I have a 2004 Chevy Trailblazer EXT. I was driving down the road and my battery light came on. I didn’t know what it meant but I kept driving. Then my lights became dim and I pulled over and parked at a church. My car wouldn’t start after I turned car off. I talked to my dad who is a mechanic and he said it was the alternator. So Monday I changed the alternator and we knew we would have to jump start my car. Well it started up with a jump start but after removing the cables it would die. Did the bad alternator kill my battery? What could possibly be the problem here?
There is an excitor cable on the back of the alternator which tells it when it ought to be charging. If this is faulty, the alternator will not start to charge the battery, even if it's a completely working unit. If changing the alternator didn't work, the likelyhood is that there is a fault somewhere in the wiring loom for this (possibly even a fuse).
Assuming the new alternator is fine, then if the battery is too flat, then the alternator may not charge even though you have jumped it.
One way is to use the donor vehicle to run its engine to charge the dead battery for 10 to 15 minutes. Then start the engine and disconnect the jump leads - if the "flat" battery now has sufficient power it should continue to run.
Of course, you could remove the flat battery and properly charge it overnight then refit and test.