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.
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A recent set--NSFW audio: https://youtu.be/_dcxcZfD6dg
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