Discharged Battery

[kalbmw]

Symptoms:
Car suddenly went dead yesterday, was able to jump start. Took it for a good drive to recharge the battery, no problems. Remote unlocked/locked it last nite (to test if battery retained charge, horn sounded good and loud). This morning battery has gone dead again. Changes I can think of in the last couple of days,
a. I switched the Manual mode on (been driving it in Auto ("A") mode all along).
b. Windshield has cracked and will be replacing it tomorrow, not sure if there are sensors in the shield.

The problem with the battery happened once 4 months ago, but went away after jump start. Battery is one year old.

Any inputs are appreciated

1995 BMW 525i

[sehiser]

Is it a sealed battery?

Once a battery goes completely dead it's pretty much shot.

I don't think the 95 525 has sensors in the window.

I'm curious about the manual mode and if it is drawing current on the battery for some reason.

I would check the current draw on the battery when the car is off. It should be next to nothing. Use a multimeter, switch it to amps and put it in line with the battery.

You would do this by taking off the red, or hot, battery cable and putting the multimeter between the battery terminal you removed it from and the battery cable. MAKE SURE YOU ARE ON AMPS. mA. Else you'll blow your multimeter.

You should see a draw of less than .0005 A (.5mA), or around that. It's going to be pretty low.

Look on your battery, you might see something like 8aH, which is how many amps per hour the battery can sustain. For instance, 8/24 = 0.33 amps per hour. So in 24 hours your battery will be dead. A car with an alarm and LCD clock can probably sit for 9 months and start right up if the battery was good when it was left there. That's about .0001 amp draw.

So basically, if you have any noticable draw on your battery then you have a short somewhere. Most multimeters can handle up to 10 amps. Your starter uses about 250-300 amps on a nice day, and 600+ on a cold day. So if you do everything right, you'll either find a draw of about .3 or .4 on your battery, which means it would die overnight. I doubt it would be over .10 because it would die before you got off work. If it's over that, it will blow the fuse in your multimeter. You can pick up a multimeter if you don't have one for about $35-40.

If you haven't worked with eletricity and your battery, have a shop check it out. Especially if your battery isn't seal or has a leak, any spark near it could cause a hydrogen explosion.

If you don't find an amp draw and your battery still goes dead, it's time for a new battery. If you find an amp draw, you have an eletrical short. I'm leaning to your manual mode verses auto, there are eletronics involved there which are low amp components, so it makes sense to me that something isn't switching off. There are a number of relays on your transmission, if there is a problem you can expect it to cost no more than $150. Probably about $45-50 for the relay and the time in the shop. I think $90-100 would be reasonable.

Scott
Toledo, OH

[redbeemer]

Battries sometimes seem to die suddenly when they reeally died a thousnad deaths.

Over time the moleclar ions crystalize into insoluble salts.
Some of these salts fall to the bottom of the battery.
When that chamber fills up they begin shroting the plates.
Each plate removed form the battery drops your power.
Entire plates can fail and entire cells can short.
Overcharging heats the battry and causes the palates to buckle.
When they buckle they short out with the same resutls.

All this drops a battery voltage down littel by little.
When you get past a certain point the battery wont take a charge.

It might just be this death by a thousand cuts and nothing specific that happended to you.

[mcg]

My 530i has 3500 miles on it, and has gone through 3 batteries.

The first one, the OEM battery that came with the car was, I think, flawed in some way. The other two were just badly used.

I drive the car only intermittently, and sometimes leave it parked in a garage for weeks at a stretch whiile I am gone. The alarm system takes the battery right down to zero.

After total discharge, they are not the same again.

Best, Michael

[kalbmw]

I will follow up with a mechanic. Interestingly I went back to auto mode and it hasnt happened ( but I am not sure this is the reason). I will let you know what I find.

[mcg]

Hello Scott,

Please amplify a little bit on this question of the normal current draw:

"You should see a draw of less than .0005 A (.5mA), or around that. It's going to be pretty low.... A car with an alarm and LCD clock can probably sit for 9 months and start right up if the battery was good when it was left there. That's about .0001 amp draw."

Is the normal draw for a car with an alarm, radio and clock going to be .0001 A, that is, 0.1 mA? Or did you mean it should it be 1 mA, maybe?

The 9 month projection for battery life with a parked 530 is way outside my experience. I get maybe 6 weeks. Naturally I am wondering if there is an ancillary short in the system somewhere.

Thank you for the normal draw info.

Best, Michael

[psychop]

9 months is a pure fiction. During such long period of a time even battery sitting
on the table will self discharge to some extent.

Service manual says that the battery on the sitting cars (ex. in the showroom)
should be recharged every 6 or 12 weeks (latter value for cars that have
special "transport" or "logistic" mode which reduces quiescent current draw
just by switching off some components).

Also i believe that the "idle" (quiescent) current of a such loaded with electronics
car would be in range 50...75mA and that value should give a clue about
projected battery life.

Remember that most of the modules are supplied continuously with power;
they do not have real "power switches", they are just turned to standby
using software command (over the CAN) or thru the control line (like switched +12
from the ignition key). This standby current sums up for all the modules and other components.

When i was still working with Mercedes cars, i remember number of Service Recalls
about modules that were defective in a way that they didn't go into standby mode
even if commanded. This included power seats modules, nav controllers, engine
electronics and others. Some of them draw huge amounts of power like 10A
continuously!

[540 M-Sport]

Any car that sits for more than a week at a time should have a battery tender hooked up:
http://snipurl.com/7cod This is the SAME as the BMW private labeled version that sells at BMW dealers for $59 to $69.

For easiest hook up, splice on a cigarette light plug like this:
http://snipurl.com/5rl7

Do this, and your battery will last much longer, and always start when needed.

[Nick_99]

I have a 05 330xi and I had same problem twice back to back each requiring a jump. Took to BMW and they have no problems and can't reproduce problem. I had a CD changer and Alarm put on the car before I picked it up. I wonder if that is it. Does the Alarm draw a lot? This is aggevating as heck. You should have to hook up a battery tender every night to your car. Heck my Toyota 4 runner limited was fine for 5 years! I have always heard BMW's electrical system had issues I am starting to believe it.

Any ideas of what I could be doing to make it run down???

Nick

[540 M-Sport]

I have a 05 330xi and I had same problem twice back to back each requiring a jump. Took to BMW and they have no problems and can't reproduce problem. I had a CD changer and Alarm put on the car before I picked it up. I wonder if that is it. Does the Alarm draw a lot? This is aggevating as heck. You should have to hook up a battery tender every night to your car. Heck my Toyota 4 runner limited was fine for 5 years! I have always heard BMW's electrical system had issues I am starting to believe it.

Any ideas of what I could be doing to make it run down???

Nick

Exactly how long is your car sitting?

Has anyone done a test of how many milliamps draw you have when the car is sitting, with ignition off?