A Mercedes-Benz hybrid vehicle such as S400 may not start if the high voltage battery is discharged. Mercedes hybrid vehicles do not have a starter. The engine is started by the electric motor which is powered by the high voltage battery, not the 12-volt battery.
Even if your dashboard lights turn on and the battery in the trunk is fully charged, the hybrid battery can still be discharged preventing the engine from starting.
You can charge the high voltage battery using a 12 volt 50 A battery charger. An 80A or 100A 12 volt battery charger would work better. The high voltage battery can be charged through the 12-volt system through the DC/DC converter.
Here is how to charge the hybrid battery.
- Pull the hood release and open the hood.
- Locate the charging terminal near the hybrid battery. You will have to remove the cap to access the positive post.
- Connect the red clamp from 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. Allow the high voltage battery to charge. It can take several hours for the hybrid battery to charge.
Do not leave the vehicle unattended. Monitor the charger to make sure it does not overheat.
The hybrid battery needs to charge to at least 15% in order to be able to start the engine. If the hybrid battery is discharged below 15% capacity, the engine won't start.
Once the hybrid battery is charged, ideally to at least 30% of its capacity, connect a scanner such as Mercedes Star Diagnostic Scanner (SDS) and clear the fault codes that are present in the Battery Management Control Unit.
Keep in mind that if the safety contractor located in the battery don't close, you will have to manually close them with Star Diagnostic Scanner.