04 X2 with small charging problem

Hi there, I'm having a small problem with my electrical charging system and was wondering if someone could help. My volt meter on my dash is showing 12-13 volts instead of it's typical 14. I've scraped the terminals off of both batteries (Perko and negative terminal on the block too) but it's not charging anymore than 12V at idle. When I give it some gas it'll go up to 13, but never more than that. If I idle for more than 2 minutes, the battery alarm comes on. Can anyone point me to the next logical spot to check? Thanks.