A Mercedes EQC 400 arrived at our workshop showing a high voltage battery warning light and just 22 miles of range. The previous diagnosis, from a main dealer, was a failed battery pack. The quote to fix it: £56,000. The actual repair cost £860. Here is exactly what was wrong and how we found it.
Table of Contents
- What the Warning Light Actually Showed
- The Diagnosis Process Step by Step
- What Is a Fusillock and Why Does It Matter?
- The Real Fix: BMS Software Reset and Update
- EV Battery Replacement Costs in the UK: What You Should Actually Pay
- Frequently Asked Questions
- Conclusion
A dealer told the owner this car needed a new high voltage battery. The estimate was £56,000.
That figure is not unusual. High voltage battery replacement in a large premium EV can reach that level when main dealers source new OEM packs and include all associated labour and programming. But in this case, the battery was completely fine. What the car actually had was a locked safety function and outdated firmware in its battery management system. A Mercedes EQC high voltage battery fault is not always what it looks like on the surface.
This is a full account of how we diagnosed and repaired this car, what we got wrong along the way, and what every EV owner should know before accepting a large battery quote.
What the Warning Light Actually Showed
The car came in as an auction purchase. The previous owners could not say exactly when the fault started. What they knew was that the EQC had been running fine, then at some point the range dropped sharply and a warning light appeared on the dash. The message read: ‘reserve level charge high voltage battery.’
On ignition, the car showed 22 miles of range. That is roughly a tenth of what this model should deliver.
The first step was a diagnostic scan. The scan returned three relevant fault codes, all pointing to the battery management system (BMS) and what it described as an ‘implausible voltage’ across the high voltage battery packs. One code was specific: an electric fuse malfunction in system 2.
Here is where it starts to get interesting.
Two BMS Units, Two Battery Packs
The Mercedes EQC 400 uses a high voltage battery with a gross capacity of 86 kWh and a net usable capacity of 80 kWh. What makes it unusual is that this pack is composed of two separate battery modules, each managed by its own Battery Management System – BMS A and BMS B.
Battery Management System (BMS): The BMS is the electronic control unit that monitors each cell in an EV battery pack. It tracks voltage, temperature, and current. It also handles cell balancing – the process of keeping all cells at equal charge levels so the pack performs as a single unit.
The two packs are connected in a combination of series and parallel configurations to achieve the vehicle’s full operating voltage and range. Two separate BMS units means two separate sets of data – and two separate failure points.
State of Charge Imbalance: 89% vs 23%
When we read live data from both BMS units, the numbers were stark. Pack A (BMS A) showed a state of charge of 89%. Pack B (BMS B) showed 23%.
This is a significant finding. In a healthy EQC, both packs should stay closely matched. When one pack charges and the other does not – or one discharges faster than the other – the car’s usable range drops to the capacity of the weaker pack.
The fault code pointing to ‘electric fuse malfunction in system 2’ suggested something was physically stopping Pack B from being used or charged normally. That led us to the next stage: getting the battery out.
BMS live data from the vehicle before repair:
| Measurement | Pack A (BMS A) | Pack B (BMS B) |
|---|---|---|
| State of Charge | 89% | 23% |
| State of Health | Good | Good |
| Cell balance | Good | Good |
| Voltage (nominal) | 395V | 345V |
| Fault codes | None | Electric fuse malfunction |
Table 1: Live BMS data from the Mercedes EQC 400 high voltage battery before repair. Source: OGS Mechanics diagnostic data, 2025.
The Diagnosis Process Step by Step
Before removing the battery, we made the high voltage system safe. This means disconnecting the 12V battery, pulling the high voltage service plug, and confirming with a meter that no live voltage is present at any HV connector. This is non-negotiable on any EV repair.
Physical Inspection Under the Car
Once the car was on the ramp, a visual inspection of the underside revealed a small amount of damage to the lower edge of the battery casing. It was not catastrophic – no cracks through the casing, no signs of fire, no visible cell damage. But something had impacted the bottom of the pack. This kind of damage is common on EVs with low-slung battery packs. The EQC sits its pack low to the floor for handling balance, which makes the bottom of the pack vulnerable to ground-level impacts.
Removing the Battery and Checking Fuses
Dropping the battery from an EQC is a significant job. The pack weighs approximately 625-650 kg. It requires a hydraulic platform table to support it during removal. The battery connects to: the high voltage cabling at front and rear, the coolant circuit (liquid-cooled), and the vehicle body via multiple securing bolts hidden behind the under-tray covers.
Once the pack was out, we moved it to a dedicated work bay. We opened the front cover expecting to find a blown fuse. The fuses in the front section all passed testing.
Here is where we made our first mistake. We were so focused on finding a blown physical fuse that we did not stop to run the full diagnostic test sequence before going further. That decision cost time, and we are documenting it here precisely because it is a mistake anyone can make on a first encounter with this battery architecture.
The rear cover is sealed with silicone sealant. After treating the sealant with a softening compound and removing all securing bolts, we used pry bars to break the seal. Inside, we found a pyrofuse with an inconsistent resistance reading – appearing near-open under load, but near-normal resistance measured off-vehicle. Something was not adding up.
What Is a Fusillock and Why Does It Matter?
A pyrofuse is a safety device built into EV high voltage systems. When the vehicle’s crash sensors detect an impact above a defined threshold, the airbag control unit sends a signal to the pyrofuse. A small pyrotechnic charge fires, severing the high voltage circuit in milliseconds. This isolates the battery from the rest of the car, protecting occupants and first responders from electric shock.
A pyrofuse that has activated, looks to a technician, exactly like a blown fuse. The circuit is open. But unlike a blown fuse, a fired pyrofuse cannot be reset by simply replacing the component.
The safety system that triggered it – called a fusillock in Mercedes diagnostics – remains locked until a specific software reset procedure is carried out using manufacturer-level tools.
A triggered fusillock can produce all of the following on a diagnostic scan:
- Multiple high voltage system fault codes
- BMS communication errors between Pack A and Pack B
- State of charge imbalance between packs
- Reduced or near-zero displayed range
- Car drives, but with heavily restricted performance
All of these symptoms look exactly like a failing or failed battery. Which is why a less thorough diagnosis jumps straight to ‘replace the battery.’
The fusillock function is password-protected in the Mercedes diagnostic system. To perform the reset, the workshop must request a time-limited access code from Mercedes, apply it through the dealer tool, and then carry out the full reset and reprogramming sequence.
The Real Fix: BMS Software Reset and Update
Once we connected the battery back to the car and ran the full diagnostic procedure with the dealer tool, the path became clear. The fault code for BMS system 2 pointed not to a hardware failure but to a software reset procedure. The description in the dealer tool read: ‘resetting the function fusillock.’
We had been chasing a physical fault that did not exist.
The reset procedure required:
- Requesting the fusillock reset password from Mercedes support
- Applying the password in the dealer diagnostic tool
- Running the full BMS reset sequence
- Completing a software update across both BMS units and related HV control modules
The software update alone took approximately 50 minutes. It covered both BMS A and BMS B as well as several related control modules in the high voltage system. After the update completed, we cleared all fault codes and turned the ignition off for a five-minute rest period.
When the car came back to life, the dash showed 219 miles of range. No warning lights. No fault messages.
And this is the part that matters most.
The car needed no new parts. No replacement battery. No new cells, no new modules, no expensive OEM hardware. The total charge to the customer was £860 plus VAT, covering the investigation time, BMS reset procedure, software update, and road test.
The original dealer estimate for a replacement high voltage battery: £56,000.
We carried out a road test of approximately 30-40 miles including motorway driving to confirm the repair held under load. The range held steady, battery balance remained correct, and no fault codes returned. In the post-repair update, more than a week after collection, the customer confirmed the car had covered several hundred miles with no issues.
EV Battery Replacement Costs in the UK: What You Should Actually Pay
Not every EV battery fault is a software issue. Some cars do genuinely need module replacements or full pack replacements. But before accepting any large quote, it is worth understanding what these repairs actually cost in 2025.
EV Battery Repair & Replacement Cost Ranges in UK Market – 2025
Estimated costs in GBP across common service options — low to high range
| Service Option | Min (GBP) | Max (GBP) | Cost Range |
|---|---|---|---|
| Software Reset / BMS Recalibration | £200 | £900 | £200 – £900 |
| Single Module Replacement | £1,500 | £4,000 | £1,500 – £4,000 |
| Hybrid Battery Replacement | £3,000 | £5,000 | £3,000 – £5,000 |
| Full EV Pack (Mid-Range) | £8,000 | £15,000 | £8,000 – £15,000 |
| Full EV Pack (Premium) | £15,000 | £30,000+ | £15,000 – £30,000+ |
Figure 1: EV battery repair and replacement cost ranges in the UK (2025). Source: (Greentec Auto UK, 2025)
The key data points every EV owner should know:
- Full EV battery replacement in the UK typically costs between £8,000 and £15,000 for mainstream models.
- The average EV battery replacement cost across UK models is approximately £7,200.
- Independent EV specialists typically offer refurbished packs at 30-40% less than main dealer prices.
A 2025 UK study found 8 out of 10 used EVs retain over 90% of their original battery health.
Global battery pack prices fell to an average of $111 per kWh by end of 2024, down from over $400 per kWh in 2012.
Before agreeing to any battery replacement, take these steps:
- Get a second opinion from an independent EV specialist
- Ask specifically whether a software reset or BMS recalibration has been performed
- Request a state-of-health report for each individual module, not just the overall pack total
- Check the vehicle’s accident history – a triggered pyrofuse explains many ‘battery failures’ that are not battery failures at all
Most EV batteries are expected to last 10-15 years before capacity drops below 70% of original. A warning light is not a death sentence for the battery. More often than not, it is a car asking for attention that a properly equipped independent workshop can give.
Frequently Asked Questions
What does a high voltage battery warning light mean on an EV?
How much does it cost to replace a Mercedes EQC battery in the UK?
What is a BMS reset and will it fix my electric car’s range problem?
What is a pyrofuse in an electric vehicle?
Can a software update fix an EV battery problem?
Why does my electric car show half its normal range?
What is state of charge (SoC) imbalance?
How long does a Mercedes EQC battery last?
Should I trust a used EV bought from auction?
How do I know if my EV needs a new battery or just a reset?
Conclusion
A Mercedes EQC high voltage battery fault can look terrifying on paper. A warning light, a range that has collapsed to a fraction of normal, and a dealer quote running into tens of thousands of pounds is enough to write off a car that has years of useful life left in it.
This case proves that the first diagnosis is not always the right one. The battery on this EQC was healthy. Both packs showed good state of health and good internal cell balance. What the car had was a locked fusillock – a safety function most likely triggered by an undisclosed accident – combined with out-of-date BMS firmware that needed a manufacturer-level reset and update.
Before any EV goes for a battery replacement, these steps should happen first: a full module-level diagnostic scan, BMS data review per individual pack, a check of the vehicle’s accident history, and a software reset procedure if any fusillock or safety lock fault codes are present.
The difference between getting that right and getting it wrong was £55,140 on this job.
