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.

 

You got a pretty bad short. Is the voltage over 12V before you turn on the ignition?

If so, it's time to break out an ohm meter and track it down.

You might try to pull out all of the fuses and put them back in one at a time with the ignition on and watch the voltage.