cold weather, electrical problems. Need help!

[///m42 sport]

so its in the 20s here in Michigan. my 318is engine starts and I notice the dash lights flash on/off. I don't think anything of it. I drive for 20 mins. All of the sudden my check engine light comes on, lights go off, but engine is still going. When I get to the house I turn off the car and restart it thinking it could be a glitch.  I notice that when I have my lights on, the check engine light comes on.  When the lights are off, engine light goes off.  Could it be the alternator? Battery? I have HID's.  Also I'm out of town right now which sucks even more.

[roundel318]

Start with the basics, aside from the battery, check the voltage from the alternater AND the voltage regulator (if it is under 14V, I understand it may be worth replacing).
Just to ensure the proper levels of voltage are being supplied, otherwise the ECU, etc could act up. Since it takes 5 minutes, you might also reseat the connector on your ECU (glove box, behind plastic panel at top). It is a large 88 pin connector that should be handled carefully when removing and reconnecting. It is recommended that you disconnect the battery (although not required) when screwing with electrical crap in the car, especially the ECU, airbag, etc.