I recently bought this vehicle. Took it for an oil change. Battery tested poor. They advised me to replace the battery before winter. 351 cca. With this lovely Indiana weather, I have used the heat and ac. I have noticed in park with either heat or ac on, itll start to idle rough and a fan kicks on. When i turn the heat or ac off, the idle goes back to normal. Could this be caused by the bad battery? I was thinking either that or alternator. Im gonna take it to test the alternator and see if its throwing any codes. Anyone have any tips, suggestions, or advice?

in defrost the a/c comes on mostly regardless of outside temp, and that puts alot of strain on the engine with the fan and compressor, especially if its a four cylinder you will notice the load

Yea, but if his A/C is on, fans are kicking on, increased load on the alternator trying to charge a bad battery will cause a low voltage condition which will cause the engine to run rough because the injectors and coils aren't getting the required voltage to operate properly. Typical of voltage drop.

Replace the battery with a new one.

Check your throttle body, it may be dirty. Its just maybe idling too low.

Replacing the battery is the first thing I'm gonna do. RPMs also increase when its idling rough. Is this typical?

i thought you had already replaced the battery, they dont run right trying to charge a dead battery like mike said the alt puts a load on the engine and everything has low voltage

No, not yet. Going to get a battery today! We'll see how it does! Thank you!