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Battery discharge overnight


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Sorry if this has been covered before. Had to replace battery because it kept going flat. Got new battery and all was fine for about 10 days but now it too goes flat overnight. Removed boot light connector, checked glove light goes off when closed and left it unlocked last night. STILL the battery goes flat. Can't keep asking my neighbour for jump start so any ideas?

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oh dennis I know the about  this but my mind is blank just now , cant think wer I seen the remedy ect. is the altenator ok sorry spellings terr. but you get the picture, kennys prob the best man as hes an elec engine r sorry bud

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Hi Dennis,


I would check that the alternator is charging the battery first.  I don't think it can be the battery.


Ir is a pity that they don't have the old style dial on the dashboard anymore.





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If you have a multimeter, you could measure the current being drawn from the battery with everything switched off and ignition off. If there is excessive current flow, it would be a case of elimination to find out the culprit.



However, I'd agree with Peter and check the alternator output first.


With the engine running, a voltage test across the battery terminals should give approximately 14,8 volts.

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Thanks guys. After the last jump start I measured the battery voltage while the engine was idling and it read 14.74V so I doubt its the alternator. Bit stuck at this point. Got someone coming this morning to do a jump start again. This time I will measure the discharge current after switching off and see what level it is.

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I think I found the problem. Some weeks ago when I replaced the DCCV valve, I noticed that electrical control of the new valve was not working on the driver's side (the old defective valve had damaged the CCM). I have not got round to extracting the CCM to repair it and install an overcurrent protection circuit as yet. So in the mean time (to switch off heating and provide A/C) I hooked up a temporary circuit from the valve control pin to ground via a fuse and a switch (the latter inside and accessible from driver's seat). This enabled some control over heating/cooling. What I didn't realise is that the +12v feed to the valve is permanent i.e. not via the ignition but straight from the battery (via a fuse) ALL the time (nasty design). So, when leaving my temporary arrangement switched for A/C and exiting the car, about 0.75A of current was being drawn from the battery when sitting on the driveway!. Hence, over a night or two it would drain the battery (and I bought a new battery for no reason arrrrgh!). Until I fix the CCM electronics I will just have to remember to switch to heat before leaving the car.

Moral of the story, don't assume any logical design with the Jag which would have ensured unnecessary circuits when parked were switched off via the ignition key.

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