How to *DISABLE* fast charging on a Treble GSI?

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AnonVendetta

Senior Member
Apr 29, 2016
1,152
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Portland, OR
First, I know that most people will be interested in having fast charging working. However, I want to know how to disable it. Mainly for battery longevity reasons. Via a Magisk module, terminal command, GUI, etc. Whatever works.

Device details:
Samsung Galaxy Tab S7+ (Model: SM-T970, wifi only variant)

Currently running the latest Android 11 BUJ3 stock firmware

Is already rooted with latest Magisk, and TWRP by @ianmacd

Device is running stock kernel (no custom kernels are available for my variant)

Stock kernel fully supports fast charging

I know that this device can run a GSI, so therefore it supports Treble. A few months ago I was able to boot Google's official Android 12L GSI, but I didn't like the experience.

In the stock firmware, there is a toggle in the Settings app that disables/enables fast charging (enabled is the default option). I'm unsure what sysfs variable (/sys/*) this changes, or if it changes a system property. But I'm sure it must interact with the kernel in some way.

On older devices I have owned, there is a file at:
/sys/kernel/fast_charge/force_fast_charge. I could change the value inside it to 1 for enable, or 0 for disable. But this file doesn't exist at this location on my tab. I couldnt find it anywhere in /sys/. So the toggle in settings must be changing something else.

Stock and GSI ROMs use the same boot.img/kernel. So if it can be disabled on stock, then logically the same must be true for a GSI.

This is literally the only thing preventing me from migrating from stock to a custom GSI ROM. Any help here is appreciated. BTW, if you're just going to post along the lines of "dude, just use fast charging"...no, just no. Don't waste your time or mine. I will not argue with these people. I've been slow charging my devices for years, I'm 110% convinced that it increases battery longevity and reduces battery heat.