Help elec nightmare

Very confused. All was working great drove home one day parked it and then tried to go start it next morning no luck. no try. figured crap battery died. my seat belt light was flickering like a bad battery. i jumped it with my ladies car and it cranks but didnt have enough juice. went to work and got my portable jumper and that starts the car iwth no issues. let the car run for 30 min. checked voltage at alt with 14.4 or so and battery at 12.3 or so. stopped engine and same isuse. so only way to get it to start is with my jump box. thought maybe the aftermarket alarm this person had installed before me was the issue..? so removed it all. still no luck. all grounds look good. its just strange that the jump box starts it with no issue but nothing else will do it. It is a 1999 ES 2.4l

This might be a basic question, but when the car is off, what's the battery voltage measuring when you try to crank?

So, after looking into it deeper decided to check voltage while trying to crank and it was dropping down to like 2 volts. thought maybe the battery is done. went to undo the battery terminals nothing weird about it and saw that the extra battery i had was not a good match . went ahead put it all back how it was and now it works good. i think it was something bogus on the terminal ends. may replace the ground side just to be safe. the bolt was able to tighten but not be super great. Thank you for the post though!

Good to hear that it was all sorted out!