Hi all, I was wondering if someone can help me.

Ford Mondeo TDCI 2.0 57 PLATE  150000 miles

Replaced 12 year old Ford battery with Lion 70ah battery, Works fine

2 weeks later Battery light comes on after driving for a couple of minutes. Replaced Alternator. Works fine again for a couple of weeks then battery red light returns same issue. Drive around for about 10 days and then the car starts sluggish. drive about 2 miles home perfectly fine. Turn ignition back on immediately and car totally dead. it seems that the car wont conk out when I m driving it. 

Wondering if its the battery not working properly but funny thing is after this happened I got the battery charged up again. Lockdown started so left the car without starting for 4 weeks and started up straight away so I don't think the new battery is duff in that respect

At odds to work out what to do here. 

Could be one of several things. Check electrical connections, especially on the starter motor, check that the alternator is putting out around 14.5v when running, also check wiring connections to it, check voltage of battery 2 or 3 days after a full charge before using the car again to see if there has been a voltage drop. Most Ford cars are now fitted with a calcium battery, have you fitted the correct one?, look in the hand book to see what the battery recommendation is.

57 minutes ago, Milkman said:

Most Ford cars are now fitted with a calcium battery, have you fitted the correct one?, look in the hand book to see what the battery recommendation is.

My thought as well, I don't know if there were L-ion battery's 12 years back and they have a different charging sequence then [acid] battery's

The battery removed was an original ford one. Yes, the Lion battery I put in is a calcium battery. Lion is a brand not to be confused with L-ion!

Have you got a voltmeter? It’s going to be easier if you do.

This very same thing has happened to my 2006 tdci Mondeo. I also changed the dead battery after it was sitting idle during lockdown, and the red battery light came on. I did the volt meter tests which pointed to a faulty alternator. 
I bought a new one and fitted that but after half an hour of driving, the battery light came on again. I got the RAC out and he did a comprehensive test which pointed again, to a faulty alternator. I’ve sent it back today and will wait for the new one to arrive and fit it.

I've just got A sneaky suspicion that it’s something else, probably minor that’s causing the issues here.