Mysterious Battery Drain

Battery Drain

Does your Mercedes-Benz battery slowly drain? Is the battery dead every morning? Does the Mercedes-Benz battery die when you have the car parked overnight or leave it parked for a few days? This step-by-step guide will explain how to troubleshoot this problem, the basics of battery drain problems, why the battery dies overnight, most common trouble spots. You try to start your Mercedes-Benz, and the car is either completely dead, or this happens: 

Battery drains happen when electronic equipment (consumer units) constantly draws current from the battery while the car is parked. A straightforward example is when lights are left on overnight. But it’s not always as simple as that. While the electronic equipment in your car should not draw any current when the vehicle is parked, there are exceptions, such as the security system in your car. Even those systems that stay on always draw tiny currents, typically less than 50 milliamperes, and should not drain the battery.

A defective electrical system, Control Unit, or a fault on the CAN Bus (Controller Area Network) also causes that can cause your Mercedes-Battery battery to die.

Tools Needed

It would be best to have a multimeter to perform a current reading to troubleshoot the current draw. The multimeter should be able to read at least a 10 amp current. Multimeters are very inexpensive and are purchased for under $20. The Mastech Digital Multimeter is what we use in this guide, it is capable of reading currents, and you can buy it online, click here to check the price on Amazon.

Top 7 Systems that Cause Battery Drain Problems

1. Seat Control Module / Power Seat Motor
2. Audio System /AGW/amplifier
3. Radio/ CD changer/ navigation unit
4. Lights, including light in the glove box
5. Heated Seat Module
6. Proximity Key / Keyless start
7. Aftermarket alarm

Before you start troubleshooting battery drain problems

• Make sure you have a fully charged battery. 
• Make sure you have the radio code written down. You may be required to enter the radio code after this test.
• You may be required to perform a window and sunroof synchronization.
• If you will keep any of the doors open, manually press the door switch. Find a way to secure the switch pushed in. Use a screwdriver to lock the door latch.
• Manually press the door switch. Find a way to secure the switch pressed. Use a screwdriver to lock the door latch.
• If you have a fuse box in the trunk, open the trunk but place the latch in the locked position.

How to troubleshoot Mercedes-Benz battery drain

Step 1

Make sure you have a fully charged battery. If your battery is over six years old, you should consider replacing it to avoid further electrical nightmares. You can save on the cost of the battery by purchasing it online. For example,  see this list of AGM Group 94 batteries for Mercedes-Benz on Amazon. Turn off the lights. Make sure the dome light is off. Remove any accessories you may have plugged into the auxiliary outlets. Remove the key from the ignition and let the car sit for at least 15 minutes. In some models, you may need to wait up to 30 minutes for all the electronic systems to go “to sleep.” This will allow the car to shut off any electronic consumers that stay on for a brief period after the vehicle is parked.

Step 2

Locate the battery in the trunk or the engine compartment. Remove the negative battery terminal to connect the multimeter in series, as shown in the next step.

Step 3

Turn on your ammeter and set it to reader currents on the A setting. Connect one test lead to the battery post and the other to the negative battery cable. You should see a current draw of less than 0.1 Ampere or 100 milliamps. If your read is over this value, we need further troubleshooting. The reading should ideally be under 0.05 ampere.

Step 4

Locate the fuse box in your car. Disconnect the fuses one at a time. As you remove each fuse, you should check the amperage reading on the multimeter. If the multimeter reading is still high, reinstall the fuse and go to the next fuse. Continue removing and reinstalling all the fuses one at a time until you pull one that makes the multimeter reading drop to below 0.5 A. Once the reading drops below 0.5 amperes, you know that the circuit protected by that fuse is drawing too much current. Then look up the fuse chart to find out which electrical system the fuse protects. For example, it may be the fuse for the seat control module. Next, you must even troubleshoot or replace the seat control module. You may want that fuse removed until you replace the defective parts causing battery drain.

Conclusion

The vehicle must be resynchronized or otherwise normalized after performing this test or anytime you disconnect and reconnect the battery. To synchronize, follow these steps: Open and close the sliding roof and power windows. As the windows and the sunroof reach fully open / fully closed positions, hold the switch down or up for 5 seconds. The steering angle sensor must be reset by turning the steering wheel to full left and full right with the engine running. Clear the BAS/ETS/ESP and other instrument cluster warnings. Follow the direction in the owner’s manual to set the clock and any settings that may have been reset during this test.

If your Mercedes-Benz battery dies, read this article to learn how to jump-start your Mercedes-Benz the right way. 

 

Things to keep in mind

You should eliminate these potential problems before troubleshooting the battery drain problem.

• Make sure no interior or exterior lights are left on during the night. Ensure no lights in the trunk stay on when parking the car.
• Check the battery terminals and ground connections. Clean battery post terminals. Use a battery terminal tool to clean the battery posts and terminals.
• Make sure you don’t have an old battery. Older batteries should be replaced due to age and wear. Perform a battery load test and inspection. AutoZone or Advance Auto Parts will do a battery test for free.
• You should do a visual inspection of the battery. Make sure the battery housing is not distorted. Make sure that all the caps are still on the battery. Check for any leakage if you have an older lead acid battery. Absorbed Glass Mat (AGM) batteries are sealed, and you don’t need to worry about leakage. AGM-type batteries are what newer Mercedes-Benz cars use.
• The voltage Regulator may be defective and needs replacement.
• Test the alternator. Make sure it is generating enough amperage.
• Check that the glove box light does not stay on when the car is parked. The easiest way to check this is to open the glove box after the vehicle is parked for a few hours. Place your hand near the glove box light bulb and feel it if it is warm. Be careful not to burn your hand.
• Aftermarket alarm system. Aftermarket alarm systems are very often the cause of battery drain. They can often discharge a fully charged battery within days, so if you have one of these systems installed, check that first. Maybe disconnect it for a few days and see how it goes. They are often connected to a live battery source with an in-line fuse. Therefore, the test performed in this article may not catch the problem. Locate the alarm fuse, remove the alarm system fuse, and test the current draw for the alarm. It should be less than 0. 1 A.
• The proximity key allows you to walk up to the car and unlock the doors while the key is still in your pocket. A radio receiver in the vehicle continuously listens for a wake-up signal from the key. As the car receives a call in the assigned frequency, it wakes to validate the key. This causes a current draw for several seconds. If the vehicle is left parked in high pedestrian traffic areas, this could be a problem. It may cause the battery to drain within a couple of days in some models. Try to turn off the proximity key function in your car by looking up the instructions in the owner’s manual.

Applicable Models

A Class

W168

1997-2004

A160 CDI

A180 CDI

A200 CDI

A170

A200

W169

2004-2013

A160 CDI

A180 CDI

A200 CDI

A170

A200

W176

2012-Present

A160 CDI

A180 CDI

B-Class

T 245

2005-2011

T 246

2011-PRESENT

C-Class

W 202

1993-2000

C 220

C 230

C 240

C 280

C 36 AMG

C 43 AMG

W 203

2000-2007

C 230

C 240

C 280

C 320

C 350

C 32 AMG

C 55 AMG

W 204

2008-2014

C 250

C 300

C 350

C 63 AMG

W 205

2014-PRESENT

W 120

1953-1962

W 110

1961-1968

W 114

1968-1976

W 123

1976-1986

W 124

1984-1995

200D

230 e ce d te

260

E 280

300 E

300 CE

300 D TE

E 320

400 E

E 420

500 E

E Class

W210

1995-2002

E 320

E 420

E 430

E 55 AMG

E 300 Diesel

W211

2003-2008

E 200 CDI

E 280 CDI

E 300 E320

E230

E 280

E 320

E 350

E 500

E 550

E 55 AMG

E 63 AMG

W212

2009-2016

E 200

E 250

E 300

E350

E 500

E63

W213

2017 – Present

S-CLASS

W 126

W 140

1993-1999

500 SEC

600 SEC

S 500

S 600

CL 500

CL 600

W 220

2000-2006

S 350

S 430

S 500

S 600

S 55

S 65 AMG

W 221

2006-2013

S 550

S 600

S 63 AMG

W 222

CLA-CLASS

C 117

CLS-CLASS

W 219

2005-2011

CLS 500

CLS 550

CLS 55 AMG

W 218

2012-Present

CLS 250 CDI

CLS 350 CDI

CLS 300

CLS 350

CLS 550

CLS 63 AMG

CLK-CLASS

208

1998-2003

CLK 320

CLK 350

CLK 500

CLK 550

CLK 55 AMG

CLK 63 AMG

209

207

CL-CLASS

W 126

W 140

W 215

2000-2006

CL 500

CL 600

CL 55 AMG

CL 65 AMG

W 216

CL 550

CL 600

CL 63 AMG

CL 65 AMG

W 217

SLK

R 170

1998-2004

SLK 230

SLK 320

SLK 32 AMG

R 171

2005-2009

R171

SLK 280

SLK 350

SLK 55 AMG

R 172

SL

129

1990-2002

300 SL

500 SL

SL 320

SL 500

SL 600

SL 55 AMG

SL 63 AMG

SL 65 AMG

SL 280

R 230

2002-2012

SL350

SL500

SL55 AMG

SL600

SL65 AMG

SL550

SL55 AMG

R 231

2013-Present

SL350

SL400

SL550

SL63 AMG

SL65 AMG

R-CLASS

V 251

2006-2009

R320 CDI

R 320 BLUETEC

R 350

R 500

R 63 AMG

R 300 CDI

GLA-CLASS

156

M-CLASS

W163

1998-2005

ML 320

ML 350

ML 430

ML 500

ML 55 AMG

W164

2006-2011

ML 320 CDI

ML 320 BLUETEC

ML 350

ML 500

ML 550

ML 63 AMG

W166

2012-Present

ML350

ML400

ML550

ML63 AMG

ML250 Diesel

GL-CLASS

X164

2007-2012

GL320

GL350

GL450

GL550

X166

G-CLASS

W460

W463

1978-2009

G 290

G 300 D

G 320

G 350

G 500

G 550

G 55 AMG