Bad batteries or current draw?
 

About 2 months ago, I replaced the battery that came in my 2005 4.8L with an Exide battery, because the old one tested with a bad cell.

Less than a month later I had to exchange the Exide for another one because that one failed with a bad cell! A few days ago the truck wouldn't start after having sat undriven for a little more than 24 hours. I had to jump start it and it's been starting fine since.

Both the battery and alternator tested good at my mechanic's shop, but he told me that he has not had very good luck with Exide batteries over the years, and when I asked him about checking for current draw/leak, he said it's hard to test for those because of the various modules on the car that are constantly drawing power.

So which is it: Bad luck with batteries? Problem with Exides overall? Current draw?

Is it in fact hard to check for current draw on our trucks, or is there a procedure for checking?

Checking total current draw is easy...turn everything off and place an amp meter inline with one of the battery terminals (start with the highest amperage rating the meter offers and work your way down).

If you want to determine the current draw of a particular circuit you can determine it by performing the above and then remove the fuse for the circuit of interest. Subtract the two and the result will represent the current draw of the circuit.

Do not attempt to start the vehicle with the meter inline.

Zulu: I think you meant to say 12 months or so? 12 years for any battery is unheard of!

What did you replace the Exide with?

What modules should I disconnect? Which ones have the potential to stay awake if there were to be a malfunction?

Unfortunately I cannot help you with this. Perhaps others here can offer their suggestions. However you could just go through each one, fuse by fuse, and see what level of current draw each has.

Checking current draw isn't quite so easy because you need to supply power to everything until sleep comes in and then have the ammeter or shunt in place without disrupting the circuit. Best way is with some kind of battery switch. It's not hard but you have to keep those things in mind.

Start with radio, NAV and telematic (blue tooth module). Pull fuses for all of them and start putting them back in one at a time. When you insert one that results in a dead battery, you have your culprit. Had a similar problem after alternator took a dump. New alternator and new battery and it kept dying over night. Parasitic drain turned out to be the telematic that wasn't "going to sleep" like it was supposed to. Removed the telematic and problem solved. Best of luck!

I understand the ammeter in series between the battery post and the battery terminal, but not sure I understand the role of the shunt in the setup. Can you please explain?


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Bone stock, no accessories added.


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Do you have bluetooth?

Just to be clear, this diagnostic should start after the truck has gone into sleep mode, at which NONE of the modules should be "alive" after 16 minutes?

On a perhaps related note, when the headlight switch is in the automatic mode (leftmost position), the lights are always ON regardless of ambient light level, they do not turn toggle, as if the light sensor/module is stuck in the always ON mode. Lights do go off when the car is shutoff. Related? Is there a separate sensing module for that function? Could it be the culprit?


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The sensor for the automatic headlights and automatic wipers is on the window by the rear view mirror. It's concealed by the black cover behind the mirror. Have you or the previous owner changed the windshield using non-BMW glass? When I changed mine it stopped working. You may also have a bad module. If you take the cover off you can unplug the sensor and see what happens. The cover is shown as number 6 here: RealOEM.com - Online BMW Parts Catalog