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  • Writer's pictureTeam Vebo

BMW Battery Drain and Discharge Issue

Updated: Oct 15, 2021

Ever wondered why when you start the car in the morning and your car is not responding to the key fob and is completely cold dead without a single sign of electrical function? Do you blame the battery for being too old? Before doing so, let’s understand this issue so it can be managed properly before replace the expensive AGM battery which doesn’t resolve the actual root cause.

In this case study, we received an F30 that couldn’t start the car after a week sitting idle. So when we reached the site, we noticed that the battery is completely flat. The battery is completely exhausted and is not able to even unlock the car. So, when we hook up to the jumper, it took a while to completely power up all the module where it took nearly 5-10 seconds. We took the car back to our workshop to continue our investigation.

Firstly, the completely drained battery failed to be charge up with high amp. Our regular battery charger is unable to charge the battery properly.

Figure 1: Battery Failed to Charge Using Regular Automotive Battery Chargers

However, one good sign is that the resistant of the battery was still within the good range which is 3-7 ohm, so there is a sign that this cell can still be charged.

Figure 2: Resistance Level Is Still Low

So, we decided to send it to battery specialist to charge it using industrial charger constantly charging with very low amp to “wake the dead battery up". And the result was positive as the battery still can be reused. Figure 3 below shows that the battery is still healthy.

Figure 3: Test Report After Charging Using Industrial Battery Charger

So, with the battery side is cleared, what is the issue with the car then? Was there a current leak that we are not aware of? Or is the alternator not charging while driving the car? We did the alternator test, and it was good. So that can be out of the picture.

Then we waited for the car to go to “sleep” and finally it just stops at 1 amp after we waited for more than 10 minutes.

We noticed that the multimeter's cable started to heat up with such draw. At this point, we noted that the abnormal current draw is pretty large and we worry it will melt the cable if this test prolongs.

What is normal current draw from a healthy car electrical system? It should be around 0.03-0.06 amp per hour only when the car is in sleep mode. Judging from the number above, it is almost 20x of the recommended current. The car is not asleep it could be still under terminal 15 mode. What is terminal 15 mode? It is good to understand that BMW have a couple of terminal mode that runs the car properly.

Figure 5: BMW Terminal Mode

So, we decided to pull some fuse and we noticed that the fuse from the front power distribution box that controls the front cigarette lighter port was drawing the current. And we decided to approach so the cigarette light port and we found a phone charger on it.

Figure 6: Cigarette Port USB Charger

We decided to connect it to the phone and guess what, it is still charging the phone despite it shouldn’t cause the car was completed closed.

Figure 7: Phone Is Still Charging

So, what causes this issue, lets perform a few tests to see what is going behind this.

First initial test shows a current draw of 10 Amp. This is the reading right after we locked the car.

Then we waited for nearly 8 minutes and the reading drops to 1 amp.

Figure 9: Reading After 7 Minutes

10 minutes later, the reading is still the same.

Figure 10: Reading After 10 Mins.

So, this time, we decided to unplug the USB Charger from the cigarette port.

Figure 11: Remove USB Charger

And we repeat the test again. At first, the car was drawing 10 amp, then it quickly drops to 1 amp within 1 minute only.

Figure 12: Current Reading Within 1 Minute

So as we waited further it continue to drop to 0.246 amp within 3 minutes.

Figure 13: Current Reading After 3 Minutes

Then we continue to wait till it drop to 0.045 amp that is slightly above 6 minutes. Finally, it is within acceptable range.

Figure 14: Current Reading Within Acceptable Range After 6 Mins

So, what happens when the phone charger was plugged in? Basically, cigarette port continues to supply power even it is not connected to any devices to be charged and the power supply is from the front power distribution box, and what power up the front power distribution box is the Front Electronic Module(FEM). Basically, this module didn’t shut down even the car is completely shut and closed, and it continues to draw 1 amp per hour from the battery.

Modern BMWs are normally powered by a relatively large AGM battery, so let’s do some calculation to see how fast this battery are being drained at this rate of draw. A BMW 3 Series (F30) uses an 90 Ah AGM battery. This meansr the battery can give 1 hour of 90 Amp continuously. This is in a perfect world with battery @ 100% charge to begin with, normal state of charge in real life of a battery installed in car might be around 70%-7% especially BMW with smart charging system.

If the car is drawing 1 amp per hour, it should take 90 amp / 1 amp per hour = 90 hours before it completely drained. Any discharge of more than 50% would be bad for the battery as this will lead to reduced CCA and possibilities of the car not being able to be started.

As a conclusion from this case study, it is strongly advised to detach any devices on the cigarette lighter port when the car is completely shut down. Above test is good enough proof that the car module will not shut down or “sleep” whenever there is a load from device even it is not attended. This would save you from catching you on surprise that you can’t start the car the next day or even blindly replace the expensive AGM battery which is unnecessary.


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