I have a 2007 Mazda 3. The new battery died after 6 months. I took the the alternator off and had it tested, it passed. I then had the battery replaced assuming it had a bad cell. When the new battery was installed the battery light would not go out. Now the new battery has drained itself in 2 days. I could have left a loose connection when I reinstalled the alternator but could there be another issue?

High voltage drop on your connections and corrosion

Who tested this alternator? 2007 is it original? I would replace just to be sure. Yeah some are going to say parts swapping but 2 new batteries and no resolution. Could be diodes in alternator and without the correct testing devices, you will not get a correct diagnosis. Bad diodes allow battery voltage to go backwards to the alternator instead of alternator to battery.

Battery connections are clean, actually very good condition. I had the alternator tested at the local auto parts store on their machine. As far as I know it is the original alternator and my plan is to replace it tomorrow. If that doesn't fix it I think it will be time for professional help.

I had a parts store (not mentioning)...Dura NOT Last....test both a starter and alternator and they told me that both were bad. I found it hard to believe and installed them in both cars and they both worked for years. I pulled them for a friend for Xmas at a pull and pay because both his cars were broke down. Had them “tested” on the way to his house. Both cars were up and running for $79. He installed one and I did the other.

I have also seen an alternator be bad that was right at the lowest voltage on the acceptable range for their make and model. I had a Sienna come in the shop that was bad at 13.2 Volts.