Measuring your car battery’s voltage can be a great way to determine just how charged your battery might be.
The ideal car battery voltage range will provide you with a relevant interval according to which you can measure precisely what the voltage says about the battery’s current charge. A perfect voltage (without any devices or electronic systems plugged in) is between 13.7 and 14.7V. If the battery isn’t fully charged, it will diminish to 12.4V at 75%, 12V when it’s only operating at 25%, and up to 11.9V when it’s completely discharged. This data and the way it relates to the design of your battery should provide you with some useful insight into the functioning capacity of your car battery.

https://shop.advanceautoparts.com/r/...-voltage-range

 

For you engine light, best to have someone scan for the code instead of taking a guess.

As for the failing alt, just because it passes a bench test does not make it good. I had an alt causing a parasitic drain on my Monte. If the car sat for 2-4 days the battery would be dead. Since the car was not driven full time or in the winter, it took a year or two to figure it out. I finally replaced it when it began giving the smell of hot/burning electronics. After that, no more problems with dead batteries.

I am very interested about the differences in the two alts. I thought for sure they were interchangeable.