Diagnosing a bad alternator?

When mine failed the auto electrician told me the alternator has a diode inside that only allows current to flow one way. When the diode fails the alternator will still charge the battery when running but with the vehicle off it then turns into a motor and tries to draw electricity (probably not scientifically accurate but you get the idea). Rock Auto has alternators at a good price and it's a pretty easy job, I would just replace it

 

Maybe I missed it in all the chatter, but has the OP said whether this is the original battery or a replacement?

If a replacement, how old is it?

 

I’ve never heard of that failure mode for a diode. Usually they open circuit and you lose one of the diode trio and the capacity of the alternator is reduced as well as AC noise being induced into the charging circuit which can shorten the life of the battery.

 

Just repeating what I was told and I do know I haven't had the battery go dead overnight since then where it was happening on a regular basis beforehand