Hi I am new to this group. I have a 2008 2.0tdci Titanium estate. I've had several problems over the years I have owned it, most I have managed to fix myself, cant afford to take to a garage. 
My problem is every now and then I come out to the car and it has had total electrical power loss overnight or when parked for a while,  the radio needs re-coding, the clock etc need re setting. but it starts up ok. Then another time I can go to it and its totally dead no dash lights, nothing.  I turn the key on and off a couple of times and it eventually the dash lights come on and then it starts.. so something is killing the power from the battery, anyone point me in the right direction of where I can start checking. ? It used to do it once in a blue moon now its getting a bit more regular. Chjeers in advance. Dave Holmes

Grounding, perhaps, if the battery was totally dead then it wouldn't crank at all. so perhaps its a short or grounding issue. you got a code reader?, 

Before doing anything, get the terminals off the battery and make sure they are cleaned up, then check the main ground strap that goes from the battery to the strut or somewhere around there, clean it up and make sure its solid connection.  Take out and reseat the connection blocks in GEM, (disconnect battery first)  after that, its going to take a lot more effort to find the cause. 

Does the central locking work when the car otherwise appears dead?

How long have you owned the car, and who owned it before you? Any strange buttons around in the cabin?

Could be a duff battery , how old is it, if it's the original it's wel due a change, assuming your talking about the car in your profile.

Thanks for the replies guys. I do have a code reader. no faults that would point to total battery failure. I will clean up the Battery terminals. Battery is fairly new. When power comes back it starts fine loads of power. What and where is the GEM to clean connection blocks??  I have owned the car for 5yrs no hidden switches, think the central locking does work.. to be honest I cant remember as most of the time its on the drive and wife seldom locks it ;o).......Cheers.

The Gem is in the passenger footwell - the "fuse box".

Make sure to take note if there appears to be any modifications to the GEM. It seems strange that you are losing permanent power to things other than just the radio, as these are normally fed from permanent live bus bars in the GEM, hence why I asked if there were any strange buttons. There are such things as "battery savers/battery guards" which will isolate the power near to the battery if the vehicle appears to be draining the battery to a point it would not start - hence causing a loss of permanent live.

OK Cheers I will give it a check out. It is weirs as it is a total loss of power, then it comes back ok???

I second giving the gem connectors a check.
It's very easy for a passenger to inadvertantly give the underside a kick and knock a plug off.

Sent from my SM-G965F (S9+)

Er ...... turned out to be a loose positive battery terminal, felt sure I had checked it before Derrrrrrr  " The Shame" ;o)

Er ...... turned out to be a loose positive battery terminal, felt sure I had checked it before Derrrrrrr  " The Shame" ;o)

🤣

Sent from my SM-G965F (S9+)