[Hex173t]

Ti2ink - I'm sort of confused about what you said here: "I installed on the back of the GMC two batteries 975 cca in series as power supply\support hook to a 12 feet jumping cables so the system always keep 12+ volts."

Can you please give some detail, like you connected them in series? As in connected (ground ->battery -> battery -> fuse block/starter/alternator connection)? Batteries in series add the voltage so as I read this, you applied a total of 24v to the car.

You must mean connected in parallel so battery capacity is doubled and volts stay at 12. But even if this is so, you must have a huge parasitic draw on the battery to have to go through that to make it until next start. And do it for 5 months. Whoa. I guess you gotsa do what you gotsa do.

 

[Hex173t]

Bottom line is you have to figure it out or take it to a mechanic.

Troubleshoot and repair. Just because you put "new" parts on there doesn't mean squat. I've replaced my alternator 3x, 2 of them within 2 weeks. They were reman, not new though.

My first alternator, the original from GM, failed by way of a shorted diode. Def the alternator

2nd stopped charging all together but it didn't directly discharge the battery like the first time. Intermittent issue, checked wiring and ecu inputs to the alternator, these were good. I documented all these things in this forum. Replaced alternator as a best guess, but as it was just installed and the fault changed, I was confident the guess was good.

Third needed like 1500 rpm to charge, T/S per the book, results pointed to the alternator, replaced and been ok for the last 18 months

Maybe grounds? Maybe bad parts? Maybe wiring/connections? Are you in the salt belt? Have people been poking holes in the wiring to check voltages? Maybe corrosion?

The system isn't very complicated, alternator, ecu, battery, wiring.