Jeep Problem. Alternator? Connection? Battery?

I have a 2003 jeep grand cherokee v8 4.7 L 4x4. i dont know if this has anything to do with it but the engine was just rebuilt. ok, so it wont start. for a week prior i would turn the key, nothing for like 3 seconds, then it would crank over and start… did that and somtimes when i turned the key it would start right up. Now, i turn the key and it did nothing, or it would click, click, click. the acessories and everything works when key is turned. check battery, 12.2 volts… cleaned posts and connections. hmm. hooked up jumper cables and it would barely crank over… left it hooked up for about 15 minutes, turned key and it started cranking over normally, started, but wouldnt stay on… thought earlier it would be starter, but it seems to work fine. thought it was battery but it says its 12.2 volts… maybe alternator? idk… it wont stay started to i caant test it… any ideas? what it could be. i know nothing about cars, so trying to locate bad connections from starter to battery or alternator to battery or ground connection will be a challenge! its very upsetting…

12.2 is low, should be at least 12.5 and 14 with the car running. I would suspect a bad ground since the engine was removed. You should keep the trickle charger on until it gets to 12.5 volts. If it starts measure the battery again and make sure your alternator is putting out the 14v.

A fully charged battery should register >12.6v. 12.8v would be ideal. What is the voltage while the starter is engaged? When it does start what is the voltage when revved to 2,000 rpm? When was “just?” The engine work may have resulted in some loose connections so how soon after the work did the problems begin?

And how did the original engine fail, BTW?

ok, so im thinking it may be a bad connection or something… again this battery is like less then 2 months old. i measured the voltage of the battery, with it sitting on the ground 12.2 volts, hooked off it read 5.5 volts and with it hooked up and the key turned to acessories postion it read 4.3 volts. something is drainging it i believe, i took the alternator to advanced auto and it checked out good… so im kind of confused… i did see some cracked plastic in the plug-in to the alternator from the battery
(red wire) if that may cause it, idk! original engine failed at around 133,000… engine starting knocking from oil depletion.

First, get that battery fully charged and load tested.

It sounds like you have a “parasitic draw”. Here is a good video on how to find it (no, I’m not “Eric”):

As others here have already stated, the battery charge condition is low and needs to be charged up using a charger and not the alternator. If the engine starts okay after the battery is charged up then check the voltage getting to the battery while the engine is running around 1,500 RPM and the lights and blower are on full. You should see over 13.5 volts if the charging system is working okay. Have the current draw checked while the car is parked and things are in the sleep mode. You should have less than 50 milliamps of current draw.