fitted brand new battery after old one died, when i turn on ignition battery goes down to 7.5 volts and if i try to start engine voltage drops to 1.5 volts, battery is fully charged.

 

Find whatever is pulling all that juice... sounds like you have a meter... start pulling different fuses till you find the circuit that is dropping your voltage, then start going through that circuit.

Sounds like maybe a shorted starter solenoid?... but the 7.5 for just turning on the ignition is not right.. .you've got a couple of things out of whack there.

 

Your ignition shouldn't drop that low without load. Test the battery before anything. You could have one or more shorted cells. That's what I would think would be most likely. Wouldn't hurt to test amperage draw from the battery, then turn the ignition on. If it's high, you can pull fuses/disconnect components until it drops low. Let us know

 

Sounds like the new battery ain't so new.. If getting 7.5 volts at the battery with the ignition on, you should see smoke somewhere..

 

im wondering if it's a new battery or a new-used battery. When I see voltages like that at work (Chrysler tech) it's normally a shorted cell.

 

Could be that or simply a new AGM battery placed that had been placed on a dumb old style charger for too long. It simply dried out...