The SUT enters S4 when the battery capacity discharges to 4% after the first entry into Modern Standyby under DC power, rather than entering S4 due to the critical battery threshold set at 2%.

Hi Liam L.W. Kuo (郭力瑋),

Thank you for the additional detail. Based on the behavior shown in SleepStudy, this appears to be related to the system reaching the firmware-reported reserve battery threshold during Modern Standby, which can trigger hibernate (S4) before the user-visible Critical battery percentage is reached.

Windows uses the battery information reported through ACPI _BIX, and Microsoft’s hardware guidance notes that the Design Capacity of Low reserve is managed by the platform and should be less than 4% of total battery design capacity while still leaving enough power to complete shutdown/hibernate and preserve the RTC.

Here are some additional checks that you can try at this point:

1. Review the _BIX battery values and confirm the reported Design Capacity of Low matches the intended reserve level for the platform.
2. Verify the EC / battery gauge reporting near the low-battery range to confirm the reserve threshold is being signaled at the expected capacity.
3. Compare the reported reserve level with the battery’s full design capacity to see whether the effective trigger point aligns with the platform target. Microsoft guidance indicates this reserve should remain below 4% of design capacity.

If a different trigger point is required, this would normally need to be adjusted on the firmware / platform side, since Windows does not expose a separate setting for reserve-battery actions.

If the system is entering S4 at 4% with “Reserve Battery Level Reached”, that generally suggests the platform’s reserve threshold is being reached before the visible 2% Critical battery setting takes effect.

I hope this helps point you in the right direction. Let me know if you need further assistance, feel free to ask me by clicking "Add Comment" or "Add Answer" if you cannot add comment so your response will be visible. Thanks for your effort.

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On Modern Standby systems, the behavior being observed is expected and is controlled by the platform’s reserved capacity for the real‑time clock (RTC) and critical action, not by a separate, user‑configurable “Reserve Battery” policy in Windows.

Key points from the platform requirements and design guidelines:

1. Firmware‑defined “Design Capacity for Low” (reserve) threshold
   • The ACPI Control Method Battery exposes a Design Capacity for Low value via the _BIX method.
   • When remaining capacity reaches this value, firmware sends a notification to Windows, and Windows performs the critical action (hibernate/shutdown) to preserve state.
   • The guidelines require that this Design Capacity for Low:
     • Is at least 675 mWh above the energy needed to complete the critical action.
     • Is less than 4% of the full design capacity.
   • This reserve is intended to guarantee enough energy for the critical action and to maintain the RTC for at least four weeks when the device is off.
2. Reserve capacity is managed by platform/firmware, not by Windows policy
   • The Windows power model assumes that reserve capacity is fully managed by the platform and is not exposed as a separate, configurable OS policy.
   • From the requirements: if a platform needs minimum battery capacity to always boot, that reserve must be managed by the platform and must not be exposed to Windows.
   • Therefore, Windows only exposes and allows configuration of Low Battery and Critical Battery levels; the reserve level is effectively a firmware/ACPI design parameter.
3. Why S4 occurs at ~4% instead of the 2% critical level
   • Because the platform’s _BIX.DesignCapacityofLow reserve threshold is being reached while the system is in Modern Standby (S0ix) on DC power.
   • When this reserve threshold is hit, firmware notifies Windows, and Windows immediately performs the critical action (hibernate/S4) from Sleep/Modern Standby, which is what SleepStudy reports as “Hibernate from Sleep without Wake – Reserve Battery Level Reached.”
   • This behavior takes precedence over the user‑visible “Critical Battery” percentage in Windows, by design, to ensure there is enough energy to complete hibernate and maintain RTC.
4. How to change the behavior
   • There is no Windows setting to change or disable this reserve‑triggered transition to S4.
   • Any adjustment must be done in firmware/ACPI implementation by:
     • Reviewing and, if appropriate, re‑tuning _BIX.DesignCapacityofLow so that:
       • It still meets the requirements (≥ 675 mWh and < 4% of design capacity), and
       • It aligns better with the desired user‑visible critical percentage.
   • As long as the platform complies with the above constraints, the exact mapping between percentage (4% vs 2%) and the reserve threshold is a platform design choice, not an OS policy.
5. Additional checks
   • Verify that the Control Method Battery implementation correctly reports:
     • Full design capacity.
     • DesignCapacityofLow (reserve) consistent with the intended percentage.
   • Confirm that the firmware is issuing the Notify at the correct remaining capacity and that no additional vendor‑specific logic is forcing an earlier transition.

In summary, the earlier S4 entry at 4% is driven by the firmware‑defined _BIX.DesignCapacityofLow reserve threshold, which Windows must honor to guarantee successful hibernate and RTC retention. Adjusting this behavior requires changing the platform’s ACPI/firmware battery reporting within the constraints of the Windows hardware design guidelines; there is no separate Windows configuration for “Reserve Battery” actions.

References:

• Power and battery user experience requirements
• Battery and power subsystem hardware design
• Modern standby SleepStudy