Mercedes-Benz Hybrid Battery Problems

This article goes over the High Voltage (HV) Battery (A100), how it works, 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 very expensive. In some cases, the hybrid battery costs more than the resale value of the vehicle. A new Mercedes-Benz hybrid battery costs over $10,000, while a used unit can be found for around $3,000 to $5,000.

Replacing the high voltage battery is relatively easy but should only be completed by a trained technician due to the high 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.

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 inside the hybrid battery. 
  • Critically Low Charge Level of the Hybrid Battery
  • Bad Li-ion Cell - One faulty cell can lead to the hybrid battery shutting off. The battery shown above has 35 cells—nominal voltage of 3.7 volts, max 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. 

Other possible problems with Mercedes-Benz hybrid vehicles are: 

  • Power Electronics Module 
  • Blown Hybrid System Fuses
    • Pyro Fuse inside the hybrid battery
    • Inline high voltage fuse inside the black box on top of the transmission housing. 
  • A/C Compressor
    • The A/C compressor is run by the high voltage system. If the A/C compressor fails, it can disable the hybrid system. 

Critically Low Charge Level

mercedes low battery voltage

One common problem we see with the high voltage battery is that the charge level drops to critically low levels, which prevents the vehicle from starting. 

In fact, the vehicle is designed to disable the engine to start if the battery charge level is below 15%. 

How to charge Mercedes Hybrid Battery

It is possible to charge the high voltage battery 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 high or low charge level than the rest of the cells.  To be balance charged, all the cells must be within 0.03 Volt of each other. 

Failed BMS on a Mercedes-Benz S-Class W221

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. 

Also, 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  Fuse

hybrid battery fuse
W221 HV High Voltage Battery A100 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 on the high voltage system. 

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 could 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. 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 to 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 in the 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, it is not recommended to fully charge the battery to 100% regularly, or the life of the battery 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 fully 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 

Here is a break down 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 level
  • 90%-100% - High charge level

In fact, if the battery charge level is below 15%, the vehicle will no longer start. While the hybrid battery discharges very slowly, it is possible that a vehicle has been for a very long time can reach under 15% 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 Volt 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 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 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. 

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


    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

Job Difficulty


🕥 Time:

4 hr -10 hr

💰 Cost:



Mercedes-Benz Hybrid Battery Problems This article goes over the High Voltage (HV) Battery (A100), how it works, 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.