OK I'm a little confused on what's going on here so maybe someone can help me. The battery on my car does not hold a charge so I know its time for a new one. When I try to start the car it cranks very slowly but does not have enough power to start, but when I remove the key it keeps trying to crank. It continues very slowly cranking for up to 2 mins with no key in the ignition before it stops. When it is hooked to jumper cables it will start right up and stay running, than turn off normally. If I start it immediately after I shut it off it starts but if I wait an hour there I'd not enough battery to start and it does the zombie crank and continues after I remove the key.

Battery not keeping charge could be alternator

Just changed alternator, and this was happening before that, I know the battery is bad but why does it keep cranking after I remove the key from the ignition on low battery?

Bad wiring? My dads econoline kept cranking because of a short in the wiring

Your new alternator, what is it? Is it a new or remanned Nissan OEM unit or an aftermarket one? J30's/300zx have a long history of doing bizarre things or alternators not lasting long that where aftermarket. Do a search, and you will find out this is a prevalent problem. I remember pages of threads from back when this forum was a newsgroup.