Why do Mercedes Benz cars have two batteries?

This blog will explain the one battery and two batteries concept in Mercedes Benz cars and answer the following questions: why do Mercedes Benz cars have two batteries? Which Mercedes Benz models have two batteries?  What happens if one of the batteries discharges? Do you need to replace both the batteries together?

Why do Mercedes Benz cars have two batteries?

Mercedes Benz cars have two batteries because the onboard electrical architecture of the cars is designed with a two-battery concept. The latest cars have one battery concept and it uses a small-sized additional battery or a capacitor. The two battery concept and single battery concept are explained below:

  • Two battery concept

Mercedes Benz introduced the two battery concept with the fifth generation of the S Class model series W221. one battery was called a starter battery and it was used only for starting the vehicle. The second battery was the main battery which supplied power supply to all the vehicle equipment like interior lights, control units and blower motors.

As the starter battery is independent it can be placed closer to the engine in order to minimise the length of the electrical line. The starter motor consumes 300 Ampere and there is a significant amount of voltage drop in the Battery. Therefore by separating the batteries, all the interior functions continue to work even when the vehicle is being cranked by the starter motor.

The charging circuit for the models with two batteries consists of a start battery relay. The relay is actuated by the battery control module and when the relay closes, the starter battery gets connected to the main battery for charging and vice versa.

There are disadvantages to the two battery systems, the overall weight of the vehicle is increased and the cost of production is also high. The starter battery had no other purpose and it was just taking up too much space. 

  • Single battery concept

Due to the disadvantages of the two battery concept, Mercedes Benz completely reworked the electrical onboard network in the models made after 2010. There were still two batteries in the vehicle, but the second battery was the buffer battery. The buffer battery had a very small capacity at 12 volts 12 ah and it was used to actuate the park gear in the automatic transmission in the event the main battery is completely discharged. 

For starting and powering the onboard electrical systems only the main battery is used. As the engine cut-off system was introduced in the models after 2013, the size of the buffer battery had to be increased. In models with ECO or engine cutoff function; the capacity of the additional battery is 12 volts 200 A. 

When the engine turns off automatically in the ECO start/stop, an additional battery relay is switched and all the vehicle electrical systems are supplied by the additional battery. 

The relay is opened again when the driver lets go of the brake pedal and presses the accelerator pedal. The engine starts automatically and the main battery is again connected to the vehicle’s onboard electrical system.

This concept reduced the weight of the vehicle and simplified the electrician wiring in the vehicle. After the launch of the C Class model series W205, Mercedes Benz further improved the single battery concept. The additional battery is replaced by a capacitor designated as C1 in the wiring diagrams.

The capacitor stores sufficient charge and deploys it to engage parking gear if the main battery is discharged. It is very small and light compared to the additional battery used in the previous models.

For the ECO start/stop function, Mercedes Benz installed a voltage sag/dip limiting device on the main battery. When the engine is turned off in the ECO start/ stop function, the engine control unit stores the position of the ignition angle. Therefore complete rotation of the crank is not required to start the engine again. 

The voltage dip limiter can limit the voltage drop at the main battery when the starter is engaged. It is limited to 10-11 v and the vehicle interior systems can still remain functional when the starter motor is engaged. Without the use of a voltage dip limiter, the voltage drop on the main battery would have been 2-4 volts with the battery voltage dropping to 8-10 Volts.

The following table summarises the key differences 

Two battery conceptSingle battery concept
Dedicated starter batteryMain battery used for starter motor
Short wire length as starter battery is closer to starterLong wire is required to connect to the starter as main battery is located further away
No ECO start-stop functionECO start/ stop possible
No voltage dip limiter on batteryVoltage dip limiter present on battery
High cost of production and maintenanceLow cost of production and maintenance 

The single battery and two battery concept is not to be confused with the (SBC) Sensotronic brake control battery found in models with SBC systems. The SBC systems used dedicated batteries and have no relation to starting the engine or providing power to the interior equipment.

Which Mercedes Benz models have two batteries? 

Mercedes Benz models with a starter battery and the main battery are as follows.

  • S Class W221 (2006-2013)
  • CLS C219 (2004-2010)
  • CL C216 (2006-2014)
  • SL R230 (2001-2011)

Models with SBC and ECO start-stop functions also have additional batteries but they are only for backup and support functions. The vehicle remains powered by the main battery.

What happens if one of the batteries discharges?

Let’s take a look at what happens if one of the batteries discharges in two battery concepts and single battery concepts.

  • If One of the batteries discharges in two battery concept 

If the starter battery is completely discharged then the vehicle will not start as the starter motor is only connected to the starter battery. When the main battery voltage levels are low, the system is designed in such a way that the starter battery can charge the main battery in emergency situations. Therefore, if the main battery is not charged by the charging circuit then both the batteries will be discharged completely.

If the battery voltage falls below 4 V then there can be permanent damage to the battery cells. 

  • If One of the batteries discharges in a single battery concept

As all the vehicle systems are connected to the main battery, a dead main battery means the entire vehicle is not getting any power supply and it will not start. In case the additional battery dies then an Additional battery malfunction message will be shown in the instrument cluster. The font signal actuation and acquisition control unit is always monitoring the voltage levels and internal resistance of the main battery and additional battery.

Conclusion

Mercedes Benz cars can have more than a hundred control units onboard and different interior types of equipment like seat massagers and blower motors. All of the onboard systems rely on the main battery for the power supply. Therefore, there are multiple batteries in the vehicle and few models with mild hybrid systems also have a high voltage battery.

What was missing from this post which could have made it better?

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