All of a sudden today when I turn on the AC my battery light comes on and then the car goes in to limp mode. The CEL is on also. What do you think?
It's an '06 R53. Thanks for your help.

 

Battery light coming on is almost always an alternator, but a battery with a dead cell or multiple dead cells doesn't really take a charge all too well.

Only thing I'd look into is if the belt is the correct size and isn't slipping on the pulleys causing you to loose alternator charge when the AC clutch collapses.

 

Find a way to measure the battery voltage (you can measure at the cigarette lighter socket with a multimeter if needed). Measure the voltage before you start the car and then again with the engine running. If you want, also measure (engine off) in the evening and again in the morning before you start it. Report back...

Many auto parts stores will test a battery for free these days, but I've had problems with false negatives. If they say it's bad, it probably is. If they say it's ok, maybe yes, maybe no...

 

The car started fine even after sitting all night so that makes me think it's not the battery. It did stumble at take off this morning at one stop sign. Could it be a spark plug or plug wire? But why would that affect the AC?

 

Bump!
I'm looking for a little help here. Does anyone have any ideas?

 

If you have a battery, alternator, or problem in the belt drive you need to look into that. The battery light comes on when the alternator isn't charging, so you need to diagnose why.

These cars are very electronically dependent, weird problems surface when you have a battery or charging issue.

 

David already told you what to do, no need for a bump. It's probably your alternator.

 

I'd check the crank pulley too. If it's separating and causing the belt to seize on the the pulleys, it could in return cause the alternator to not charge the battery. You probably can recognize it more with the ac on since its putting more load on the engine.

 

Alternator or crank pulley - I'd bet alternator. The car goes into limp mode since the alternator isn't providing voltage, so it's preserving the remaining battery power to get you safely to the shoulder or to safety. It won't run more than about 30 miles, and it automatically shuts down nearly everything (wait until it shuts down your power steering, that's a treat on the Dragon, ask how I know!).

Get it replaced. It's like 3 bolts and 2 plugs. Unfortunately, you have to remove much of the front end to get TO those bolts and plugs! BUT... you don't have to drain the coolant or a/c, they swing out of the way. Check YouTube and NAM for write-ups.

Also check the crank pulley while you're in there, they "go" about 100-120k and since you'll have the belt off anyway just do it.