Find solidjake's vid of cluster test...voltage is #9...you can see voltage of car off and on...and you should still do this...or if you have multimeter, just use that and report voltage you have.
If you have new bat, but alt is bad, you'll run into this issue again.
But, hope you just needed battery...although, it could have just been how you were using your battery too...too many short trips, not enough long ones.
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!