Something is draining the battery on our 2002 bmw z3.the battery and alternator checks out good,what is draining the battery??

If the alternator is charging the battery it will not be the fuse. The alternator would simply stop working if the fuse has blown.
If the battery isn't holding charge you might have a power drain somewhere in the vehicle. Something in the electrics may not be turning off correctly and draining the battery. Do you have any warning lights on the dash? Have you access to an OBD2 scanner to check for any Trouble Codes?
If it is a power drain the simplest fix is to get a good Auto Electrician to check the vehicle. Finding a power drain or an electrical short can be a real pain in the tail bone job because there is so much electronics on a car. Paying an expert to do it is a lot quicker and easier as they have the right tools to do it.

The problem is that an electrical component in the BMW that should be dormant when the vehicle is shut down is actively drawing power.
The only way to track it down is to have tests run to see which circuit is drawing excessive power because there are numerous components that might be the cause.

You can learn more here...........
Solving Automotive Electrical Problems

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You have a Drain on your Battery. The Nominal drain during the night is 50mA. Anything more than that and you have a issue.

Put a Multi-Meter on your Battery (Between your Battery and your One Terminal's and then one on the Wire itself (Disconnect the NEGATIVE Clamp). MEasure the AMPERAGE DRAW...

Then disconnect one fuse at a time and if the number does not go down. When you get the DRAMATIC DROP, then you have found what system has the draw (test all just for a Piece of mind).

Then once you discovered which system, then you need to dig into that system to define it down to the problem and fix it and then you will have your issue resolved.

Good Luck

check your ignition system
hope you find it

I hate to say this, but I doubt the radio or battery has much to do with it.
This is just a wild guess, but I would suggest you get a voltmeter and check the alternator output.
It should be 12.5 at idle, but over 13.5 when reved up.
If the radio is weak output, it seems to me it is getting less then 12 volts input, or else it had blown internals.
Another possibility is that a door speaker wire may be shorting and draining radio power.
Hard to guess.

There is nominally a fuse, relay, and a switch associated with your reverse lights. Possible repair listed from 'easy to hard' by escalation

easy:
disconnect your negative battery cable, check the fuse block. It should be a 20A fuse(#F203). Some models use a 10A fuse(#F26). Replace as needed
Reconnect the battery, test.
if that doesn't work...

moderate.
have already checked the fuse, check to see if you have a "reversing light relay" (K6325) (My 2001 z3 does not) If present, disconnect negative battery cable and replace the entire relay. It is not worth trying to refurbish them.
Reconnect the battery, test.
if that doesn't work...

hard...
If both the fuse and relay are in good order, it is likely a fault in your wiring or the "reversing light switch"(S8511)

On the manual transmission:
The switch is located on the left of the gertrag transmission
The switch is located on the right of the ZF transmission

unscrew the switch from the transmission
install the new switch, tighten to 15 ft-lbs torque

On the automatic transmission:
If replacing the fuse and relay did not work there is an issue with your transmission range switch, various shift solenoids, shift linkage, etc. I highly recommend a dealer or qualified facility's examination.


I hope this helped!

it seems like something is draining your battery while the car sits overnight. causing the battery to get weaker and weaker putting stress on the alternator making it fail prematurely. the dealership will do a paracidic drain test where they'll find out what is draining your battery while your car sits overnight. it could be a bad module somewhere that's staying on when it shouldn't be.

i would say its a dead (flat) battery. if its more than 4 or 5 years old, it might be time to replace it.

try jumping it with another vehicle. if it jump starts, then replace the battery.

if its a newer battery (less than 4 years old), then something is draining the battery or the alternator is not recharging it.

hi,i have bmw z3 coupe 1999,2,8.i was having same problemes,and it` cost me money by mecanic by geusing(starter,altermateur) like 1200$,belive me the probleme was the battery, get a new battery,and make sure the+ i mean the red line in the battery is good and try to move it to make sure good luck..

look at the dome light switch on the drivers side for corrosion etc. make sure it isnt pulling the circuit down