Battery discharge control: hold silently reverts after 60 min on Fronius GEN24 (one-shot write vs. SunSpec InOutWRte_RvrtTms)

[marknisk]

Describe the bug

With batteryDischargeControl enabled, evcc correctly puts the home battery into hold mode when smart-cost (or plan/fast) charging starts — but the hold is written to the device once and never refreshed. On a Fronius GEN24 the SunSpec model 124 revert timer (InOutWRte_RvrtTms, register 40358) is 3600 s, so the inverter silently reverts to normal self-consumption exactly 60 minutes after evcc's single write. evcc's internal state still says "hold", so it never re-writes, and the home battery then discharges at full power into the vehicle charge for the rest of the session.

Any battery hold longer than 60 minutes is affected; shorter charges appear to work, which makes this easy to misdiagnose.

Observed behaviour (telemetry, 30 s resolution, times UTC)

Vehicle smart-cost charging at ~11 kW, home battery at 48%:

Time	Event
21:00:41	Charging starts; one sample of ~7.7 kW battery discharge
21:01:11	Hold applied — battery power drops to ~1 W, SoC steady at 48%
22:00:41 → 22:01:11	Exactly 3600 s after the hold write: battery jumps to ~7.7 kW discharge (its maximum), grid import drops ~11.7 → ~4.2 kW, charging continues at ~11 kW
23:13	Vehicle reaches its 80% limit, charging stops; battery has drained 48% → ~23%

Smart-cost remained active the whole session (all hours ≤ the 0.12 limit), the loadpoint kept fast-charging, and there is no battery mode change or error in the logs — the release happens inside the inverter, not in evcc.

Root cause

1. core/site_battery.go → requiredBatteryMode(): keepUnlessModified returns api.BatteryUnknown while the current mode already equals the required mode, so applyBatteryMode is skipped on every subsequent cycle. On current master this is reinforced in applyBatteryMode by the site.batteryModeApplied cache ("don't re-apply the mode the battery is already in").
2. The fronius-gen24 template's batterymode hold case writes StorCtl_Mod=2, OutWRte=0 and does not touch InOutWRte_RvrtTms.
3. The GEN24 (tested: Symo GEN24 Plus 8.0, FW 1.39.5-1) has InOutWRte_RvrtTms = 3600, so the storage-control settings expire after 60 minutes without a refreshing write. Per SunSpec model 124 this revert-timeout behaviour is standard, so other SunSpec storage devices are likely affected too.

Expected behaviour

The hold should persist for the whole charging session. Possible fixes:

• periodically re-apply the active battery mode (a refresh watchdog similar to what External control of battery charge power (experimental) #32848 / Battery: external charge power limit (experimental) #32931 introduce for external battery control), or
• have the templates set/clear InOutWRte_RvrtTms when engaging/releasing a mode (with the trade-off that a crashed evcc would leave the battery locked if the timer is disabled).

Version

0.313.3 (behaviour verified unchanged in core/site_battery.go on current master)

Configuration

meters:
  - name: battery
    type: template
    template: fronius-gen24
    usage: battery
    host: <inverter>
    port: 502
    id: 200
site:
  meters:
    grid: grid
    pv: pv
    battery: battery
loadpoints:
  - charger: wallbox   # ocpp template
    vehicle: bmw_ix    # cardata template
    mode: pv

batteryDischargeControl: true and smartCostLimit: 0.12 set via UI.

Log details

Log level INFO — the session only shows (times local, UTC+3):

[lp-1  ] INFO 2026/08/21 00:00:41 start charging ->
[lp-1  ] INFO 2026/08/21 02:13:11 stop charging <-

No battery mode: errors, no planner warnings during the session. The hold engage/release timing above is from the site's InfluxDB export (batteryPower, batterySoc, gridPower, chargePower).

[github-actions]

Investigated and can confirm the root cause with high confidence.

Confirmed in code:

• core/site_battery.go → requiredBatteryMode(): once the site's tracked mode (site.batteryMode) already equals the required mode (e.g. BatteryHold), keepUnlessModified returns api.BatteryUnknown, so updateBatteryMode() skips calling applyBatteryMode on subsequent cycles. applyBatteryMode additionally short-circuits via the site.batteryModeApplied cache ("don't re-apply the mode the battery is already in"). So the hold write genuinely happens only once per mode transition, with no periodic refresh.
• templates/definition/meter/fronius-gen24.yaml → the batterymode mapping for case: 2 # hold writes only 124:0:StorCtl_Mod=2 and 124:0:OutWRte=0 (SunSpec model 124). It never sets InOutWRte_RvrtTms (register 40358), which per SunSpec model 124 is the storage-control revert timer. If the inverter's revert timer defaults to a non-zero value (e.g. 3600 s as reported), the device reverts to normal self-consumption once that timer elapses, independent of evcc's internal state.

This matches the reported telemetry precisely: hold engages, battery holds for exactly the revert-timer duration, then reverts even though evcc never logs a mode change.

This looks fixable, but the two possible directions have different trade-offs (periodic refresh watchdog vs. template setting/clearing InOutWRte_RvrtTms), and picking one has cross-device implications (other SunSpec storage devices, behavior on evcc crash/restart while holding). That warrants a short design discussion rather than a blind mechanical fix, so I'm not opening a PR for this.

🤖 Generated with Claude Code

[andig]

@marknisk could you give PR a try? Reading watchdog timeout will only happen at configuration time though...

[iseeberg79]

Are you sure InOutWRte_RvrtTms is having a default value in your setup, ie. never changed it? There are scrips setting values out there as well.

From what I read the default of InOutWRte_RvrtTms for Gen24 is zero, ie. not resetting the registers.

The current template is not using a watchdog because of this?

Your issue is getting solved by setting InOutWRte_RvrtTms 0 once, returning to default.

As an alternative it may have changed since documented. Maybe it's worth checking this and probably better add the prerequisite of InOutWRte_RvrtTms being zero in the documentation of the template?

Perhaps somebody using same hardware may double-check.

[tilalx]

Confirmed on my hardware (Symo GEN24 Plus): InOutWRte_RvrtTms = 0 is the default and currently set value on my system.