I was working on my Focus mk2.5 with the radio and a/c controls unplugged to fit a new head unit and steering wheel control interface.

with the a/c controls unplugged, the fan runs at full speed when the ignition is on Acc only when powering up the head unit on its loom to check functions before putting into the dash.

during this work my battery went flat and I heard a beep as the car suddenly shut down on low voltage. Battery voltage was down to 5v after a very short time (15-20 mins) and despite charging it on a reconditioning cycle and testing the battery on my ctek charger it was in need of replacement.

I put in a new battery (yuasa 72a/h as original) and everything came back to life, except interior and boot space lighting which are both dead

I was careful to connect battery neg last as you should with the new battery.

I have pulled fuse 104 to check it and I see that it is fine and tested the bulbs in case they were all blown but cannot see what else could stop the interior lights.

could the battery saver relay have failed?

 

 

 

 

 

Tried to make sense of the wiring schematic but bit tricky using my phone. 
 

Do you have footwell lamps? If they’re out as well then maybe the battery saver relay. Same for the glovebox lamp.
 

Just not sure if this is a physical relay or internal to the BCM. If it’s a proper relay you could try swapping it with another relay of same type or try shorting the connection for the contacts - one being the output side of F104 the other being pin 7 of C98 or these below

 

1 hour ago, IanCollis said:

Battery voltage was down to 5v after a very short time (15-20 mins) and despite charging it on a reconditioning cycle and testing the battery on my ctek charger it was in need of replacement.

Keep this bookmarked for future reference. A 'below' battery voltage may still be salvageable, new 'smart' meters are too clever for their own good!

 

15 hours ago, StephenFord said:

'smart' meters are too clever for their own good

I presume you meant 'smart' chargers. Freudian slip?

Don't get me started about 'smart' meters . . . . or 'smart' anything for that matter

Thanks for the suggestions. It’s all fixed but the fault was actually down to me not plugging things back in properly.