draining battery

ok. got myself completely confused over this. cars been fine for ages but the battery will just not hold its charge while the car is sitting for a period of time now. i'm pretty sure the alternator is fine as i have no problems with starting the car after going for a drive even with full beams on and stereo. there just seems to be something taking power while its sitting even though im pretty sure everything is off. the only things taking any power will be a single LED from the immoboliser and the clock on the dash. i've tried 2 different batteries so i dont think that it is a battery problem. any enlightenment would be greatful or suggestions.

im sure i've forgot a few things shall add when i remember.

cheers for your time.

How old is the battery, the first time it went low was there a valid reason.

It could be the fact the battery has just given up and you need a new one.

first time the battery went was due to me leaving the lights on. i then stuck a new battery in (from my focus) and charged the old 1 overnight. and it was fine then put the old battery back in for a bit and all was well with normal driving then it lost its charge completely over night being in a hurry that morning i put the focus battery in again and drove the car for 20 mins then left it sitting all day and came back to that battery being flat.

Make sure the battery has a charge on it, doe not have to be completly charged and remove the negative terminal. Place a multi-meter or test light between the cable you removed and the battery post. POS of the light or meter touching the disconnected cable end and the NEG of the meter or light to the battery post. With everything off and the ignition off the meter or light should show no current running through them. Now the clock will draw a little for a few seconds at a time, but there should be no constant draw. I cant say about the immobilizer, maybe you'll have to de-activate that some how.

If there is something drawing power from the battery, start removing fusses. Remove them one at a time watching the light/meter. When the current stops flowing after removing a fuse, find out what that fuse covers and you should find the problem.

thing is with a battery that has put a few years under its belt, draining it can then just be the straw that breaks its back. So would consider at least getting it tested, most places will do that for you free.

 but if your confident its not the battery then as advised start removing fuses while measuring the draw. To see what is the cause.

I did the same left my lights on, drained the battery and then after that point when ever left over night just wouldnt have enough oumph to turn the engine and start it. New battery fixed that for me.

just pretty sure its not the battery seeing as it drained the other battery aswell to completely dead in a day.

This sounds familiar to what happened with mine, first thing I would say is although your say you think the alternator is fine, double check that when the engine is running you are getting the correct voltage (should be as near to 14v as poss think mine was 13.7) between the alternator + and the battery -. although you say the car starts fine after you have driven it with full beams on, the alternator may just be keeping the battery at its current level and not actually charging it. Hence why overnight it is draining away as there is nothing to maintain it.
What make of batterys have u been trying? I had problems with the Halfords ones…..replaced with a Varta and will never buy anything else but from now on.

the original battery is a bosch 1 thats pretty big and the other i've no idea of the make but had it in the focus for a few years and no trouble whatsoever.

Happened to me - I guessed it was the battery was I didn't know the history, changed it for a new one, but then the new one was faulty…. replaced that and now its great!