Hey I have a Mk 2 Focus TDCI and i'm having an electrical issue, I was driving yesterday when I had my battery light on my dash come on, I also lost all auxillary electronics in the car, radio was unable to stay on for more than a few milliseconds, dash cam stopped receiving power, satnav wouldn't charge either, however I still had all dash instruments and headlights as well as the red battery warning light, once I stopped I was unable to restart the car either there just wasn't enough power to start it.

Luckily I was at somewhere with a battery charger so we removed the battery and allowed it to charge for an hour, plugged it back in and it managed to start the car, drove home for about 30 minutes with no issues, dash cam was working (I kept the radio off and sat nav out of concern for draining the battery out again) but as I said after about 30 minutes I noticed the dash cam go off (it's setup to shut down when the car switches off/looses power) then about 5 minutes after that the battery warning light appeared on the dash again.

Now this is the bit however that is confusing me, my knowledge of electronics is limited but if the battery was dud, surely it wouldn't have been able to charge/hold a charge from a mains charger to allow me to restart the car again, however the research i've done and the information i've found tells me that if it was the alternator that had gone bad, when the battery died I would have lost all electronics, now i've been told that Diesels are self sustaining once started but if the alternator had gone shouldn't I have lost dash instruments and headlights, which all still worked.

2 hours ago, Jaiimez said:

now i've been told that Diesels are self sustaining

That was only true for the old mechanically-injected engines with a manual fuel cut-off for stopping it. Modern common-rail engines such as your 1.8TDCi use electronic fuel injection so a loss of electrical power will stop the engine. Even with mechanical fuel injection if it uses a solenoid-controlled fuel cut-off (so it stops when you turn off the ignition) it won't be able to run without electrical power.

Your description of the problem sounds like a dead alternator to me.