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Dreaded Battery Revisited

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Hi Guys-


I am on my 5th rear engine Porsche, currently that would be a 2001 Turbo.  It is the best Porsche I have owned, as a matter of personal opinion, having driven more that 100 of these things- including a fair number of the newest 991s.  I love this 996TT, but as always, there are a few quirks.  I am on my 2nd Bosch battery- beginning yesterday.  Now it may be that the battery was bad, as my shop indicated today when I brought it in- no power- not even enough to test it.  After 2 hours of charging- it would not hold a charge and they replaced it for free.


My question is at the end of all this is, is there something, possibly stray voltage, that could cause a battery to go completely dead after 3 months of normal driving, or could this have just been a bad battery.  I purchased the car last fall with essentially a " power kill switch" in the interior, similar to what you would see in some of the Ferraris or Maserati's.  Even with the key out of the ignition- there is always power to this.


I am sure if my alternator were bad, no matter how good my battery, the car would not run- but would a bad alternator cause what should have been a perfectly good Bosch to go bad in 3 months. 


I do plan to take it in and have it looked at- just thought I would get some early advice from the real experts.


In advance- thanks.

Edited by flatsixing
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If you're going to bring it to a shop, it should check your charging system and any battery drain.


Do you remember if the voltmeter on the dash reads 13.5-14.5v all the time whenever the engine is running? If so, it's unlikely your alternator. Maybe you have a drain somewhere, especially if you have aftermarket electronics installed on the car. If the battery is overcharged or undercharged, it can deteriorate quickly.


What does the "power kill switch" do?

Edited by Ahsai
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A power kill switch does not always kill total power to the vehicle.  It is sometimes fitted with a fused bridge of 10 or 15 amps.  The reason for this is that when the kill switch is in the off position low current items, such as an alarm, cigar socket, internal lights or parking lights, will continue to work as the current passes through the bridge.  However, if an attempt to start the car is made when the kill switch is off the low current fuse will blow as it has insufficient capacity to energise the starter.


I have had a battery fail completely, but not for the last 30 years or more.  It went completely dead and would not accept a charge and it was a fairly new battery in my father's car.  So it does happen, but I have not heard of it in more recent times.


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Thanks for the replies everyone- appreciated.


Ahsai- I will confirm this- thanks.


Hilux2400, this is precise what it does, thanks for the post on that.


White987S- there really is not a week that the car is not driven, even winter when the roads are dry.  Might be a good option to consider though.


I will report back my findings!

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The kill switch as such (that it still leaves the battery connected) will not be able to address a parasitic drain. Our battery is about 70Ah in capacity. Even with the normal ~50mA drain, it only takes about 30 days to drain half of the capacity. If the drain is at 1A (e.g., a bad alternator diode that drains the battery), that's like 1.5 days to drain half of the battery.


Bottom line is still the same, the shop needs to check the battery, the charging system and any battery drain as a complete diagnostic.

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