Read up all (?) of the other threads on flat batteries but still I've got a problem. Had a flat battery 3 weeks ago in my 9 yr old 2.0 Z3. Original battery was suspected so replaced with Varta 70A/hr equivalent. Still went flat after a few days. suspected boot light and sure enough the switch didn't cut it off when shutting so I removed the bulb. I drove for 40 mins today, parked for 6 hours, then drove for another 30 mins. No lights on, no heating. A good sunny day. Came home and after 1 hour couldn't turn the engine over. Its back on charge now, and just changed to a trickle charger for over night. Checked voltage with voltmeter. Was 12.3V when it couldn't turn engine over. Now at 13.7V with charger on. I did check it with engine running and it peaked at about 14.2V so I suspect alternator is OK. I've set passenger bay light to permanent off now. What else could be causing this? Duff battery? If there is another "popular" leak path (like the bootlight) does anyone know where it is, so I can check?
With the exception of the boot light is every thing else working as they should.The wiring loom where ot goes into the boot is one area that gives problems.
To check if you have a short causing your problem,turn every thing off ,unbolt thr pos terminal on the battery ,offer the pos cable to the pos terminal on your battery and note if it sparks ,if it does this indicates a short or a drain on the battery.
Does the starter turn over,loose starter connection maybe.?
The spark test that maxman suggests is dangerous. Batteries give off hydrogen gas and to create a deliberate spark is not too wise (especially, in an enclosed space). The venting tubes should keep you safe, but you can't be too careful.
The test to do is to disconnect the negative wire. Connect an ammeter in series between the wire and the battery and note the value. The value should be around 0.0x Amps.
If the value is more, take one fuse out at a time until the current value drops. When it does, you've found the circuit that's causing the drain.
The fuse number should tell you what each fuse does, but you can use the circuit diagrams here to trace the circuits that each fuse protects.
If the battery is in good condition and fully charged it should last for many weeks as only the clock will be drawing power, I very much doubt if the small amount of driving 30min and 40min with a start in between would be enough to give the battery a decent charge from flat, see how it goes after your over night charge and if the checks as suggested by pingu fail to find any thing, and if it still gives problems get the battery rechecked.
I lost all power from the battery on mine last year.I had an outo elctrician look and the battery was charged, I thought it would be the dreaded wiring loom at the boo tbut it turned out that even though they looked clean there was a layer of corrosion at the terminals . All it took to remedy it was to use a bit of sand paper to clean the contacts .
Its worth a try as mine had an intermitant fualt before comlete failure.
best of luck
I have been diagnosed with ADHOTS. Attention Deficit Hyperactive Ohh That's Shiney
You are wasting your time making or accepting guesses on this one! The only way is to check IF the battery is being discharged, and if over 30 mA do some logical diagnostic work to find yout where the discharge is going on.
You have to buy a decent multimeter - about £20 at Halfords, and see what is going on.
You may not have a discharge, but a bad battery connection or engine earth.
One of the few worthwhile checks you can make is to turn on the parking lights and see if they go out when you try to start up - if they do, suspect the battery and/or its connections, but if they stay on, the battery will be Ok and the engine earthis a good suspect. As it bolts on to the RH engine mount, whihc is aluminium, high resistance is a common proble - take it off, clean the surfaces, and fit with a bit of Vaseline on them.
But - for anything elese, use a meter, and not guesswork!
OK - set the multimeter to a low current range (or if it is self-ranging, as many digitals are) to 'Amps' and connect it in series withthe battery. This means that youi disconnect the earth cable, and connect the meter with its positive lead to earth, and its negative lead to the battery neatgive terminal.
The meter will now be bale to read the current being passed by the battery, which with everything turned off shuld be about 30 milliamps - or 0.03 amps if using a digital.
If more than this - and it would be appreciably higher to discharge the battery in the time indicated - remove and replace one fuse at a time and see if the reading changes - as described above.
When you find the fuse(s) which drop the reading to about 30 milliamps, you will need to disconnect whatever the fuse feeds, and again check the meter reading.
Do things slowly, write down what you do and the meter reading, and report back.
My guess is that either there is somehting wrong with the battery (very doubtful, as both the old and new batteries had the same problem) the battery charger is not working properly (again, very doubtful) there is a defectibe piece of electrical equipment (doubtful again, as it would be drawing lots of current to kill a fully-charged battery so quickly) or there is a source of high resistance between the batttery and the starter motor - quite likely, such as bettery terminals, engine earth strap, starter motor terminal etc.
Yep, well explained Mike. Incidentally though, my battery standby discharge current is around 140mA, sounds like quite a lot but always been like that.
140 mA means that a 70 ampere-hour battery - if fully charged and in warm condiitons - will be deas in 3 weeks. In an average UK winter call it a fortnight.
Higher than normal discharge rates can be due to after-market things such as stereo systems, alarms, GPS etc - it's surprising how much they can draw.
When BMW launcjed their uber-touring motorcycle, the K1200LT, its 30 AH battery was being flatenned in less than a week, due to the effort of keeping the radio memory alive!
Gee another day at school,
I can understand the alarm drawing current, but I never realised that a stereo or Radio drew current when switched off, or a GPS but I have never had a vehicle with GPS.
Electrics was always my weak point, as all of our w/s electrical was sent out to the Sparky, so I became lazy an left it up to them
In any engineering area, you never get something for nothing, and if you want to keep the memory (station frequencies and functions etc) of your stereo alive, then it takes power. Big amplifiers (ugh!) which stay on standby when not in use are another big long-term consumer of battery power.
Mike, My Ford Fairmont Radio keeps the presets when the battery is disconnected I do have to enter the code, I have not tried the Z.
My cell phone [Samsung] and lap top also keep all the info when battery is removed.
All my mains powered stuff do the same I realise these probably have a built in battery.
Depends on what technology is used for the memory. Some systems need a constant supply and even have capacitors or rechargeable batteries to cope with short term disconnects of the battery. Other systems (usually more modern) will use flash memory which requires no power at all to maintain it's state.
It sounds like your car is drawing a fair bit of current, so it should be easy to find provided you have a bit of patience to sit there with a multi-meter pulling fuses. Multi-meters don't cost much these days and handy to have around for all kinds of jobs. At least once you isolate which circuit the load is on you can start trying to figure out what the problem is.
Regarding the boot light - how did you determine that it didn't go out when you close the lid? The plungers you see on bottom the lid are NOT switches, they simply debounce the lid when you close it. The actual microswitch is part of the lock mechanism and is depressed by the funny little conical bit on the lock.
Cheers R.
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
Regarding the boot light - how did you determine that it didn't go out when you close the lid?
..the simple (often overlooked way) is to feel the lens after 10 mins or so with the boot shut. If it's warm when you open it again then the light will have been on!
(?) of the other threads on flat batteries but still I've got a problem. Had a flat battery 3 weeks ago in my 9 yr old 2.0 Z3. Original battery was suspected 
so replaced with Varta 70A/hr equivalent. Still went flat after a few days. suspected boot light and sure enough the switch didn't cut it off when shutting so I removed the bulb. I drove for 40 mins today, parked for 6 hours, then drove for another 30 mins. No lights on, no heating. A good sunny day. Came home and after 1 hour couldn't turn the engine over. 
Its back on charge now, and just changed to a trickle charger for over night. Checked voltage with voltmeter. Was 12.3V when it couldn't turn engine over. Now at 13.7V with charger on. I did check it with engine running and it peaked at about 14.2V so I suspect alternator is OK. I've set passenger bay light to permanent off now. What else could be causing this? Duff battery? If there is another "popular" leak path (like the bootlight) does anyone know where it is, so I can check?
