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Parasitic Battery Drain

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My Boxster is a MY03 2.7 with Tiptronic transmission. It has been owned by me since new and, up until recently, has been largely trouble-free. The car is as delivered from the factory, without any modifications or electrical gadgets added.

The problem is a significant battery drain, which has arisen totally out of the blue.

With a fully charged battery, the starter is sluggish. After a short drive, it only just manages to fire what is then a hot engine. My CTEK charger does not indicate any charging problem and it goes through the normal charging sequence.

When the battery is fully charged, it is showing 13.1 V. At idle speed, it shows 14.0 V. This idle speed reading drops to 13.80 V after a short drive.

I've pulled the fuses and B1 (instrument cluster, Tiptronic, diagnosis, PSM button) and D8 (radio) are the culprits.

With both fuses in place, the multimeter is showing a current of 1.14 A. Pulling the B1 fuse drops this down to 0.54 A. Then pulling the D8 radio fuse, the reading drops to .08 A, which I believe is then more or less normal.

Nothing has been touched on the car that might have given rise to this problem.

There are clearly some very knowledgeable people who contribute to this excellent forum. I'm hoping that one or more of you can point me in the right direction to hopefully solve the problem.

I have a Durametric cable, but no laptop to hook it up to. This can be arranged if any readings are needed.

Thank you.


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First of all, your car should have a “normal” parasitic draw of 40-60 mA with everything off and the key out of the ignition (a common testing mistake), and you are obviously way over that. That said, you should also note that some systems stay active for a short period after the car is first switched off, so you sometimes need to let it sit for a short period before testing. Try letting the car sit for a bit, key out, and try the current draw test (ammeter between the disconnected + battery cable).

I am also circumspect about the alternator voltage, 13.8V seems a bit low. Could just be a voltage regulator on the way out, but could also be corrosion on the battery cable ends, an easy DIY fix. You may want to have both the battery load tested and the alternator tested (in the US, most auto parts stores will do this for free). If the voltage regulator is going bad, you can buy a replacement from any VW or Mercedes dealership; they use the same part number and are much cheaper than Porsche for the same part.

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It takes about an hour for the drain on the battery to reach its low point. There is a table here that shows the progression from about a 900/1000s AMP draw down to 30/1000s for the early 986s. JFP may well have a more accurate figure for later years but notice the progression in the table as sensors and security systems go into deep standby mode where they check less often.

Is 13.8 really low? I've seen other posts which cite 14.4 but more often or not the posts say that 13.8 is about right. At what RPM are you testing?

Of interest is the voltage after 4 hours. Fully charge it, disconnect it and let it sit. Then test it. Lets see if it exceeds 12 volts then with no draw.

You don't say how old the battery is or if you have ever done any maintenance on it. Given where your profile says you live ... the heat can be as tough on a battery as cold.

If the battery is one that has any possibility of being opened to examine the acid/water level I'd look there first and use distilled water to fill it if low. And I'd clean the terminals and the clamps and tighten them down. I'd do the load test and the charging voltage test which require access to a special tool (about $100 US) but any mechanics shop should have one, and any place that sells batteries would too, I'd think.

My bet is the battery is bad. You don't need a special Porsche-branded one. In the US there are dozens that work just fine. Just pay attention to the size and the CCA ratings.

You may lose the setttings on the '03's windows when you swap the battery but the owner's manual has an easy no tools proceedure you can do to re-establish them. You have a MOST Radio so no need for radio codes.

Good luck.

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Thanks very much both of you.

When taking the readings, the key was out but I didn't wait the 2 hours or so for the switch from standby mode to shut down. For the voltage output test, the engine was at idle speed.

The battery terminals are clean and the battery itself (MOLL) is about two and a half years old. I regularly check the electrolyte level and regularly hook up the CTEK when the car is not in use.

As suggested, I'll recharge the battery, let it sit for 4 hours and rake readings again. A load test will then be the next step.

I wonder if an alternator regulator replacement is a job within the capabilities of a moderately mechanically adept person? In the past I've changed the window regulator on the car myself without too much hassle.


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Changing out the voltage regulator has been written up a couple of times, it is a simple DIY project. The regulator is inside the back of the alternator. In the US, most auto parts stores will also happily test the alternator out of the car for free as well.

For a system in good shape, we typically look for idle voltages of 14V or slightly higher. When the regulator or diodes fade, we usually see voltages in the mid 13's or lower, which is starting to get weak when you consider that there is no real current draw (head lights, stereo system, etc.) on the alternator during the test. Often, just switching on a couple of these systems will pull the voltage down further, which is not good.

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Listen to JFP, he runs a P-car shop. He knows.

What it comes down to is battery or alternator or connections. A simple test is the load test on the battery after an external charge. If it passes, then it isn't getting charged sufficiently by the car's charging system (alternator/regulator). If it fails, then QED.

Here is the sort of device that does the load test. JFP pointed me to one several years ago.

If the regulator is the culprit, it is a Bosch and commonly used on lots of German brands (VW, Benz, etc) so is widely available.

Bosch F 00M 145 350

And the sagas I read say the replacement is trivial, it is getting the alternator in and out (behind the panel behind the seats) that is a bear.

Edited by mikefocke
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Thanks once again.

The battery was fully charged yesterday and showed 13.2V. After 4 hours of standing (disconnected / in the icy cold), the reading showed 12.7V.

I'm still curious as to why the radio has something to do with this. Having checked the current draw with the radio fuse pulled and everything 'shut down/key out', it's below 100 mA.

I haven't started the car yet but shall be doing today. I'll start searching after the weekend for a load tester as I'm sure it will come in handy in the future.

Thanks for the Bosch regulator part number.

(I need to change my profile. I lived in Bahrain up until 18 months ago, but now live in Germany)


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Well, another day. Having left the battery disconnected overnight (in sub-zero conditions), the voltage had dropped from 12.7V to 12.55V.

The starter was very, very laboured but the car eventually fired.

At slow idle, the reading was 14.4V. However, over the space of 10 minutes of slow idle, the voltage had dropped 0.3V to 14.1V. At 1500 rpm, the voltage remained at 14.1V. With the headlights and heater then turned on, the voltage remained constant at 14.1V.

The Actron load tester is not available here in Europe. I have instead found one from Bosch (BAT 110) which has additional features to check charging and starter systems.

As the car is under 6 inches of snow at the moment, with more forecasted over the next days, this project will have to be put on hold.

Your guidance has been very much appreciated, and I shall let you know the outcome for interest sake.


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As Mike mentions in the link in his post, you need to leave it at least 61 minutes with the car locked before you can take a true current drain reading. The manual suggests to leave the front lid open, and activate the catch with a screwdriver. Then lock the car, and disconnect the battery to take the readings. (If you have a siren, best disconnect that first!)

There is a useful table in the workshop manual which shows drains from individual components:


On the subject of radios, I recently helped out with an '03 Boxster which was flattening the battery in a day or so. We found (eventually) that the CDR23 was drawing about 0.6 amps continuously. It was actually warm to the touch when we extracted it.

Having said that, the Voltage Regulator pack on the alternator is the most common cause, as JFP and Mike have said. Part numbers are:

Bosch Part number: F 00M 145 225 or F 00M 145 350.

Porsche Part No:

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I'm still curious as to why the radio has something to do with this. Having checked the current draw with the radio fuse pulled and everything 'shut down/key out', it's below 100 mA.

Simple: The radio draws current to retain your personal settings (saved stations, etc.).

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Thank you, Richard.

0.6 amps is about what mine is drawing. I'm not electrically-minded at all. The radio is rarely used so, if I just pull the fuse, am I correct in assuming that that would eliminate 0.6 amps of drain?

The car was locked, with the bonnet resting on the latch when taking the reading. I read your post re the CDR23. It also made reference to a 'Diagnosis' function. The other circuit on my car that seems to be drawing current is B1 (instrument cluster, Tiptronic, diagnosis, PSM button). I wonder if there are similarities.

Once the weather improves, I'll be doing a load test on the battery and have the alternator checked out.


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As the table shows, the radio should only draw 0.001 A (after 61 minutes, to be sure). If yours draws 0.6 A, it is way too much. I would pull the fuse D8 for the time being, and see what happens. It won't do any harm, apart from maybe losing your presets, as JFP says.

As for the cluster, were you measuring the drain after 61 minutes? If not, you need to test it again after that amount of time.

It would be worth doing the test, and watching the draw go down as per the table. Once the 61 minutes has elapsed, if you are getting a much higher drain, that is the time to start pulling fuses, but keep the car locked when you do it. Maybe do it with the windows and top down, double-lock the car to deactivate the motion sensor, and you can then reach inside and pull fuses. Not easy to do out in the snow - find yourself a warm garage!

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Yes, the cluster was measured when everything was shutdown. However, the battery was fully charged last night (with the radio fuse already pulled, the car locked apart from the bonnet and the battery disconnected) and still the starter was very, very sluggish when I came to reconnect the battery and start the car this morning. All very perplexing but, without winter tyres, I can't use it anyway, or get to a garage for a load test.

I've got the warm garage, but it's a case of Sod's law. No sooner than the winding mechanism went on the 36-year-old Mercedes that I inherited from my father (window now fully down and garaged by necessity) that the snow arrived, along with the Boxster problem!


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Having switched the batteries, as Mike suggested, the problem appears to be the Porsche Moll battery. The starter cranks strongly and the engine fires immediately. Hopefully the problem has been solved as I wasn't looking forward to pulling the alternator out.

I'm not at all impressed with the Moll battery and shall go with a Bosch S5 as its replacement. The battery in the Mercedes is an 8-year-old Unipart Samson that my father put in. Over a period of 5 years it sat in the car (disconnected) and was only charged once a year whilst I was back in the UK on leave.

It would appear that I have wasted everybody's time. My apologies.


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