Battery is at 12v while sitting. When ignition is engaged voltage drops to 4v and wont engage starter. However when jumping the car, it starts. So obviously there is a horrible draw in voltage somewhere, any ideas what could possibly cause this?

 

Are you measuring right at the battery (probes on the lead posts that are part of the battery)? If so, the battery is severely run down, and / or defective. If something in the car were able to pull the voltage on a good charged battery down to 4.5 volts, that something would quickly get very hot and catch on fire.

Jump-start the car and disconnect the jumper cables then measure at the battery again.

If it is more than 13.5 volts, the charging system is working, so the battery should have charged up, meaning the battery must be defective.

If it is less than 13 volts with engine running, the charging system is not working, causing the battery to run down. In that case, you may still need a new battery after fixing the charging (usually a bad alternator).

 

Thanks alot MK! Got a new battery and everything is good! just wasnt sure it was the battery since it tested at full voltage when the car was off. And yes i was measuring at posts and at power inverter inside the car. Again thank you very much! (Kyle)