Flickering battery light during idle

[Loperamide]

When the engine is idling the battery light mostly flickers. Sometimes it is ok.
I can make it do it by turning fan speed up/rear screen/lights on etc. If i rev the engine t he light disappears. I have had battery and alternator tested and both are ok. Voltages also ok during engine off/engine on. The only thing someone suggested was the engine was idling at too low speed, around 450 rpm. I can increase this idle speed by turning on the AC button which then increases the idle speed to around 550 ish, and the light disappears.
My question is how do i increase the idle engine speed without using AC button?