Mercedes hybrid vehicles do not have a starter. The engine is started by the electric motor, powered by a high voltage battery, not the 12-volt battery.
Although your dashboard lights are on and your trunk battery is fully charged, the hybrid battery can still be discharged, preventing the engine from starting.
You can charge the high voltage (HV) battery using a 12 volt 50 A battery charger. An 80A or 100A 12 volt battery charger would charge the HV battery faster. The high voltage battery can be charged through the 12-volt system through the DC/DC converter.
- No other fault codes related to the hybrid system other than the low battery state of the HV battery.
- External engine starter charger or connect to another vehicle using jumper cables. 12 Volt and 50 Amps or higher output.
How to charge a hybrid battery
- Pull the hood release and open the hood.
- Locate the jump-start terminals for the low voltage system (12 volts). On Mercedes-Benz S-Class, for example, you will have to remove the cap to access the positive post.
- Another option is to connect the external power source, such as jumper cables from a running car directly to the main 12-volt battery (this method may not be recommended by the manufacturer but we have been able to use it successfully).
- Connect the red clamp of your 12-volt charger to the positive post.
- Connect the black clamp from your charger to the ground post near the strut tower.
- Get inside the car and turn on the ignition; all dash lights will turn on.
- Connect a diagnostic scanner to the OBD-II port.
- Go to control units and select DC/DC converter.
- Once you connect to this control unit, you should see the Actuation sub-menu. If you don't, your scanner may be too basic, and you need a more advanced scanner.
- Under DC/DC converter control unit, you can turn on high voltage battery charging. While the battery is charging, you should see Boost Mode activate. Allow the high voltage battery to charge. It can take a few hours to charge the hybrid battery.
- It can take several hours for the hybrid battery to charge.
To verify the hybrid battery is charging, go to COMAND > System > Hybrid system. It shows the small battery symbol with an up arrow. If the up arrow is not showing, the hybrid battery is not taking charge.
The hybrid battery may not charge if there are active codes for the high voltage system.
Do not leave the vehicle unattended. Monitor the charger to make sure it does not overheat.
Depending on the model, the hybrid battery needs to charge at least 15% to 28% to start the engine.
If the hybrid battery shows 0%, the vehicle will no longer charge that battery.
Once the hybrid battery is charged, ideally to at least 30% of its capacity, connect a Mercedes Star Diagnostic Scanner (SDS) and clear the fault codes present in the Battery Management Control Unit.
Keep in mind that if the safety contractor located in the battery doesn't close, you will have to close them with Star Diagnostic Scanner manually.
Hybrid Battery Charge Modes
The hybrid system on Mercedes-Benz vehicles operates in one of the following modes.
- Boost mode - The high voltage battery is charged using the main 12-volt battery or external 12-volt power. Energy is converted from 12 volts to 120 volts via the DC/DC converter.
- Buck mode - During this mode, power is transferred from the high voltage battery to charge the main 12-volt battery.
- Passive - There is no power transfer between the high voltage battery and the 12-volt electrical system.
- Risk of death or injury. Working on a high voltage system can be dangerous. Only trained technicians should work on a high voltage system.
- The vehicle should not be left unattended during the charging process.
If you are having issues with the hybrid, please "contact the YOUCANIC" team with any questions that you may have.
We can help you troubleshoot the problem FREE of charge as long as you send us the codes. We need the codes from these four modules:
- Engine ECU,
- DC/DC converter,
- Power Electronics Module.
Once we have the codes, we can tell you if you have a bad power electronic module, hybrid battery, or both.
If your vehicle is at the dealership or mechanic, ask them for a list of CURRENT codes and descriptions and send them to the YOUCANIC team. Screenshots from Xentry or Snap-on scanner showing current codes work best.