Just make sure if it is the alternator(have it tested at a shop, usually free of charge) make sure you get a new alternator instead of a rebuild. ANd try avoiding draing the good battery. Its bad enough the alternator is going out, no need to buy an extra battery.
"So I'm drifting away like a feather in air, letting my soul take me away from the hurt and despair.
So I'm, keepin' it vertical forever elevator, riding the escalator to something that is greater.."
Official E46 OT Podcast