Dropping charge between alternator and battery. Car won't start.

[d.f.]

Not sure where this thread should live so apologies if this is the wrong section to post in.

My car is not starting. Once jumped she drives like a dream, but stop her and she won’t start again. All power gone.

We have measured 14.5V coming from the alternator (it is a brand new alternator). At the battery end however it is getting 12V. So we are dropping current along the way.

Tomorrow we will trace the loom and see if there any faulty connections.

Has anyone experienced this? Is there anything we should be looking for/testing?

The only other thing I can add is that a warning light on the dash beeps and comes on. It’s yellow and looks like a lamp. Trying to find an example online but can’t seem to.