'02 Taurus Lights get brighter on the gas

[cvg2k31]

I have an 02 Taurus SEL 24 Valve. While at idle or at a stoplight all lights dim slightly and the heater fan speed slows. Once you are back on the gas and driving the lights brighten just enough to notice and the heater fan speed increases. So what's the deal bad alternator or bad battery or what? The heater fan speed increase is very obvious and is what lead me to check how the lights reacted in the same situation.

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[PAracer]

Unless this has just started and is getting worse, there really is no problem. Because the alternator is spun by he accesory belt, the speed at which it spins must be fast enough to charge the battery. Your idle speed just isn't quite high enough to hold a high voltage. If your Taurus has a volt gauge, you would be able to see this while driving. I notice this in my SHO as I have a set of underdrive pullies. The idle is normally around 800, but the battery cannot pick up a charge until after 950.