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almost new battery discharging

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1999, 996  140k

I think I have a problem with my battery discharging so much that the car wont start if I leave it sitting for more than 2 days.  I cant find any obvious thing that might be draining it, ie, interior lights etc.


Is there a procedure to detecting weather the alternator is charging or something is draining?

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The voltmeter in the dash should move to about 14v after cold start then it may taper off to 13.5v or so as you drive the car. It should be between 13.5v to 14.5v whenever the car is running.


You can take the battery to any auto parts store for testing.


For battery drain, open the hood and trip the lock with a screwdriver to trick the car to think the frunk lid is locked. Then close all the doors and arm the car and wait for at least 20min. Use a dc clamp meter on one of the battery cables to check for drain. It should be like 30mA or less.




Edited by Ahsai
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I would first start with a system voltage test.  With the engine off, your battery should show around 12.2 Volts, running that should be in the 13.5-14V range.  Next I would look at parasitic draw on the battery.  To do this, put your multimeter between the positive cable and the posititve battery cable, set on amperage (low range).  After the car has sat for about an hour, it should read 40-60 mA; any more than that and something is draining the battery.   You can figure out what circuit is involved by pulling the fuses one at a time until it drops into range.  The last circuit pulled is your suspect.

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Tricky business...  do you have a  multimeter?  You really need one to chase down the drain.  If you have or by now borrow a multimeter you can disconnect the positive side of the battery and set the multimeter to DC Amps.  Put one lead from the multimeter on the positive battery post and the other lead on the removed battery cable.  Also, you'll need to have the multimeter leads plugged into the correct plug inserts as well as have the multimeter dial set on DC A 10A scale.


If the above shows more than a ma draw then you can start pulling fuses.  No need to pull head light fuses since you can tell if they're on or not.  It's the alarm, radio, interior lights that can do this.  Remember after you turn off the key it will take some time for the computer, interior lights to settle down.  I've waited for an hour before I got a .06ma draw.  With the interior lights just after closing the door, I showed .55A, then in time, it settled down to .06ma.  Also, the alternator can leak back through the diodes so you can disconnect the lead(s) to the alternator.


I found out recently that if you reach in and turn the key on to check mileage, what ever, then turn the key back off without opening and closing the door the interior lights stay on.  I've never left them on long enough to find out if they time out.  Always remove the key from the ignition.  


I'm no expert on this but have stumbled through with a similar problem...  Lastly, have you had your battery load tested?


Good luck,



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Does your car have the smokers package, meaning do you have a cigarette lighter?

If so, check the lighter plug.  It may be that the lighter is engaged, not working, but still draining the battery.

Just a thought...probably not correct but worth a check.

I use my lighter plug to run a GPS, or to backfeed a trickle charger to the battery when away and not driving for more than a week.

Good luck.  Post back your findings, please.

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From what others have found - the most simple test is to get the car stone cold (after overnight sitting), open the engine lid and place your hand on the alternator.  If it is warm, or hot, the diodes in the alternator voltage regulator are bad.  This has been known to cause significant voltage drain in a short period of time.  The voltage regulator can be replaced without a whole new alternator.

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Thanks to all of you for several easy suggestions as to how to track this down.  I'll post up as the problem reveals itself.


Right now the battery is disconnected and I'm changing the water pump and thrermostat.  It wont be until after I complete this task that I start tracking down the battery drain.  I'm waiting on a 160* thermostat in the mail.

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