Ok..I took my car to advance yesterday cuz my battery seemed like it was dieing. They tested it and said it was a bad battery.. Put a different battery in it, But its alot smaller(alot less amps). They tested it and said it all tested good, alt was fine but the battery was alittle low..Charged it up good, Drove it alot today then it just dies while im driving tonight cause the battery died

So I get the battery jumped, Enough to get home. Put it on my home charger and it says the alt is bad. Which Im guessing it is, cause the battery dies and when you pull the positive off while its running it turns off. I just replaced the alt tho like 3 months ago. Could it be anything else? Starter maybe? wiring? or the fact its a too small of a battery?