Can the Mercedes auxiliary battery be charged?

                                                     

This article will explain the use of an auxiliary battery in a Mercedes Benz passenger car. And answer the following questions: Can a Mercedes auxiliary battery be charged? What is one battery concept in a Mercedes Benz ? and what to do when the auxiliary battery has malfunctioned?

Can the Mercedes auxiliary battery be charged?

Yes, An auxiliary battery in a Mercedes Benz can be charged. An auxiliary battery is connected to the main battery in the vehicle through an auxiliary battery relay. This relay is actuated by a master control unit that controls the onboard electronic network in a vehicle. This master control unit is also called as Signal actuation and acquisition control unit or SAM for short. 

One of the functions of sam is to monitor the voltage and temperature of the auxiliary battery via an electrical signal line. When the electrical voltage of the auxiliary battery goes below the specified value of 11- 14 volts. The auxiliary battery relay is actuated and the auxiliary battery can be charged by supply from the main battery. 

There is no risk of discharging the main battery as the function of charging the auxiliary battery only starts when the vehicle is running and the main battery is being charged by the alternator.

Why is an auxiliary battery used in a Mercedes Benz vehicle?

Mercedes Benz cars have an ECO (engine cut off) start-stop feature, when the vehicle stops at an intersection or red light then the engine is automatically turned off. As soon as the driver lets go of the brake pedal, the engine is started again and the journey can be resumed. 

Auxiliary battery play an important role in providing power supply to the vehicle onboard systems like blower motor of climate control, audio systems etc. An auxiliary battery is also used for the emergency Parking function when the main battery is discharged.

There is a 3-4 volts voltage drop across the main battery when the engine is cranked as the starter motor can draw up to 200 amps. Due to this voltage drop, The power supply can drop below 8 volts and there is no sufficient power to support other onboard systems like an ac blower motor, ambient light, radio etc.

The auxiliary battery is therefore connected to the vehicle power supply circuit by closing the auxiliary battery relay and the main battery can be used to start the engine without any effect on other vehicle functions.

The auxiliary battery is also used to provide a power supply to the transmission auxiliary oil pump as the mechanical oil pump in the gearbox is not working since the engine is turned off.

It is important to note that the auxiliary battery only supports the power supply of the vehicle during the cranking of the engine. Hence, the size and the capacity of the auxiliary battery is very small.

The auxiliary battery used in vehicles with eco start and stop function is 12 volts and up to 200 amperes. Few older models without eco start and stop function have an even smaller battery which is used for putting the vehicle in P when the main battery is discharged. 

What is One battery concept in a Mercedes Benz vehicle?

Mercedes Benz used a two battery concept where one starter battery located in the engine compartment is used only to start the engine. A bigger main battery usually located in the trunk is used as the main power source for the vehicle. 

These two battery concepts not only resulted in higher vehicle weight but the cost of replacing the batteries was also high. Therefore one battery concept was designed. Unlike the name ONE battery concept, there are also two batteries in the vehicle but the second battery is much smaller and the Starter motor uses current from the main battery. 

One battery concept has greatly reduced the weight and repair cost of the vehicle while allowing the use of the ECO start and stop function to reduce fuel consumption and protect the environment.

The latest modification in one battery concept is the use of a voltage dip limiter.

A voltage dip limited is connected to the positive terminal of the battery and controlled by circuit 50 (starter circuit) relay. A voltage dip limiter limits the voltage drop across the main battery when the engine is cranked.

It has two separate electrical pathways, one path has a resistance builtin while the other is directly connected to the starter motor. When the vehicle is normally started current flows from a pathway which has no resistance as maximum current is required to start the vehicle for the first time. 

During eco start and stop, the Engine control unit knows the position of the crankshaft and only slight rotation is required to start the engine again. The run period of the starter motor is significantly less in the ECO start-stop function. Hence the second resistive track is used to supply the starter and the engine can be started without turning off the ac blower motor and other interior equipment. If the engine is not started within 2ms then the voltage dip limiter is again switched to the first path and provides full supply to the starter motor.

Thanks to the use of a voltage dip limiter the voltage level at the main battery can be maintained at 10 to 11 volts during starting the engine. The additional battery is no longer required to supply power supply to the vehicle. A small capacitor is used instead to put the vehicle in P when the main battery is disconnected or discharged.

The capacitor is connected at the same location as the auxiliary battery. Hence no further modification to the vehicle onboard electrical network was required.

Malfunction of the auxiliary battery.

Auxiliary battery malfunction message is shown on informative display in the instrument cluster when the internal resistance of the battery exceeds 0.0 ohms. The internal resistance of the battery is defined as an opposition to the flow of current within the battery. The internal resistance also depends on the temperature of the battery. When this resistance is exceeded the auxiliary battery must be replaced.

Removing the battery and charging it with an external battery charger does not rectify the problem. The eco start-stop function can no longer work if the battery is not replaced. Continuing the use of an auxiliary battery with high internal resistance will also affect the main battery and can result in damage to the main battery. 

Mercedes Benz does not use flooded lead-acid batteries. Always replace the batteries with authorized AGM (absorbent gel mat) Batteries.

Conclusion

As the auxiliary battery is only used for a limited time, there is no need to trickle charge or charge it with an external battery charger. The auxiliary battery usually lasts for 3-4 years and if there is a malfunction it must be replaced. Latest Mercedes Benz vehicles do not use Auxiliary battery. Always follow safety instructions when handling the battery and get it replaced by an authorized workshop.

FAQs: Can the Mercedes auxiliary battery be charged?

Can you recharge an Auxiliary battery?

No, the Auxiliary battery cannot be recharged and used again in the vehicle. Once the voltage level drops below 4 volts and internal resistance is exceeded, replace the auxiliary battery.

How do you charge an auxiliary battery in a car?

The auxiliary battery is charged from the main battery in the vehicle. The Main battery is connected to the alternator. Hence always check the condition of the main battery while replacing the additional battery.

How do I charge my Mercedes battery?

Mercedes battery can be charged by connecting the external charger to the vehicle jump post. If the vehicle is not being used for a longer period a trickle charger can also be used to maintain the good health of the battery. The charging current must not exceed 60 amps as it can cause damage to the battery cells.

What type of battery does Mercedes Benz use?

Mercedes Benz uses AGM (absorbent gel matt) batteries. Some models also have the option to install a lithium-ion battery.

Can an auxiliary battery drain the main battery?

The auxiliary battery can drain the main battery if there is a malfunction in the auxiliary battery and if it has not been replaced.

Leave a Comment