for a reason of its own. What it means is narrower and still real:

• the depth the pack reaches on a heavy evening is set by the house, not by anything an operator can configure; and
• peak_soc_floor reads like a system-wide guarantee it is not. A floor that binds one consumer and not the other is worse than no floor if its name does not say so — which is precisely what SAFETY-25's other half fixed.

3. Why SAFETY-24's floor cannot be extended to cover this

The peak guard files a lo limit — a lower bound on the setpoint. That asymmetry is what makes the guard trustworthy: a reviewer proved across 19,800 combinations that it can never lower a target, never cap a discharge, never contradict the arbiter, and never file a negative floor. peakguard.claim() leaves hi at +inf as a fact about the object rather than a rule someone has to remember.

A lo limit is structurally incapable of stopping a discharge the loop itself wants. So the same property that makes the guard safe makes it useless here. Raising peak_soc_floor cannot become the answer; §2b is that statement measured.

4. The floors we already have — and this would be the fifth key

SAFETY-24 recorded a decision to keep maintenance_soc_floor separate with an explicit revisit trigger: "if a third floor appears." The trigger has already fired, unremarked, when OPT-04 shipped arbitrage_soc_floor.

option	default	bounds	priority of the consumer
maintenance_soc_floor	11	the monthly recalibration's drain target	P_MAINTENANCE 60
peak_soc_floor	20	the peak guard's forced discharge	P_PEAK 50 (a lo limit)
bms_soc_floor	10	nothing — it describes the pack, so rule 8 can refuse a floor that cannot bind	—
arbitrage_soc_floor	20	arbitrage discharge	P_ARBITRAGE 30
the loop	—	nothing	P_LOOP 10

Two of them ship the same default (20) from different cards for different reasons, which is how a value that looks deliberate becomes a coincidence nobody dares change. SAFETY-27 owns the naming question and this memo does not pre-empt it — but any answer to §1 that adds a fifth key should land after SAFETY-27's decision, or it adds a fifth accreted name to four.

5. The options

Option A — record the BMS as the deliberate backstop; add nothing

Say plainly, in DOCS.md and in freeze.py's finding, that demand-driven discharge is bounded by the pack's own cut-out on purpose, and close the question.

• Protects: the whole point of the product. Every watt the loop discharges is self-consumption the house would otherwise have imported; a reserve is literally energy bought at the import tariff and held back.
• Costs: on a heavy evening the pack ends at the BMS cut-out with no EMS-side record of why. That is the SoC-drift condition MAINT-01 exists to correct, and the first symptom — a battery that is never available when it is wanted — is indistinguishable from a coulomb counter that has drifted. It also means the morning's peak-guard capacity is whatever last night left, which the peak guard cannot influence.
• Cost of building it: zero. It is already the shipped behaviour, and after SAFETY-25's honesty half it is already documented.

Option B — a hi limit at P_LOOP, with an enforced ordering (recommended)

A new option loop_soc_floor. While soc <= loop_soc_floor, main.py files Claim.limit("reserve", P_LOOP, hi=0.0). Zero forbids discharge and permits charging, which is the mirror image of the guard's lo = 0 and one-directional in the safe direction.

⚠️ This shape has one trap and it is fatal if missed. arbiter.resolve() applies every limit at or above the winning set claim's priority and takes the most restrictive, with no notion of one limit outranking another — that is deliberate and is what makes the safety bounds unconditional. So when the loop wins at P_LOOP, a reserve hi = 0 and the peak guard's lo = excess > 0 are both applied, lo > hi, and the arbiter resolves a CONTRADICTION to 0 W: the battery parked, in the one quarter of the month that sets the bill. This is the same crossing-limits hazard shaping_suspended() already exists to avoid for the maintenance charge-only clamp.

The fix is an ordering constraint, and it is provable: require

bms_soc_floor  <  loop_soc_floor  <=  peak_soc_floor

Then whenever the reserve binds (soc <= loop_soc_floor <= peak_soc_floor) the guard is already floor-limited and files lo = 0.0, so lo == hi == 0 and the result is 0 W with no contradiction. Refuse any other ordering at startup, in CFG-01's manner and with rule 8's message shape. A loop_soc_floor above peak_soc_floor must not be merely discouraged; it is the parked-battery bug.

• Protects: the pack from routinely sitting on its own cut-out, which is the MAINT-01 drift condition; and it gives the operator one number that means what its name says.
• Costs: a fifth SoC key (see §4), a validator rule, and — the real cost — every watt below the reserve becomes billed import. Set too high it turns a working battery into a smaller one.
• What it does not touch, correctly: a maintenance DRAIN sets at P_MAINTENANCE 60 and arbitrage sets at P_ARBITRAGE 30, both above P_LOOP 10, so a limit filed at P_LOOP is not applied to either. The monthly recalibration still drains to maintenance_soc_floor; arbitrage is still bounded by its own key. That is the right answer and it falls out of the priorities rather than needing a special case.
• Recommended starting value if this option is taken: a low one — three points above bms_soc_floor, i.e. 13 %, well under peak_soc_floor's 20. The purpose is to keep the pack off its own cut-out, not to reserve energy for anything, and an over-tight reserve is the version of this that fights MAINT-01 from the other side: it shrinks the band the pack actually cycles in, which is the condition a coulomb counter drifts in. It does not matter that 13 sits above maintenance_soc_floor's 11 — the maintenance drain sets at P_MAINTENANCE 60 and a P_LOOP limit is not applied to it, so the two never interact. The number itself is part of the owner's answer; what this memo offers is the derivation and the ordering constraint that makes any number in that range safe.

Option C — a set claim above P_PEAK

File Claim.set("reserve", 55, 0.0) when the pack is under the reserve. Because clause 2 applies only limits at or above the winner's priority, the guard's lo at 50 would then be dropped and there is no contradiction to engineer around.

• Protects: the same as B, without the ordering rule.
• Costs: it is a stop, not a bound. It parks the battery outright rather than merely forbidding discharge, so it also blocks the charging that would end the condition, and it silently outranks peak shaving in a way that is invisible on the priority table. This project's rule is that a floor about energy is not a stop of last resort (peakguard.py, point 1). Not recommended.

Option D — clamp inside control.compute()

• Costs: it puts precedence back inside a strategy, which is the exact thing arbiter.py was built to end, and it would be invisible to explain(). Not recommended.

6. Recommendation

Option A is honest and cheap; Option B is what I would build. Concretely:

1. Now (shipped with SAFETY-25): the honesty half. peak_soc_floor, its documentation and the status page say plainly that it bounds the peak guard only. This is required under every option and it is the actual dishonesty on the live system today.

2. Owner's call: A or B. If B, take it after SAFETY-27's naming decision so the fifth key lands into a scheme rather than onto a pile, and after MAINT-01 has run one real cycle — until then nobody has observed what this pack's SoC reading does over a full cycle, and picking a reserve against a reading that has never been recalibrated is picking a number against an instrument we have not checked.

3. Whichever way: the tripwire in test_peakguard.py part 10 stays until the question is answered, so the day someone builds a loop-side floor it goes red and has to be updated deliberately, rather than the exposure quietly changing under a green suite. It has two halves and they cover the two places the floor can land:

   • 10f pins the claim-source set. Every claims.append(...) in Controller.cycle() is read off its AST and must be one of eight named sources, and real cycles — spied at main.py's own resolve, swept over the whole 0–100 % SoC range in both the idle and the floor-limited state — must file exactly those. Option B is a ninth source, and so is Option C's set claim, so filing either takes part 10 red on the line it is written. Option A files no claim at all — it is the honesty half already shipped, and part 10 is the record it leaves behind.
   • 10a–10c replay the shipped code, so Option D — a clamp inside control.compute() — moves the replayed numbers instead.

   ⚠️ 10f is load-bearing and was added late. PR #31 shipped this section claiming the tripwire without it, and the reviewer disproved the claim by building Option B in main.py: the entire suite, part 10 included, stayed green, and mutation 13 went on printing "nothing prevents it, on purpose" on a tree where something did. Do not remove 10f while this question is open.

7. What the replay is and is not

The committed captures carry one series — signed net power at the meter — so the pack is modelled, and the model is stated rather than hidden:

• the captured w is the load presented to the meter; the modelled battery's delivered power is subtracted from it, one telegram late (an actuation delay);
• the loop's command is the shipped control.compute() at the shipped tuning, resolved through the shipped arbiter.resolve() against the shipped peakguard.claim(), at a cycle_floor_s tick;
• SoC falls by the delivered discharge integrated over the real telegram timestamps, on a pack of 300 Ah × 51.2 V × DoD 90;
• the BMS is modelled as a hard cut: at or below bms_soc_floor the modelled battery delivers 0 W whatever is asked of it. Without that the run would be unphysical — the BMS really does stop the pack, and the whole finding is that it is then the only thing that does;
• nothing recharges the pack inside the window. One evening, one direction.

The fixtures cannot express a low pack on their own — there is no battery or SoC telemetry in them — so the starting state of charge is a fixture parameter, not a measurement. It is chosen as one point above peak_soc_floor, which is derived from the shipped option rather than picked. Everything downstream of that one number is the real capture and the real code.