I’m accidentally left the key in the run position and went inside for about 5 hours. The battery is now completely dead. I hooked another car up to jump it with jumper cables and I have power but the starter won’t do anything. Does the battery need to be fully charged first before it will start even with the jumper cables?

 

Dead battery is a dead battery. Trying to boost it won't work until the battery starts to get a better charge. Using booster cables it could take up to 15-30 minutes to bring the charge up enough to crank the engine over. You might get lucky and have enough charge in about 10 minutes. Either way, putting a battery charger on it is better and having a battery charger on it when your done with the car at the end of the day is even better. 2 amp trickle charge overnight.

 

Thanks,I was assuming that might be the case and figured unless the voltage was exactly correct perhaps the computer might not work correctly. It was acting like VATS. No starter clutch or anything. Waited 10 minutes and it fired up.