Battery Drain

I think my car has a battery drain. I have to boost it every 2-3 days in the winter if I don't drive it, even less if it's extremely cold. When my car is off, I can hear a hum coming from the radio area so I think this might be the cause of the drain (I'm assuming it's trying to keep it warm).

When I bought the car it came with an aftermarket radio (Clarion). I can live without the radio, so I was thinking of pulling the fuse. But when I look at fuse box diagrams, the radio fuse is also for various other things.

The battery is a year old and I had it tested, so I know it's not that.

It's a '96 328i.

I was hoping to get some clarification or suggestions.

Pull the radio itself and disconnect it from the harness (and pray it wasn't ghetto-wired in).

Thanks for the reply. I pulled the head unit. No ghetto job that I can tell. Hopefully this solves the drain.

I had a battery drain once, there was a short in the trunk wiring. The trunk light would stay on even after I shut the trunk lid. Try it at night ... turn your car off, pull down the back seats and see if the trunk light is on. Worth a shot

good luck!

I think you're talking about the a/c control unit fan. Mine turns off about 5 mins after I turn off my car.
It's most likely your battery.

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Here is a little trick to find the source of any battery drain. Buy 2 alligator clips and a 10 watt, 1 ohm resistor from radio shack. Connect the resistor between the alligator clips. Disconnect the negative terminal from your battery and put one alligator clip on the battery terminal and one on the negative battery cable that you disconnected. Now take a volt meter and put one leg on each side of the resistor and read the voltage. Leave it all connected and start pulling fuses one at a time. When the voltage drops, you found the bad circuit and you have a place to start looking. Ive used this method many times and it is a good place to start. You can rig up the apparatus I described for $5 plus a voltmeter.

Originally Posted by Nicky3

Here is a little trick to find the source of any battery drain. Buy 2 alligator clips and a 10 watt, 1 ohm resistor from radio shack. Connect the resistor between the alligator clips. Disconnect the negative terminal from your battery and put one alligator clip on the battery terminal and one on the negative battery cable that you disconnected. Now take a volt meter and put one leg on each side of the resistor and read the voltage. Leave it all connected and start pulling fuses one at a time. When the voltage drops, you found the bad circuit and you have a place to start looking. Ive used this method many times and it is a good place to start. You can rig up the apparatus I described for $5 plus a voltmeter.

You're doing it the hard way. For $3 you can buy a digital multimeter and measure the current directly.

I'm having the same problem with my 99. We just bought it used and thought it was kinda weird that the seller gave us the receipt for the battery. Left it parked in the garage and didn't drive it the next day. Went out to start it today and the battery is dead. I've verified that the trunk light is going off when it's closed. I put an amp meter in series with the battery. With ALL lights off, including the trunk light, it's pulling .53 Amps. I don't know if this is acceptable or not. Seems to me that even a small current like this (about 1/2 amp in my case) would drain the battery in 24 hours. Any ideas?

The drain should be a tenth of that or less. A half amp over 24 h would run down a new, warm battery perhaps 20%. From this great distance, I might suppose the battery was very cold or has suffered deep discharges.

Originally Posted by loveour99328

I'm having the same problem with my 99. We just bought it used and thought it was kinda weird that the seller gave us the receipt for the battery. Left it parked in the garage and didn't drive it the next day. Went out to start it today and the battery is dead. I've verified that the trunk light is going off when it's closed. I put an amp meter in series with the battery. With ALL lights off, including the trunk light, it's pulling .53 Amps. I don't know if this is acceptable or not. Seems to me that even a small current like this (about 1/2 amp in my case) would drain the battery in 24 hours. Any ideas?

I'd recommend trying the above approach of pulling fuses and retesting the draw on the battery each time until you find the circuit that is responsible for a large portion of the battery draw. Good luck and let us know how you make out!

Ok, so, after doing more reading on other threads I realized I wasn't waiting long enough to see the current draw when the car "goes to sleep". The 530 miliamps I was seeing was immediately after everything was turned off. My actual current draw after setting "asleep" for quite a while is .07 (only 70 milliamps). Does this sound normal? Like I said, we just bought it used so really don't know any history on it. We do however, know the battery is only 4 months old. I'm not hearing any noise from inside the car like something might still be running (like someone else noted). Should I be concerned about the 70 milliamp draw? Any help would be much appreciated! We need this car to be reliable.

70 milliamps is about normal for drain when the car goes to sleep. Even though the battery's only 4 months old it may be bad. The tests that parts stores do on batteries typically only find gross faults, not internal drains that sap storage capacity. Measure the battery voltage after the car has sat for a bit and has gone into sleep mode. With only a 70mA drain it should be around 12.4-12.5 volts.