Battery Draining Fast? Common Causes & Fixes

Hello everyone,

Since I've reached the end of my rope, I'm hoping that you can help me.

The problem is as follows:

For a few months, I've been having a problem with my Polo 9N (year 2004) where the battery is completely drained after about 2 days, and sometimes the clock in the dashboard doesn't even work anymore.

So, I got myself a decent ammeter with alligator clips and measured the quiescent current. Even when the vehicle is locked, the quiescent current doesn't drop below 600mA after more than 20 minutes. That's easily ten times what a Polo should consume.

A diagnostic scan using VCDS reveals only two errors in the radio control unit:

00457 - Body control module (J519). 004 - No signal/communication - Intermittent.

as well as an error that is caused by the antenna not being connected.
However, there is no direct connection between the body control module and the radio. All signals, such as ignition and lighting, are transmitted via the CAN bus, which works perfectly. However, it is still possible to hear something even when the system is supposedly turned off.
that the CD drive causes the radio to turn on for a few seconds every now and then.

When I disconnect the 15A fuse labeled "Radio/Navigation" during the quiescent current measurement, the quiescent current drops to 500mA, which is still much too high.
If I now disconnect the SB50 15A fuse for the central locking system, the quiescent current drops to below 10mA.

If I remove the SB50 15A fuse, the central locking system works, but the radio no longer turns on the CD drive. This suggests that the power supply to individual components, specifically for the radio to remain on, is dependent on the SB 50 fuse.

The question is, what's behind SB50? Just the central locking system, or also the comfort control unit? What exactly is being powered? I can't really imagine that it's the entire KSG (Komfortsteuergerät - comfort control unit), because the power windows are fused separately. However, I haven't tested whether they still work when SB50 is disconnected.

I hope you can help me, thank you in advance.

"In the past, pulling a fuse was a useful method for identifying a faulty component. However, since all devices are now interconnected via CAN, this method is no longer effective. When a fuse is pulled from a CAN node, it often disrupts the entire CAN bus, preventing it from settling down. As a result, the power consumption remains high or even increases compared to before."

VAG recommends a waiting period of more than 20 minutes (ideally 2 hours) before measuring the voltage drop across the still-connected circuit breakers. (The current can then be calculated from the voltage and the circuit breaker rating).

With VCDS, you can also check the CAN gateway to see if all bus systems are in a sleep state (by looking at the measurement blocks). If this is not the case, each potential cause must be disconnected individually and removed from the gateway control unit until the component responsible for preventing the system from going to sleep is identified.

Since you already found the information about the radio/navigation system in the vehicle's configuration sheet, I would disconnect it for now and code it as "not installed" in the gateway module.

And then, during the power measurements, also wait for a minimum of 90 minutes.

Regards,
Guste.

Thank you for your reply.

Since the quiescent current drops immediately when the fuse is pulled, the CAN bus must completely shut down. Otherwise, I would have to see a higher current consumption than 10mA, right?
Can you provide me with the formula that allows me to calculate the quiescent current using the voltage drop (presumably the difference between two voltage readings?) and the fuse rating?

How can I check the Mean Values (MWBs) with VCDS/Vag-Com if the ignition is not turned on? Wouldn't the bus systems be active in that case? Without the ignition on, I can't establish a connection to the control units.

DOW wrote:

Can you tell me the formula that allows me to calculate the quiescent current using the voltage drop (presumably the difference?) and the fuse rating?

Ohm's Law:wink:
(Static) Current = Voltage drop (across the fuse) / Fuse resistance.
The latter is likely to be a bit tricky to determine, requiring the use of a micro-ohm meter or passing a defined current through it and measuring the voltage drop.

Quote:

How can I check the Mean Values (MWBs) with VCDS/Vag-Com if the ignition is not turned on? Then the bus systems are active. Without ignition, I cannot establish a connection to the control units.

So, at least the Body Control Module (BNStG) remains in the diagnostic dialog when the ignition is switched off...

DOW wrote:

Since the quiescent current drops immediately when the fuse is pulled, the CAN bus must completely go into a low-power state; otherwise, I would have to see a higher current consumption than 10mA, right?

*Clears throat* I confess: I probably skimmed over that section in the first post. My mistake. You are, of course, right; at 10mA, everything is stable, and you can safely ignore my comments about bus stability.

DOW wrote:

Can you tell me the formula that allows me to calculate the quiescent current using the voltage drop (presumably the difference between two voltages?) and the fuse rating?

No, it depends on the fuse rating and type (a large FKS fuse or a small FK1 fuse), but VW has a great table for that. You mentioned 15A, so I'm assuming it's an FKS fuse. For those, a voltage drop of 1mV corresponds to 226mA. Therefore, you'll need a more sophisticated measuring device with µV resolution. The voltage drop and current flow are directly proportional.

DOW wrote:

How can I check the module work bits (MWBs) with VCDS/Vag-Com if the ignition is not turned on? Then the bus systems are active. Without ignition, I cannot establish a connection to the control units.

If you open the correct measurement block of the gateway, you can then turn off the ignition. The communication remains active as long as you do not end the diagnostic process. Only then will the gateway go into sleep mode (the current flow is naturally higher due to the active diagnostic process).

The SB51 fuse only protects the connection to the radio/radio-navigation unit connector T8 (8-pin), specifically pins 5 and 7.

However, the radio or navigation system sometimes takes longer to restart. So, you might really have to wait for 2 hours and repeat the test.

Regards,
Guste.

I can confirm that the waiting time was very long.

Everything is on and off, but the laptop is connected to the CAN-GW, and all three buses are quiet: 600 mA.
Diagnosis complete: 122 mA.
I really waited for 2 hours: a few mA with regular jumps to 25 mA (which could be the activated rain sensor).

Compared to the Passat 3C.

If I have inadvertently altered the values from memory, please let me know and provide corrections .

The measurement itself is quite simple:
Loosen the battery terminal nut and press a test probe through the terminal onto the lead post, holding the other probe against the terminal. Then lift the terminal up.

guste100 wrote:

No, it depends on the fuse rating and type (large FKS fuse or small FK1 fuse), but VW has a great table for that. You mentioned 15A, so I'm assuming an FKS fuse. For those, a voltage drop of 1mV corresponds to 226mA. Therefore, you'll need a more sophisticated measuring device with µV resolution. Voltage drop and current flow are directly proportional.

I have a Voltcraft VM 130, which has a resolution of 0.1mV for DC measurements and a basic accuracy of ±0.5%. It seems I can't measure accurately enough with it. I would only be able to detect changes in current in steps of 226mA. Unfortunately, even more expensive Voltcraft devices don't have such a measurement range.

Quote:

If you open the correct measurement block of the gateway, you can then turn off the ignition. The communication remains active as long as you do not end the diagnosis. Only then does the gateway go to sleep (the current flow is naturally higher due to the active diagnosis)

.

Do you know the correct MWB (Material Waste Bin)? I don't have any descriptions of the data that each individual MWB displays.

Quote:

The SB51 fuse only protects the connection to the radio/radio-navigation unit connector T8 (8-pin), specifically pins 5 and 7. However, the radio or navigation system sometimes takes longer to restart. So, you might really have to wait for 2 hours and repeat the test.

Sure, those are the safe pin and constant power connections. However, they are connected to my Quadlock because I have an RCD 200 installed. The problem also existed when the navigation system was still connected using the standard ISO connector.

But what's even more important: Where is the SB50 going? Because when I draw power from it, the radio also stops working completely, even though it can still be turned on. So, I'm not breaking the circuit to the radio. The current then drops rapidly to about 10mA, even though the constant power supply and the safe pin are still connected to the radio and "consumed."

"@dieselmartin, I already have the 600mA draw, even when the laptop isn't connected to the CAN-GW. The 600mA draw makes more sense in your case, since you don't have a bus idle state."
122mA is quite a lot, but your 3C will also have significantly more comfort features than my little Polo .

I performed the measurement exactly as you instructed.

Despite initial hope, the situation is disappointing. I had reached the point where, after a KSG reset (removing and reinserting the fuse), the CD drive no longer starts up. I thought this would solve the problem. I was wrong. With the power supply connected, I'm getting a quiescent current of 620mA, even after more than 30 minutes.

I came across this thread:

http://www.audicabrio.info/co/showthread.php?tid=17189&pid=152678

However, even pulling the radio fuse doesn't change the amount of current required.
If the door control units (i.e., the central locking fuse) are disconnected, the quiescent current immediately drops to 100mA. Therefore, the issue must be related to the door control units. The body control module (BCM) itself is still active, as the electric windows continue to function, but the central locking system does not.

I haven't found any errors in the control units except for the radio.

Quote:

Sunday, April 11, 2010, 15:22:24. VCDS-Lite Version: Release 1.0 Control Module Part Number: 6Q0 035 152 B Component and/or Version: Radio DE2 0004 Software Coding: 0020200 Workshop Code: WSC 31414 2 Faults Found: 00856 - Radio Antenna 011 - Open Circuit - Intermittent. 00457 - Network Control Module (J519) 004 - No Signal/Communication

So, in some cases, there is no communication with the BNSTG. (J519) However, it appears that everything is secondary, because even after disconnecting the radio fuse and recoding the gateway, there is no change: approximately 700mA with the central locking system active, and 100mA without.

The door control units are integrated into my central locking system (KSG), so they remain active. Only the actuators of the central locking system should be protected by the SB50 fuse, right?

DOW wrote:

...approximately 700mA with voltage conversion, 100mA without. The door control units are integrated into my central control unit (KSG), so they remain active. Only the actuators of the central locking system (ZV) should be protected by the fuse SB50, right?

Hi,

"However, even 100mA is still significantly too much. I'm not sure if pulling the fuse might just be masking the underlying problem."

Unfortunately, the exact function of fuse SB50 cannot be determined from the wiring diagrams. The fuse connects to the DWA ultrasonic sensor (if installed) and to the comfort control unit. However, the wiring diagrams do not specify what the comfort control unit uses this voltage for, or where the currents flow within the unit. Since the T23 connector of the comfort control unit, to which fuse SB50 is connected, also connects to the contacts of the central locking motors, it is likely that these motors are also connected to this circuit.

Do you have the possibility to replace the engine control unit (ECU) with one from a different vehicle?

Even though I don't think it will help: Did you really wait for 2 hours with all the fuses blown while taking measurements?

Best regards,
Guste.

I just measured the resistance between SB50 and ground. The result, with the ignition key removed, is:
Constantly 5.26V.

How does this help me?

The power windows must be connected to the fuse box labeled "SB 50" because when I pull that fuse, the central locking system stops working, even when you turn the key in the door lock.
However, the window lifters are still working, and the central locking system is still accessible via VCDS.

I could install the KSG (dashboard control unit) from my girlfriend's car (9N3 Polo), which also has central locking, radio, but no alarm system.

I don't have a DWA.

I didn't perform the 2-hour test because: If the main fuse is removed, the car still starts without any problems even after a week. However, if the fuse is intact, the car will be completely dead within a maximum of 3 days.

DOW wrote:

I just measured the resistance between SB50 and ground. The result, with the ignition key removed, is: Constant 5.26V

Did you remove the fuse for that?
Then, for interpretation, all voltage values (with the ignition on and off) at both terminals of the SB50 would be helpful, which is a total of 4 voltage readings.