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Boxster 986 Battery discharge problem


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Dear all,

As a new member I just love this forum, you guys seem to know everything about the workings and problems with Boxsters. :renntech: I have done a search but couldn't find a definitive answer from previous posts, Here we go then

I have had my Boxster 986 2.5 manual for just overa year now and in general I just love it, however there is a problem that just keeps coming back.

I drive about 500 miles a week generally in 2 or 3 journeys , so they are quite long trips that gives the battery plenty of time to charge up. The problem is that when I don't use the car even for a short time, the battery completely discharges and I have to go through the painful process of hitching up a live lead to fuse C3 to open the front to then jump start the battery.

It is worth noting at this point point that my battery is only 6 months old and is the highest power one I could find.

I parked the car up on Friday night and by Sunday morning (3am) the alarm was going off signalling that the power was drained from the battery. :censored:

I put an electrical tester accross the battery terminals today with the engine running and it was reading 14.2 volts which I think means that it is charging up OK. I am guessing that there is something draining the battery when the car is inactive, I can't believe that the alarm system would drain it that quickly.

Can anyone tell me what the likely cause is and also how do I test the electrics, I have a tester but am no expert so I pretty much need basic instructions.

Thanks a lot again, :cheers:

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Disconnect battery (-.) Connect ammeter (multimeter on DC amps or current) between battery (-) and ground cable. Read milliamps (hopefully not amps) and post here.

If you get more than a few milliamps, begin pulling and replacing fuses, one at a time, reading any change in current drain, until you find the circuit causing the drain. The kind of drain you have is likely a bulb somewhere. Be careful working with and around the battery.

Good luck.

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2 other things come to mind as well - alternator (but seems like it might be okay) and your ignition switch. Eventhough you are taking out your key, your switch may be bad and is getting the car to 'think' the key is still in there. Ignition switch is pretty inexpensive (about $25 or so) and a known weak link in the boxster. As long as you're a contortionist, you can DIY it. Does your dash (i.e odometer) stay lit well after you remove the key (i.e. 10 minutes)? Do your accessories still work as well? After checking the trunk lights, I'd take a look at this as well. Good luck and let us know.

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I had the same problem with my first replacement battery.

I would drive it during the day. Park at night. In the morning it was dead. Hooked up an amp meter and there was no unusual power drain. This started a few months after I put in the new battery.

Put the charger on the battery then drove to the parts place where I bought it. Guy hooked it up to the tester and I had a bad battery. Sometimes even a new battery can be defective.

Guy gave me another battery but this time I had it tested before I left the store.

The 2 little trunks bulbs draw very, very little power, even if that was the problem. Plus the computer shuts them off after 2 hours anyway even if they are on.

Take your battery in and have it tested, but make sure it is charged first.

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Dear all,

As a new member I just love this forum, you guys seem to know everything about the workings and problems with Boxsters. :renntech: I have done a search but couldn't find a definitive answer from previous posts, Here we go then

I have had my Boxster 986 2.5 manual for just overa year now and in general I just love it, however there is a problem that just keeps coming back.

I drive about 500 miles a week generally in 2 or 3 journeys , so they are quite long trips that gives the battery plenty of time to charge up. The problem is that when I don't use the car even for a short time, the battery completely discharges and I have to go through the painful process of hitching up a live lead to fuse C3 to open the front to then jump start the battery.

It is worth noting at this point point that my battery is only 6 months old and is the highest power one I could find.

I parked the car up on Friday night and by Sunday morning (3am) the alarm was going off signalling that the power was drained from the battery. :censored:

I put an electrical tester accross the battery terminals today with the engine running and it was reading 14.2 volts which I think means that it is charging up OK. I am guessing that there is something draining the battery when the car is inactive, I can't believe that the alarm system would drain it that quickly.

Can anyone tell me what the likely cause is and also how do I test the electrics, I have a tester but am no expert so I pretty much need basic instructions.

Thanks a lot again, :cheers:

Nik, lots of good advice here especially about the amp meter, if you use a volt meter the battery will always give the full 12 volt reading right up until just before the battery is completly dead, starting the engine and getting a 14.2 volt reading does not indicate your alternator is OK, the amp output is the important reading.

The 12 volt car system is not really 12 volts, the alternator will output 15 volts or more, it has to, to get charge into the battery, think of the voltage as a sort of pressure, if the alternator pushed 12 and the battery was 12 it is a sort of stalemate, the push from the alternator has to be greater in order push charge into the battery.

The switch over from battery power to alternator power is to the user invisible but all the time the electronics measure voltages, the alternator powers your car and pushes voltage into the battery when the output from the alternator exceeds 12.7 to 13.3 volts, output depends on rotational speed and load - ie what have you got switched on, the alternator is a complex device it has a series of diodes, (A diode will only allow current to flow in one direction) if a diode goes bad the alternator can draw current and try to behave as if it is an electric motor (it will try to turn itself) this will flatten your battery.

There are 3 fields in the alternator all diode controlled, one can fail but the output with a volt meter connected to the battery will give the impression all is well, what is happening is that a pulse of charge is set up as one of the three fields is not working, voltage will look fine but the amperage output will be severly reduced, typically the starter motor will draw in excess of 150 amps when engaged - ohms law in which watts equals amps times volts - 12 volts times 150 = 1800watts - a lot of power drawn, also typically the battery is 80 amp hours (all batteries have a rating of amp hours) this means the battery will deliver 80 amps for one hour before it is flat, therefore drawing 150 amps to start would flatten an 80 amp hour battery in half an hour (if nothing else was powered by the battery - but other things are powered such as ignition and other circuits), however if the engine is cold, the oil thick the starter will exceed 200 amps and that is why you could not turn the engine in the real world for half an hour.

Because of the initial draw of power to start, voltage is reduced to other areas of the car, this is counteracted by voltage stabilisers, most of the electrical equipment in the car is regulated to voltages between 5 to 10 volts so they contine to work when voltage is reduced - you dont want your water temperature or fuel gauge to read lower when the battery becomes discharged. (this is why i say the 12 volt car system is not really 12 volts)

To get back to your problem, im not sure what equipment you have to test the system, a simple way is to remove the obvious - take out the bulbs in the two boots, fully charge the battery put a clamp meter over the positive lead (no need to disconect anything) the clamp meter will indicate what amperage is being drawn by the car, the reading should be very low, just milliamps, if above this disconnect one fuse at a time and recheck the meter - you will then be able to identify what is drawing the current- If this still does not find the culprit disconnect the electrical connection to the alternator - this will test the condition of the diodes - if they have failed and the source of your current drain.

Sorry about the long winded reply

Glyn

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Take your battery in and have it tested, but make sure it is charged first.

Probably the best advise.

Many many thanks guys, you've been really helpful, i'll get the battery checked tomorrow and if not start removing fuses. ill be sure to let everyone know what the result is as it may help someone out in the future :clapping: :clapping:

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Take your battery in and have it tested, but make sure it is charged first.

Probably the best advise.

Many many thanks guys, you've been really helpful, i'll get the battery checked tomorrow and if not start removing fuses. ill be sure to let everyone know what the result is as it may help someone out in the future :clapping: :clapping:

Yes, please do keep us posted. Nothing worse than being left hanging By a thread) :jump:;) !

Edited by PrinciPorsche
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