Pontiac G5 only starts when jumped. Battery tested out fine, Alternator tested out fine. Not running any codes...Now what?

taking the battery leads of a running alternator is one sure way of destroying the alternator
if you think a battery is dead you have it load tested and replaced if it doesn't meet the test
what you should be doing is taking the vehicle to an accredited auto electrician and having the battery load tested and the alternator and voltage regulator professionally tested for proper operation

Connect a amp meter in series at the + terminal on the battery and note the drain. there should only be a small drain caused by the clock ECT, Check for a light staying on in the trunk or glove box. Are you sure the battery is holding a charge?. connect a volt meter across the battery terminals and start the engine. voltage should be around 14 volts if alternator is ok.

a few tests to see if alternator has gone are. start the car and the red light should go off on the dash. start the car and take a lead off the battery ( the car should still run ) and start the car in the dark.turn head lights on then hit the gas,the lights should go brighter as you rev...hope this helps

You need a new alternator , Check your amps coming out of the back of your altenator you will find it is not sending 14 volts when its running. The reason it starts and runs for a bit after it sits or charges is that the car is running on the battery if you keep this up you will not only need a new altenator but a new battery

Well a mechanic would test the battery for a dead cell, then test the charging system on the car.
Does the charge warning light come on ?

First, you can't and shouldn't test an alternator by disconnecting the battery cable while the engine is running. You run the risk of cooking the alternator and the computer. Thats how we used to test generators back in the last century.
Is the check engine light on when the engine is running ? Is the battery charge warning light on ?
There should be a fuseable link between the alternator and the battery that could keep the alternator from charging the battery.
The ignition module would have no effect on the charging system.

The battery may have a bad cell. If a jump gets the vehicle started, replace your battery and you should be fine.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

Hello! I do not believe this is a security issue...The Battery Light is telling you that the charging system has a fault...One of three possibilities exist...# 1...The battery can no longer hold a charge...
# 2...The Alternator is defective...Or # 3...The cables that interconnect power and ground are corroded and resistive...Do you have a voltmeter and a wee bit of experience using one?...If not send a comment...Set the meter to rear +12 volts...Place the meter probes directly in the center of the battery post...Record the reading...Jump start the vehicle...Quickly remove the jump cables and while the engine is still running, take the same reading again...The second reading must be 1.8 to 2 volts higher than the first...This tests the alternator...If alternator is good...Have the battery tested...Clean cables (on both ends) if necessary...

When the battery, alternator and cables are correct, the battery light will go out...At that point in time the engine should start...If not, then we have other issues and I'll fix them if necessary...

Guru.......Saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

These are all signs of a bad alternator. The car is running off of only the battery. The alternator charges the battery and keeps the car running. When the alternator quits charging the car will run fine until the battery dies. Then you can jump the battery and charge it, and the car will run fine until all of the electronics and lights run the battery down again.

Place the negative cable on the frame of the car to ground it. It's best to do this even when there is a negative post present to protect the car and the alternator.