[Feature Request] Smart Charging - Implement battery idle mode

When I am using the smart charging feature on a FP4 and the battery reaches the stop trigger level, the phone is starting to draw power from the battery again and discharges it, while still plugged in.

In comparison, the Advanced Charging Controller App (AccA)|Advanced Charging Controller offers the possibility to switch to its so called "battery idle mode". That means the phone draws its current power consumption from the power source and does not actively empty the battery again (self-discharge happens, of course).

https://github.com/VR-25/acc#frequently-asked-questions-faq

What's idle mode, and how do I set it up?

It's the ability of running off the charger. The battery behaves as if it were physically disconnected from the device. The primary indicator of idle mode is charging current around 0 mA. One can customize idleThreshold (absolute value).

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IMPORTANT: lithium batteries have the longest lifespan when kept about 40-60% charged (about 3.7-3.9ish Volts). That said, try not to use idle mode above 60% too often.

This seems to be an ever better way to save the battery health. But the hardware has to support it (can be checked programmatically), so the function could probably not be provided in general. It still could be an interesting feature for anyone not willing to use Magisk with the corresponding module.