This article covers the High Voltage (HV) Battery (A100), its work, and common problems.  The first-generation 128v 6.5ah hybrid battery is used in BMW ActiveHybrid and Mercedes-Benz S400 W221 and uses thirty-five Li-Ion cells similar to these Saft VL6 cells.

The high-voltage battery is costly. In some cases, the hybrid battery costs more than the vehicle’s resale value. A new Mercedes-Benz hybrid battery costs over $10,000, while a used unit can be found for around $3,000 to $5,000.

Mercedes Hybrid Problems

Replacing the high-voltage battery is relatively easy but should only be completed by a trained technician due to the increased risk of a fatal accident.


  • This article is for information purposes only. The hybrid battery is hazardous and can lead to fatal accidents.
  • A hybrid battery should not be opened up or repaired by untrained individuals.
  • Lithium-ion batteries are a safety hazard and extremely dangerous if overcharged.

Hybrid Mercedes-Benz Common Problems

MB Hybrid Battery

The most common problems with Mercedes-Benz S400 hybrid vehicles are the failure of the power electronic module and the failure of the hybrid battery. To fix either one of these problems at the dealership, you will spend between $9,000 to $12,000.

  • Battery Management System (BMS) Failure.  BMS is integrated into the hybrid battery.
  • Critically Low Charge Level of the Hybrid Battery
  • Bad Li-ion Cell – One faulty cell can turn the hybrid battery off. The battery shown above has 35 cells—a nominal voltage of 3.7 volts and a max of 4.1 volts per cell.
  • Contractor Issue – Contactors open and close the high-voltage circuit. They are located inside the high-voltage hybrid battery and can fail.
status of contactors

Other possible problems with Mercedes-Benz hybrid vehicles are:

Critically Low Charge Level

mercedes low battery voltage

One common problem with the high voltage battery is that the charge level drops to critically low levels, which prevents the vehicle from starting. The vehicle is designed to disable the engine start if the battery charge level is below 15%.

How to charge Mercedes Hybrid Battery

Charging the high-voltage battery is possible using a 75Amp 12-volt charger connected to the vehicle jump-start terminals. Follow our guide on how to charge Mercedes-Benz Hybrid Battery.

Mercedes hybrid battery charging

BMS Failure

Mercedes Hybrid Battery BMS

Inside the hybrid battery, you will also find the BMS (Battery Management System – N82/2). The BMS manages the charging of the hybrid battery. A critical function is to ensure that each of the 35 li-ion cells is balance-charged, meaning that none of the cells should have a higher or lower charge level than the rest.  To be balance charged, all the cells must be within 0.03 Volt of each other.


Common fault codes that show up as stored or current in the battery management module (SG-BMS):

  • 180F00 – The high-voltage battery malfunctioned during the recovery phase.
  • 0BBD00 – The limit value for the difference between the high-voltage battery module cell voltages was exceeded.
  • 0BBE00 – The cell voltages in the high-voltage battery module are different.
  • 0A7D00 – The charge level of the high-voltage battery module is low.
  • 180C00, 180500 – The hardware monitoring function has detected a fault in the control unit.
  • 0A1F04 – There is an internal fault in the control unit battery. There is an internal fault.
  • 0AE886 – The temperature sensor D of the high-voltage battery has an electrical fault. There is an incorrect signal.
  • 0ACA86 – The temperature sensor C of the high-voltage battery has an electrical fault. There is an incorrect signal.
  • 0AC586 – The temperature sensor B of the high-voltage battery has an electrical fault. There is an incorrect signal.
  • 0A9B86 – The temperature sensor A of the high-voltage battery has an electrical fault. There is an incorrect signal.
  • 0A1F96 – There is an internal fault in the control unit battery. There is an internal component fault.
  • 0A1F00 – There is an internal fault in the control unit battery.
  • 0A7F00– Hybrid Battery Pack Deterioration
  • 161A52 – ECU In-Plant Mode Active not activated
  • 0AE500 – Hybrid Battery Negative Contractor Control Circuit Range Performance
  • 0ADA00 – Hybrid Battery Positive Contactor Control Circuit Range Performance
  • 0ADE00 Hybrid Battery Negative Contractor Control Circuit Range Performance

When you connect a multi-system OBD-II scanner and read the BMS Control Module codes, you communicate via the CAN network with the N82/2 module inside the hybrid battery.


The YOUCANIC Full System Scanner can read and clear fault codes through every vehicle’s control module.

s400 hybrid precharge high voltage battery

The BMS monitors the status of the interlock circuit, the temperature of the cells, the voltage of the hybrid battery, and the contractors’ status. The BMS also monitors the cooling system for the hybrid battery, which runs off the electric A/C compressor that also cools the cabin.

Hybrid Battery Safety Fuse

hybrid battery fuse

Inside the hybrid battery, you will find a fuse in series between cells 18 and 19. The fuse’s main function is to avoid overheating and the explosion of the hybrid battery if there is a short circuit in the high voltage system, which is possible in case of an accident.

If the first generation high voltage battery, the battery found on the W221 unit needs to be replaced if the fuse has blown because the battery is not designed to be opened or repaired. In the newer W222 S-Class, the battery can be opened by a trained technician and can be repaired.

Contractors (A100s1)

hybrid battery contractors

Contractors A100s1 are installed inside the hybrid battery. Think of the contractors as an on / off switch inside the hybrid battery controlled by the ECU and BMS.

interlock insulation resistance contactor

When you turn on the ignition, the contractors close, which powers up the high voltage system.

MB Hybrid Battery Contractor

When the ignition is turned off, the contractors open up, and the system automatically turns off.

Why does my hybrid battery not charge 100%?

Hybrid battery not charging to 100%

The ideal charge level for the hybrid battery is not 100%. Li-ion and LiPo batteries’ ideal charge levels are 50%-70% of maximum capacity.

In this range, the HV li-ion performs the best and lasts the longest. Do not expect to see a 100% charge on a hybrid vehicle. Even on fully electric vehicles, regularly charging the battery to 100% is not recommended, or the battery’s life will decrease significantly.

Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. It is better not to charge because the high voltage shortens the battery life. You will notice that your hybrid battery will fluctuate between 40% and 80%, which is perfectly normal.

HV Battery Charge Levels

hybrid battery cell voltage check

Here is a breakdown of the optimal state of charge (SOC) level on a li-ion battery:

  • 0-30% – Extremely low charge level
  • 30%-50% – Low charge level
  • 50%-90% – Normal charge, operating level
  • 90%-100% – High charge level

The vehicle will no longer start if the battery charge level is below 28%. While the hybrid battery discharges very slowly, it is possible that a vehicle that has been parked for a very long time can reach under 28% SOC.  The hybrid battery on a W221 has 35 individual cells rated at 4.1 Volt maximum. Typically those batteries operate in the 3.5V to 3.7V range.

Therefore the maximum charge level reaches a voltage of 12.9.5. Simultaneously, the extremely low range when the battery reaches 0% is, in fact, 115 volts or 3.28 Volts per cell. Even though the battery voltage can drop below 115V, it is considered fully discharged and will engine start will be disabled. At 80% SOC, the voltage of the hybrid battery should be 130 volts.

How to troubleshoot a hybrid Mercedes-Benz

Even if there is damage or burn to the board, the YOUCANIC scanner can access the module and determine the error.

  1. Connect the scanner to the OBD port.
  2. Use Electronic Motor Control Unit Menu to check Motor ECM faults.

Procedures for using a scanner to diagnose HV battery not charging.

  1. Connect the charger’s positive terminal to the 12v input Battery input and connect the negative to the ground.
  2. Connect the scanner to the ODB2 port and turn on the ignition.
  3. Check the “Status of Contractor on the menu.” If the contractor is open, then most probably, this is the cause of the battery not charging.
  4. Check above via the DC-to-DC converter Control unit Menu.
  5. Read Data Stream
  6. Check the operating state status; if the status is passive, the system is not charging the HV hybrid battery.
  7. Verify the status of “Voltage of High voltage On-Board Electrical System.” It should be between 48 v to 120.

Follow the procedures to charge the HV system if the “Voltage of High voltage On-Board Electrical System” status equals 0.

  1. Clear the codes
  2. to go to the battery management system.
  3. Go to the actuation test.
  4. Go to the actuation of contractors menu.
  5. Press f4

These procedures will generally change the status of contractors from open to close if there is no other error is associated with it.
The “Voltage of High voltage On-Board Electrical System” would slowly rise.

How to ship a high-voltage lithium battery?

The high-voltage lithium-ion battery is classified as a hazardous material. They should be handled with care and should not be dropped. Your country may have different regulations, but the most common classification for a hybrid battery is the UN 3480 Lithium-ion battery, Class 9, PG II.

It is recommended to ship a li-ion battery at 30 to 40 % state-of-charge rather than a full charge. Do not ship a hybrid battery if the housing is damaged.

Get Help

mercedes hybrid problems

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,
  • BMS,
  • 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 the list of codes and send them to us.

Manufacturer Part No

  • A221 343 09 00,221 343 09 00,A2213430900,2213430900
  • A221 902 13 00,A2219021300,221 902 13 00,2219021300
  • A221 343 08 00,A2213430800,221 343 08 00,2213430800
  • A2213400500,A221 340 06 00,2213400600,A2213400600

Buy the Mercedes-Benz S400 hybrid battery and power electronic module. 


Year    Make    Model    Trim    Engine

  •      2013    Mercedes-Benz    S400    Hybrid Sedan 4-Door    3.5L 3498CC V6
  •      2012    Mercedes-Benz    S400    Hybrid Sedan 4-Door    3.5L 3498CC V6
  •      2011    Mercedes-Benz    S400    Hybrid Sedan 4-Door    3.5L 3498CC V6
  •      2010    Mercedes-Benz    S400    Hybrid Sedan 4-Door    3.5L 3498CC V6

We hope you find the Mercedes-Benz Hybrid Problems guide helpful. Check these troubleshooting and repair guides for more help on your Mercedes-Benz.

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  1. Hello . Can you help me. I have a Mercedes W221 Hibryd in the works. The problem is that my inverter is out of order. The car was moving at the time. I fixed the inverter, but now I have a problem with the contactor counter, it is 0. How can I fix this? I tried programming the BMS module but it didn’t restore the counter. if i take the battery apart and read 95256 can you reset the data or is it possible via vediamo or monaco?

    1. No one has been able to reset counter with vediamo or monaco at least so far. If you can do it, you will be the first one.

  2. hello mercedes s400 fault code help code 00aa400

    1. Please click on the Support menu, login and post a picture of the code so that we can see the code defintion and help you more. You also need to check a few things in the Battery Management Control (BMS) unit to really get the whole picture.