Use cluster test to see if it's battery, alt, or both. The alt isn't a charger, so if you've had a bad battery for a while, the alt could go bad.
Voltage off should be around 12 V and on around 14V.
Your starter isn't an issue at all.
I'm thinking just battery, but you need to check the voltage to begin.
Search Cluster test.
In NJ--Tierney's in Montclair, Sept. 25, 7:30 p.m.
In NYC Sat, Sept. 30, 8 p.m. at Broadway Comedy Club! Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Comedy can be a great cheap date!
Mangofy your E46!