Nvidia 840M throttle in AC mode but not battery mode

For the last few months I've been encountering a unusual issue with my 840m: when I'm not plugging in my AC charger, everythings works fine, but when I plug it in the gpu starts to throttle a lot, even when the temperature is a little far from temperature limit
I do notice that the higher the temperature the more likely it starts to throttle (around 75 degree), but since it doesnt happen in battery mode I dont know how to solve this
I've already set power mode to high performance in both windows and nvidia control panel, but no luck
Also this happen with all windows versions (8.1 and 10), all drivers

This is heaven running on battery

And this is heaven running on AC power

Yes, I did overclocked it but only to make the issue more clearly seen, it happens on default clocks as well as when underclocked, and sometimes doesnt on high temp at all

In GPU-Z PerfCaps Reason was: Pwr, which I believe is PowerLimits problem
How can I fix this and what's causing this problem?

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Nvidia 840M throttle in AC mode but not battery mode