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Old 06-23-2011, 12:20 PM   #1
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Can alternator diodes "leak" and discharge the battery?

My battery went dead after the car (2000 S) sat for a couple of days. The previous owner had put in a Walmart battery and it was 5 years old. It didn't seem to respond to the charger the way I expected so I bought a new generic battery at O'Reily's (sp) and everything was good. Then after a month the battery was really low when I went to start the car. I charged it up, which took a really long time on a 6 amp charger and the car worked fine. The starter really cranked fast. After about a week the fast cranking starter was still starting fine but not cranking as fast.

I bought a voltmeter that plugs into the cigarette lighter and it said about 12.3 volts with the car shut off. That was after pulling the passing lamp switch for about 10 seconds and then releasing it. When I started the car the voltmeter showed almost exactly 14 volts. During a 30 minute drive it went down to about 13.8 and then 13.7 after I turned on the headlights. On other cars I'm used to seeing above 14 whenever the motor is running. Still it looks like it should be enough to keep it charged.

So I'm unsure about the next course of action. The guy at O'Reiley's said they have a tester and I guess I should do that. But I was reading another battery thread and there was talk about "diode response" and I don't know what that means.

Also, what should an amp meter read if I unhook the cable at the battery post and run it through the amp meter with the car off? Seems like there's a lot of stuff still running on these cars with the switch off, like the evaporative emissions check. I notice the hood & trunk lights will shut off after 2 hours so maybe I'd have to wait that long to see a true "shutdown" condition if the other car components follow the 2 hour rule.

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Old 06-23-2011, 12:52 PM   #2
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The car should only be pulling 40-60 mAh when everything is off; if it is pulling more, you have a parasitic drain.

Your alternator should run in the low to mid 14 volt range at an idle, your sounds like the voltage regulator is on the way out. It is a replaceable part, about $40-50.
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Old 06-23-2011, 02:26 PM   #3
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If you have a manual for the car, definitely look in there for some info along with what I'll share.

The car does stay very powered compared to more conventional cars, with a lot of systems staying active even while the car sits not running. For example, I'm sure you've found that after about 5 days your keyless entry will be deactivated and you will need to key into the car. So things like that will deactivate over time, but in the meantime they do run the battery down. Power windows stay active for up to two hours in case you have a passenger sitting in there while the car's off, as well as the seat adjustments if you have electric ones. The trunk lights should shut off after about 30 minutes, so if they're still on beyond that I'd find that a little abnormal.

Anyways, leaving the car for a month, without hooking it up to a maintainer, and locked with the alarm active will definitely run the battery down. So if you're going to leave the car for that long of a period, put it on a maintainer (not a charger) at least overnight after maybe two weeks at the most just to keep the battery strong. If the battery goes through several deep cycles of charge and discharge it really kills the lifespan, which is why those $200 Porsche batteries sometimes don't even make it a year before getting toasted when someone puts the car away for half the year and doesn't touch it again until that other half of the year comes back around.

If the car's driven regularly enough and there's no major electrical issues to speak of you should never have a problem with the battery getting run down. I just replaced my alternator and battery recently because the alternator failed and the battery had deep cycled so many times that it could no longer take a charge even with the good alternator. Now the car starts strong every time, even after sitting for a week, so maybe you do have an issue, but probably, and hopefully, not one quite as expensive as mine was.
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Old 06-23-2011, 06:01 PM   #4
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i had the same issue. I think all boxsters do.
i used this to stop the battery from going dead no matter how long it sits.
http://www.bldsafety.com/Battery-Protector-Stnd-W-Led-p/s-1105034.htm

easy fix
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Old 06-23-2011, 09:10 PM   #5
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Quote:
Originally Posted by J Tinsby
... It would be great to use a real meter and not a digital one, if you can find one today... Good luck!
No it wouldn't! An analog gauge should NEVER be used on microelectronic circuitry because it draws too much current to actuate the gauge and can damage the circuitry which wasn't designed for that amount of voltage.

Wet cell battery technology is well designed and understood. Each cell is specifically designed to produce 2.1-2.2 VDC. You have 6 cells in series, so the total voltage on a healthy battery s/b 12.6-13.2 VDC. Any more/less, and you have a less than perfect battery. Anything under 12 VDC s/b replaced.

To use a DMM, disconnect the battery and measure the voltage across the battery terminals, looking for that 12.6-13.2 VDC discussed above, in any case record the value.

Now, reconnect the terminals and measure again. The difference is the draw by the car's ancillary systems. If greater than 1 VDC, you have an issue.

Now start the car and let it idle 1 minute. Check across the battery terminals again and you should now see 13.5-14.5 VDC (that's the rating on the Bosch alternator). If not, the weakest link in the charging system is the diode pac, so this is likely the culprit.

The up side is that it is both cheap and easy to replace with the alternator in situ because it's external.

Cheers!
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Old 06-24-2011, 04:05 AM   #6
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Tell me more about replacing the voltage regulator with the alternator in place. I'm pretty sure I'd want to do this since the part is pretty cheap and especially if you're saying it's not difficult. Do you have a link to a DIY? What part exactly am I looking for and where can I get it?
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Old 06-24-2011, 05:31 AM   #7
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Update - I see Pelican has the VR available for order so I know where I can get the part but I'm not sure about being able to replace it with the alternator in the car. i didn't find any DIY that speaks to that. They all say you have to take it out first.

Another update - Just got back from O'Reillys and had them do a battery and charging system test. They said everything was OK. Charging system voltage at 13.6, which seems a little low to me. They could be right though if there's a parasitic drain with the car off. I have a digital multimeter with a miliamp range so I might be able to check the basic system if I can find a way to clamp the test lead probes to the cable and terminal. My meter doesn't have alligator clamps, just sharp probes.

I'm not too sure about how to test the car since the interior lights will be on when I open the door and I don't see a way to disable them. They'll go off after a long time but it will take forever if I have to wait 2 hours each time I pull a fuse.
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Old 06-24-2011, 05:39 AM   #8
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The alternator has to come out of the car to replace the voltage regulator.
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Old 06-24-2011, 04:54 PM   #9
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Agreed

Quote:
Originally Posted by Lil bastard
No it wouldn't! An analog gauge should NEVER be used on microelectronic circuitry because it draws too much current to actuate the gauge and can damage the circuitry which wasn't designed for that amount of voltage.
Cheers!
I wouldn't put an analog voltmeter in a circuit smc's in it. But the gauges on the dash are analog and magically they don't hurt anything. Putting the meter in line like an ammeter shouldn't effect anything, at best it would be a floating ground connection, you might lose the meter in a worst case that's all.

No worries looks like he is replacing the regulator anyway.

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Old 06-25-2011, 05:57 AM   #10
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I've ordered the new VR and sure hope it fixes the car. Yesterday with the AC on the accessory voltmeter I bought occassionally showed less than 12v at idle.

About the gauges - I don't know about Porsche specifically but most modern cars don't operate the gauges from directly connected sensors. The gauges are controlled by the ECU, which decides what position to point the needle based on the sensor reading but the ECU is programmed to avoid alarming the driver with mildly high readings (temp) or low readings (oil & volts). The ECU puts the needles in pleasantly reassuring positions unless the underlying system gets really bad.
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Old 06-25-2011, 07:57 AM   #11
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One point worth considering is that the dash displays in these cars are notoriously inaccurate to begin with. The coolant temp typically displays near 180F, while the DME says the actual temp is more like 210F; this same display is also non-linear in accuracy (it is further off at some temperatures than others).
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Old 06-26-2011, 08:51 AM   #12
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I haven't received the voltage regulator yet but I did perform a parasitic battery drain test today. The car had been sitting for many hours and I was careful not to open a door or do anything to "wake it up". I saw about 17-22 milliamps when I disconnected the battery cable and hooked the battery post to one side of the meter and the cable end to the other side of the meter. So I'm assuming I can rule out parasitic losses.
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Old 06-26-2011, 08:59 AM   #13
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That is correct; normal draw can be as high as 40-60, so if anything you are on the low side.

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