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lots of things,how long does it take to drain,overnight or days or weeks?
 

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You need an ammeter to measure the current drain from battery. After the car engine has got to mormal running temperature (This will prevent glow plugs being supplied when ammeter is connected and current check is done) Disconnect battery positive and connect ammeter in series (Ideally it would be a multi range ammeter where you can start with the capability of reading a high current (Guess at 10AMPS), then switch ranges on the ammeter to read the lower current drawn as systems go to "sleep".

Turn OFF all electrical items, Lights/Fans/Radio/Heated Seats/Windows etc then turn ignition to "RUN" position i.e. warning lights lit on dash. DO NOT TURN THE ENGINE OVER OR START THE CAR OR YOU LIKELY BLOW THE AMMETER UP AS THE STARTER MOTOR TURNS.
Turn ignition OFF and note current drawn from battery when ignition is initially turned off, this should be about 3.5 Amps. Monitor the current over the next few minutes and as devices go to sleep the current should slowly fall to about 0.03 Amps and remain at this constant level. This standing current of 0.03 Amps will half flatten a fully charged 95A/Hr battery in about 2 Months. If the standing current is much greater than 0.03 Amps (Which is 30 milliamps) you would have to pull fuses one at a time until you find out the circuit that is drawing excessive current.
If the standing current remains at 0.03 Amps while ignition is turned Off you have a duff battery. (You could do with a "Max/Min" current meter which would allow you to leave it connected for a long time while it records the Max/Min currents reached. You could come back after a few hours and see the Maximum current drawn at any single point in time)

Hope this helps.
If you can get to Doncaster I could do the checks for you.
 

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What time delay have you got set for the items which would normally end when the ignition is turned off? Do you have headlights turning on when you unlock the doors, and/or headlights remaining on after locking the doors? They're all adjustable, and can cause the battery to drain, especially if it's getting on a bit age wise.
 

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You can just use a bulb and a Bit of wire, Take off the positive side of the battery clamp. Wire a bulb inline between the now disconnected + terminal and the battery. And again just take out fuses one at a time till the bulb goes out. Then you'll know where the problem lies.
Good luck!
 

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Monitor the current over the next few minutes and as devices go to sleep the current should slowly fall to about 0.03 Amps and remain at this constant level. This standing current of 0.03 Amps will half flatten a fully charged 95A/Hr battery in about 2 Months.
Hi, I am also having a problem with my new battery going dead.

I monitored the current as the car when to sleep from 4-5 amps down to 0.3 amps (300 milliamps).

So, I just wanted to verify your value of "0.03" which is 30 milliamps, which is suprisingly low. For relevance, the LED that flashes in the dash to indicate that the alarm is armed requires about 10-15ma just to light up. And if the total correct sleep current is in fact just 30 ma, I would expect the amp meter to jump up and down from 15 to 30 milli amps as the alarm led blinks on and off.

So, it there any change you might misplaced the decimal position by one digit and the idle current is 0.3 (300ma) and not 0.03 (30ma)?

Update:

I can confirm that my 2012 300c will draw 50-60ma when:

1) close all doors and hood/trunk
2) make sure no lights dome/glovebox are on
3) lock car with fob
4) move the keyfob more then 100' away from car
5) wait full 5 minutes (it will first be at 4amps, then 2 amps within a minute, then 1 amp, then .75amp, then .5amp for a while, then .05amp (50ma) at 5 minutes).

So, the 30ma is in the same range of the 50-60 I observed and maybe the increase is due to an aftermarket alarm.
 
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