4.9l V-8. Why won't it start?

Battery cable connections ? Need to hook up scan tool to see if the modules on the vehicle are communicating ! Voltage drop test starting system . Check START 1 relay in underhood fuse/relay box ! Check PCM/BATT fuse 10amp ! Check power at relay ! This video shows how ! VEHICLE RELAYS Operation Diagnosis If you don't know, best take to a ASE certified technician before you break it ? How to Test and Troubleshoot Starter Problem

run fault codes to find any problems in the CPU/ ECM as jumping a battery incorrectly causes a power surge which generally wrecks the units. If it checks out ok then use the reader to reset the CPU as the program was probably corrupted.

have a load test done on the battery as there is a dead cell evident. If the battery is replaced have the alternator checked for output and your problems should go.

My first guess would be a dead cell inside the battery, or a blown fuseable link between the battery and the fuse box.
Someone would need to check the battery connections and fuse box with a meter.

not sure what you said but.
seems the battery is discharging,? 12.6v is fully charged,
own a voltmeter?
try a good loaned battery , does it start the car,?
i think you meant?
i can jumper cable start my car at any time. (off another running car)
but after running my car, the battery discharges and it will not crank at all. right?
this means ,the charging system on your car is bad.
if the starter cranks great, then the engine is flooded or is in great need of a tune up (sparkplugs etc)
if flooded try the super secret in the op guide, crank with throttle wide open , this magic trick cuts all fuel injection allowing the flooding
to purge away, and eventual starting.
my guess 2 is the car wont start, so you crank the battery flat
you jumper start it and it runs. (luck)
then day 2, same deal cranks ok but wont start.. ever tune up the engine?

if the lights come on you'r getting juice from the batt,if they go out when you crank the motor over batt is bad or you have bad batt connections

check your courtesy fuse it might of blown in a spark caused from the jump.

you have a dead cell in your battery witch will not turn the starter motor try switching the old batt w/ the jump batt and that mite do it ... i would start there b4 i checked any wires..

yur not supposed to and no yu dont need to reset them to the best of my knowledge my truck a 97 dodge ram 2500 v8 sat for like 8 months dead battery and had no problems but yes disconecting negative will remove power from the computer message me back with any questions

I would first check the condition of the battery. If the battery is okay, that is it puts out enough current to operate the starter. I would then check for a short. The battery would be dead in a couple of hours only if there is a short or another load on the battery.