Mercedes-Benz Hybrid High Voltage Battery: Resistance Check
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One of the components that can fail and lead to no-start conditions on a Mercedes-Benz hybrid vehicle is the high voltage (HV) battery.
At a Glance
- Test: Resistance check of the hybrid high voltage battery
- Tested on: 2010 Mercedes-Benz S400 Hybrid (battery under the hood, passenger side)
- Result: Under 2 ohms passes; over 2 ohms points to a bad battery
- Tools: Scan tool for BMS codes, digital multimeter, 10mm socket, HV gloves
- Skill level: Trained technicians only
- Note: A passing reading does not rule out an internal battery fault
This short guide shows how to check the resistance of the high-voltage battery. This post is for information purposes only; trained technicians should only perform these steps. When performing this procedure, wear high voltage insulated gloves and eye protection.
High Voltage Safety First
- Hybrid systems carry several hundred volts DC, which can be fatal. If you are not trained in high voltage work, stop here and use a qualified technician.
- Wear insulated high voltage gloves (rated for at least 1000 V) and inspect them for holes before use.
- After the system is disabled, wait the time listed in the factory procedure so capacitors can discharge, then confirm zero volts with a meter before touching any orange cable.
- Never cut or pierce orange cables, and keep tools and jewelry away from exposed terminals.
How to Perform Resistance Check on Mercedes-Benz Hybrid High Voltage Battery
The first step is to use a good scanner, such as a YOUCANIC UCAN-II Pro scanner, to read the Battery Management System (BMS) codes on supported models. Also, go to live data to check the actual value of the resistance: item 087- isolation resistance.
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- Disable the high voltage system following the factory procedure. On many models this requires a professional bidirectional scan tool.
- Disconnect the 12-volt battery in the trunk.
- Open the hood and locate the hybrid high-voltage battery on the passenger side. On some Mercedes-Benz models, the hybrid battery may be in the trunk.
- Unplug the electrical connector from the high-voltage battery.
- Loosen the 10mm bolt and disconnect the orange cable connecting the A/C and power electronic module.
- Pull the black ring up on the orange cable from the DC/DC converter, then pull out the cable.
- Set a digital multimeter to measure resistance (ohms). The reading should be under 2 ohms.
- Measure the resistance between the two terminals shown in the picture.
If the resistance is below two ohms, the battery is ok. If the resistance is above two ohms, the battery is not good and needs to be replaced.
Remember that even if the battery shows under two ohms, it may still not charge and be faulty due to internal failure. This test was performed on a 2010 Mercedes-Benz S400 hybrid.
Get Help
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. If you want to ship the car to Baltimore, we can repair it at a fraction of the dealer’s cost. We love these cars and believe it is a sin to pay over $10k to keep them running.
Frequently Asked Questions
Can a bad hybrid battery cause a no start on a Mercedes S400 Hybrid?
Yes. A failed high voltage battery is one of the components that can cause a no start condition on Mercedes-Benz hybrids.
What resistance reading means the hybrid battery is bad?
In this test, a reading under two ohms is considered OK and a reading above two ohms means the battery needs replacement.
If the battery passes the resistance test, is it definitely good?
No. A battery can read under two ohms and still fail to charge because of an internal fault.
Which module codes should I send YOUCANIC for help?
Send the codes from the engine ECU, DC/DC converter, battery management system (BMS), and power electronics module.
Check these troubleshooting and repair guides for more help on your Mercedes-Benz.
Related Guides
- Mercedes-Benz Check / Test Battery Voltage with Multimeter
- Mercedes-Benz Check Battery Voltage No Tools Required
- Mercedes-Benz Force Charge 0% Hybrid Battery
- Mercedes-Benz Check Battery Voltage on Cluster
