Hello I have a Ford Fiesta and last night I parked it for the night and came back out in the morning to go to work and it was dead no lights were left on key was left in the ignition but in the off position so I tried to jump it but it wouldn’t go above 5.4 volts so I thought that maybe one of the cells in the battery died so I got a new one and it had 12.6 volts before putting it in as soon as I hooked it up it read 5.4 volts also when the key is off the radio lights flicker and one of the relays clicks repetitively also there is a noise coming from inside the dash does anyone know what it could be? 

This is a video of what it’s

doinghttps://youtube.com/shorts/Q5osQvMlHJc?feature=share

flat battery or time for a new one🤔

Replaced battery still did the same thing 😖

2 hours ago, dezwez said:

flat battery or time for a new one🤔

You might have an electrical drain somewhere something is cause it. Anyway before that did you checked all the fuses?  You might have a short somewhere and a fuse may  blow up. If it's a fuse find out what that fuse is for and from there try to see what's wrong. From the fuse box sounds  it's seems there is a relay that is trying to get on or it resets. I might be wrong but I hope it helps. 

So just to confirm, you measured a new battery at 12volts then as soon as you fitted it put a meter across it again and then measured 5 volts???

Also do you always leave your key in the ignition?or is this a new thing you are doing?

The fault is almost certainly the alternator rectifier pack has gone short-circuit (you need a new alternator). 

Do NOT leave the battery connected or the short-circuit current through the battery will damage it and you will need yet another new battery.

With the battery fully disconnected from the car, use a digital multi-meter and measure the resistance between the negative battery lead and the positive battery lead. I expect you will find it to be less than 100 ohm.

Still with the battery disconnected, remove the main positive cable (big heavy cable) from the alternator. Remeasure the resistance between the battery leads.

If your new battery has been left connected for more than 5 minutes with this short-circuit then it may well be damaged permanently.

See below for an example of a schematic of a rectifier pack. The picture is of an actual rectifier pack.

If I remember correctly (which is by no means certain) the way I tested alternators to see if they were discharging the battery through the rectifier pack was to :  disconnect all wires from alternator.  Use multimeter on resistance setting with one probe on the main power output connector on the alternator and the other probe on the alternator metal body. And then do the same with the probes swapped round.   One way the resistance will be total (infinity) the other way will be low.  If you don’t get infinity one way then power will discharge from battery back through the alternator. 

The rectifier converts ac to dc and is made up of diodes.  But think of the rectifier pack as a one way valve that lets electricity pass from the alternator coils to the battery but stops the electricity going back the other way