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Battery Dies after 1 week


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I've been trying to isolate the cause of this w/o much issue. My battery has been dying, but not totally.

2001 Boxster, manual with 37K miles. Car runs great. But if I don't drive it, the battery dies in 7 days. Guaranteed.

If I drive the car daily, or every 3 or 4 days, it's perfect. Drove the car last Tuesday to work (45 minutes one way) and back. Nice highway ride. Went to drive tonight (7 days later), and it BARELY started. The battery usually isn't "dead" though. I had door lights, dashboard lights up, etc. Even was able to let the computer check the oil level before I lit it off, and the fan on the AC ran as I didn't shut off the climate control last time. I was lucky and it started, but it was SLOW, and had the fires not lit when they did, I'd have been hearing the 'ol battery-is-dead clickety-click within another second or two I think. It's like the battery has enough voltage, but maybe not under a high-current draw like when starting. But it has enough current to light up the dash lights and the interior lights.

Last time it died, it was really dead and I didn't have lights at all. That was the only time it was that dead. Normally, I at least have dash lights and things look normal, until I try to start the car. If I put the battery on a charger, it'll top off and charge, and as long as I drive, it's fine.

I keep the car in the garage, top is up, and since it's in the garage, I don't lock it, so the internal alarms aren't on or anything. The only thing I can think of, is I do leave my radar detector plugged in to the 12Vdc. I can't see a tiny little radar detector pulling the battery down that hard.

Battery is a Duralast that was with the car when I bought it in April of this year, but the sticker on the battery says 11/08, so it's pretty new.

Other than taking the battery back to AutoZone to check it, (maybe I got a bad cell, and it's causing a slow discharge on the battery???) anything I can look at? Is this normal for a Boxster? It sure doesn't seem to be quite right.

I know driving fixes it, (which isn't a bad thing) but sometimes that just isn't an option in Wisconsin. :) Pretty sure alternator is ok, because even though it was nearly dead tonight, once the engine caught, I drove around for about 45 minutes, and it lights off fine, w/o issue, so the alternator is charging.

Very perplexed. But I still love my Boxster. :)

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Try locking it and disconnecting the radar detector. Locking the car should bring it into a power saving state.

Does your alarm horn work? The alarm horn, located by the battery, has a constant current to it and are know to go bad and drain batteries...

A.

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Minimum power consumption be achieved with the doors locked by the remote control, remove the detector power lead also during long term parking. If this arrangements present no solution, let the battery check and the car on power loss.

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If you lock the car it will shut down the electronics after a time to conserve energy. You then must use key to enter.

I would not think the radar detector would pull that much either. But maybe your detector pulls more amps than mine did. My detector would not even auto start because it said it did not have enough power. I later rewired detector to switched power.

You might have a leak in the system, a multimeter and a lot of patience will help locate it. The other possibility is that you new battery is not up to snuff. I'd get it load tested at two different sources.

The battery maintainer, sometimes called a "trickle charger" even in enthusiast magazines, is a good investment. I keep one on my car the minute I pull into the garage.

Be aware that the trickle charge mode on a normal (not chipped) battery charger may constantly try to charge the battery instead of only when it is necessary.

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You need to check what is draining the battery - The car should be able to be left for weeks without a problem, only if you drive it rarely should you need to resort to conditioners.

Get a DC ampmeter - or take it to a workshop with one - this simply measures the amperage drain going from the battery, Its just a C clip that is placed over the battery lead - it measures without direct contact with the cable, Your car should never draw more than 40 milliamps when everything is turned off and locked up.

If you have a greater draw than this (measure after the interior lights have gone out, and if you have the door open - which you will need to do to find which circuit is drawing the current - simply use a screwdriver on the door lock to simulate the car is shut - ie push the part of the lock that engages with the B post catch inwards until it clicks shut)

Now with the meter in place pull one fuse at a time and check the meter readings - do this until you find which circuit is drawing the power - then you will need to determine which component on that circuit is drawing it.

Tedious - well yes a bit - but the rewards will be you will have confidence in that the battery wont be flat everytime you go to it

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The diagnosis

approach I would use is to run the car ~50 miles at speed so you know the battery is changed well.

Use a multi-meter to verify that the change is around 12 volts by measuring across the battery.

Disconnect (unplug) the radar detector.

Wait 1 week.

Start the car.

If you get a normal start, you know the problem is with the radar detector being on and the "lighter" socket being always powered on (my other cars this last isn't true...the socket turns off but not with the Porsche). So turn the detector off next time so it doesn't draw current.

If it isn't the detector, you are going to have a much more complex process to diagnose by having the battery tested...by measuring the current draw while one fuse at a time is removed to see what circuit is creating the draw.

My car starts normally after 2 weeks of sitting but I don't have any external devices hooked up.

My bet is it is the detector.

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Back when I used the lighter plug I always pulled the detector plug out slightly to cut the power to the detector.

Be glad the plug stays hot with engine off. It makes it much easier to use a battery maintainer.

I just posted on a BMW site a host of problems last week the wife's E70 X5 exacerbated because I could not use the lighter plug with the engine off. I was not near the start of that thread. BMW apparently has a big problem with alternators and/or charging systems. :angry:

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Thanks for all of the tips guys! I'll start digging. It's been raining here for a few days so it's been sitting. Interesting that locking the car will cause it to use less juice. I never knew that. I assumed leaving it unlocked would be better, as no need for the alarm. I will try a few of things things and see what I can find. Thanks!

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The diagnosis

approach I would use is to run the car ~50 miles at speed so you know the battery is changed well.

Use a multi-meter to verify that the change is around 12 volts by measuring across the battery.

Most Starting Car batteries have a fairly high reserve capacity or amp-Hour rating.... Most modern radar detector draw minimal current, but if your battery was marginal, I guess it could drain it in a week.

A multimeter should read between 12.6-12.8V an hour or 2 after u turn off the car if it is close to 100%... 12.0V would be a pretty flat battery.

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The diagnosis

approach I would use is to run the car ~50 miles at speed so you know the battery is changed well.

Use a multi-meter to verify that the change is around 12 volts by measuring across the battery.

Most Starting Car batteries have a fairly high reserve capacity or amp-Hour rating.... Most modern radar detector draw minimal current, but if your battery was marginal, I guess it could drain it in a week.

A multimeter should read between 12.6-12.8V an hour or 2 after u turn off the car if it is close to 100%... 12.0V would be a pretty flat battery.

might check that the courtesy lights inside the front and rear "trunks" are really going off

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You could put a volt meter on the battery and start pulling CB's (Sorry Fuses). When you notice that there isn't a draw with a particular circuit breaker you have found the source. Then find the fault within that circuit. Easy if you have a helper.

post-16847-1251239162_thumb.jpg

Edited by seafeye
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