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Mystery battery discharge and solution

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Just over a week ago I got a call from my wife that the Boxster was completely dead and the key was stuck in the ignition. The car had been sitting for 5 days, but the battery is fairly new, just over a year old and I use the CTEK battery maintainer during winter storage. The car sat for 5 days over Memorial Day weekend in May and there were absolutely no issues. I tried using the procedure in the manual to open the front compartment lid with no luck. I did not use jumper cables, but a battery charger. I called the dealer and got a recommended towing company.

The next day the tow driver showed up and suggested we try to jump the car. He used a portable battery pack about the size of a briefcase. He plugged the power pack into the cigarette lighter socket and opened the hood using the switch on the floor by the driver's door. He then proceeded to jump the car with the power pack connected directly to the battery.

I drove the car to the dealer without turning the engine off and left it for them to look at. Later that afternoon I got a call from the service department. They could find nothing wrong with the car and completely tested the battery.

I spent the last week using a digital voltmeter looking for any sign of a discharge after the car sat overnight with no luck.

I have concluded there is an intermittent fault in either the car or the battery.

As a solution to this electrical gremlin, I just ordered a Beatit 18000 mAh portable jump starter from Amazon and a cigarette lighter plug adapter. Cost total $75. Welcome to the 21st century. Jumper cables are evidently obsolete.
A couple of suggestions/questions:

Did you check whether the instruments turned off after removing the key? If they did stay on its most likely a problem with the ignition switch.

Even if it wasn't the case I would install a new ignition switch. It is one of the known electrical fault causes and its quite inexpensive and relatively easy to replace.
I did not remove the key after the car started. I will keep an eye on the behavior after the car shuts down in the future to make sure there are no instruments lit on the dash. Thanks.
... in the manner you describe.
One was a bad rear trunk latch that allowed the trunk light to stay on.
Test both trunk lights by opening the lid and then using a screwdriver, close the latch and watch for the light to go out.
Happy Boxstering,

Pedro Bonilla 1998 Boxster 986 - 284,000+ miles: http://www.PedrosGarage.com
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

Racecar spelled backwards is Racecar
"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"
"If you wait, all that happens is that you get older"... Mario Andretti
"Being second is to be the first of the ones who lose" ... Ayrton Senna
and that wasn't it. Interestingly, the front trunk light goes out by itself after a couple of hours. That makes the battery maintainer more efficient since the front trunk lid is open to access the battery. That's not to say the rear trunk light isn't the problem, just that it happens to be working today.
No mystery. Do not leave the key in the ignition. Turn the ignition off and remove the key and it is best to then lock the car. This puts the car's electronics into their lowest power state the soonest. The security system consumes a bit of power but because it is active when the car is otherwises inactive it is designed to use little power. I've left my Boxster parked for up to just short of 4 weeks with no issues.

When you use the car be sure you use the car long enough the battery gets recharged to replenish the charge the battery loses over time and that it loses when you start the engine. Based on the volt meter in my Turbo it can take 10 minutes in mild weather (with no electrical loads like lights or AC on) to 15 or even more minutes of engine run time before the battery appears to be topped up.
Just to be clear, my wife inserted the key before she realized anything was amiss. My plan right now is to keep an eye on things to see what I can learn. Pedro's comment was very helpful. He has seen the problem on more than one car caused by a faulty rear trunk lid latch that did not shut off the light. On a scale of 1 to 10 I place the importance of a rear trunk light at about 1. I am tempted to remove the bulb from the rear trunk if I can't discover something else as the cause and the problem returns.
Sorry. I misread your post. You can disconnect the rear trunk lid light just to be sure. i've never heard of this being the cause of a dead battery but I know the techs pop the front trunk lid light out and disconnect it first thing many times when I bring my cars in for service.

Be sure you have no parasitic electrical loads in the car. I'm thinking of a phone charger plugged in. I "killed" my Boxster's original battery by leaving the car parked a week with an OBD2 code reader plugged in. Went off and forgot about it.

Another time prior to that my VW Golf was manifesting signs of a battery drain. Turned out someone -- maybe me in "exploring" the new car -- turned on a small overhead light in the back seat. I finally spotted it one night. Turned it off and the battery drain symptoms went away.

Sometimes the problem is with the ignition switch. Switches that have to bear the load of a heavy key chain appear to fail most often, but just time and normal wear and tear can take their toll.

The car has no signs of water in the cabin? Door bottoms are dry?
The car was parked in my garage, so water was not an issue. Most of the time the car sits for less than a few hours unless it's parked at home in the garage. My plan right now is to park the car with the hood unlatched in my garage and see if the problem appears again. The front hood light turns off automatically after a couple of hours so that will not be the cause of any discharge. I can check the battery condition with my voltmeter.
I thought I would try out my new emergency jump starter by discharging the battery by leaving the rear trunk unlatched. I found that the rear trunk light turns off automatically after a few hours just as the front trunk light does. There must have been an upgrade from earlier Boxsters to the 987.2.
I would
Boxsterra - 11 months ago
measure current across each of the fuses. That will tell you which circuits are consuming power, assuming the drain isn't intermittent.

That said, since the key got stuck in the ignition the drain is 95% likely to be caused by a bad (electrical portion of the) ignition switch. You can get a replacement part for about $8.
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