Charging/Starting/Alarm problem

911 996 Carrera 4 1999. The car doesn't get used all the time, and sits on a driveway. There seems to be a problem holding the battery charge. If the car isn't used for a week, the battery goes flat, causing the alarm to go off, and I haven't found a way to stop that - usually it stops by itself after a while, but I've had to get up in the middle of the night to fiddle about with it until it stops. I can re-charge the battery in an evening, and the car will start OK afterwards, and will continue to be OK as long as I use it every day. Both battery and alternator are only around 18 months old. The car has been checked over by a garage that specialises in Porsche, and they have found nothing wrong with the charging, and no evidence of untoward battery drainage. Does anyone have any ideas, please? Many thanks Ed

996's have always suffered from battery drain. The alarm and other backup systems do place a large draw on the system. This is normal for this car. What you really need is a battery optimiser. Check out CTEC. Mike

You really need to do a load test to see if the drain is abnormal. This involves disconnecting the negative terminal of the battery, and putting a multimeter in circuit to measure the current drain. When disconnecting the battery, switch the ignition on, and then off again, but leave the key in the ignition - don't withdraw it. Then disconnect the negative of the battery and connect the multimeter in the circuit. Unplug the front boot light, switch off the radio, and shut the doors, and switch off anything else that is active. You then need to measure the current drain over the next 31 minutes. It will start at around 1.0 amps, but should drop to around 40mA at the end of the 31 minute period. (This only applies up to MY2002).

Thanks Guys. Richard - I've had problems with the car again this week - used it a couple of times for short journeys to and from work, but yesterday morning the battery was too flat to start it. This morning, the battery wasn't just flat - it was completely dead, and with bonnet locked shut. Luckily, for some reason, the engine cover was free, so I was able to connect a charger and get the bonnet open. Now charging it, and will try your multimeter test once charged - although I must say, I had expected that the garage would have done that when they had it, and gave it a clean bill of health! Will post results, hopefully tomorrow. Ed

I haven't ad my 2004 my 996t very long, just coming up to 12 months. I'm not even sure if the battery was new at purchase. Like you I keep the car on the drive and it has gone for up to six weeks and will still start. After a two and a half week lay up whilst it has been snowing, I fully expect it to start. Maybe I am, or have just been, lucky

Thanks for your reply 924nutter - that's exactly what I would hope for! Richard - I finally got around to doing the battery drain test. I had to borrow a suitably meaty multimeter - mine only goes up to 250 mA. When I started the test, the drain was 1.34A, but this decreased to 0.67A once I disconnected the front boot light. Then, it just stayed at that level - at 35 minutes and still at over an hour. So, I guess there's some timer system, intended to cut the power to some systems after half an hour, that isn't working? At that rate, I suppose it's no surprise that the battery is flat after a few days.

After about 10 minutes, try pulling Relay 2 out of the relay board under the carpet panel behind the rear seats. The oxygen sensor heaters are held energised for a few minutes after the ignition is switched off, which causes quite a hefty drain. After doing that, I would try leaving it until 35 minutes have elapsed, and start pulling fuses one at a time, to see if one has the effect of dropping the current to a sensible level.

Thanks for this Richard. I need to have some coherent plan of action here. Presumably, you'd normally expect some system(s) to cut out after 30 minutes? I presume that's why you suggested the 31 minute check? But you're suggesting checking the oxygen sensor heaters after 10 minutes. I'm confused. I have to say that my inclination is to take the car to the garage again (this isn't a Porsche main dealer - just a normal garage that handles Porsches, and therefore reasonably priced). At least I can now tell them that I've measured a battery drain that's far too high, and get them to track down the problem. Don't get me wrong - I'm very grateful for your advice, but given the likelihood of having to replace an expensive component anyway, and of my being unable to arrive at a conclusive diagnosis myself, I might as well hand the problem over straight away. Ed

I've given you duff information already - it looks like the oxygen sensors may cut off after 20 minutes, not 10. This is the load drain table from the manual, so you can see what each system draws, and the times at which the current drops. Bear in mind that on -01 cars, it should all stop draining after 31 minutes.

Thanks again Richard. However, I'm further confused. I followed your directions to the letter earlier - I turned on the ignition and then turned it off, but left the key in the ignition. However, I notice that on your attachment, it says 'Read off the measuring range only when 31 minutes have passed since the vehicle was locked '. I never locked the car! Does this mean my test yesterday was invalid? Maybe the drain is always that high as long as the key is in, and car is unlocked? Should I repeat the test with it locked? Ed

I don't think it will make a great deal of difference, but you could try it with the car locked. If the meter is not already in circuit, you will need to leave the key in the ignition while you put the meter in place, and then remove the key and lock the car. I would leave the window down, so you can lean in to remove the key, and activate the boot catch with a screwdriver before locking it with the boot open.

Thanks again. I've repeated the test with the key out and the door locked. This time the initial battery drain after a few seconds was around 130mA, but this had decreased, by the half hour stage, to only 40mA. So, contrary to my first conclusion, I now reckon that's OK. I now suspect that I have a duff battery - it's fine in the milder weather we're having at the moment, but it seems to give up very easily in the very cold conditions we had a couple of weeks ago. I guess the best plan, in that case, short of replacing what is a relatively new battery, would be just to disconnect it altogether if there's another very cold snap and not use the car for that time. Ed

40mA is normal, so I'm sorry for sending you on a wild goose chase! You can disconnect the battery, but bear in mind you won't be able to lock the passenger door. If you keep it inside, just get a battery conditioner and leave it connected. Lidl do one for about £15 which is excellent. I have a couple of them.