This Mercedes EQC drove into the workshop on its own power, fully charged, showing 94 miles of range. It should have shown over 200. No accident. No warning signs. Just a car that had been sitting for a while. Part 1 covers the full diagnosis, what we found inside the battery, and how far we got before the next problem showed up.
Table of Contents
- The First Clue: A Misleading Full Charge
- What Nobody Tells You About the Mercedes EQC Battery
- What the Scan Revealed
- Why the Official Mercedes Fix Failed
- Opening the Battery: What We Found Inside
- The Manual Recharge and the BMS Data Transfer
- Frequently Asked Questions
- What Happens Next: Part 2
The car came in with a malfunction warning. It had drive. It showed a full charge. And it showed 94 miles of range.
For an EQC owner who knows their car, 94 miles is an immediate red flag. These cars are rated for well over 200 miles. No accident history. No flood damage. Not an auction vehicle. The owner confirmed the problem had developed after the car sat unused for a longer period. Until that point, the car had driven without issue.
This is not an isolated case. It is becoming a recognisable pattern on the Mercedes EQC, and the root cause sits inside the battery architecture, a design that most owners do not know exists until something goes wrong with it.
This is Part 1 of a two-part repair story. It covers the discovery, the diagnosis, the dealer tool failure, what we found when we opened the battery, and where we got to before reassembly. Part 2 covers the result.
The First Clue: A Misleading Full Charge
Definition: Range is the estimated distance an EV can travel on its current charge level. On a fully charged Mercedes EQC, the expected real-world range is 200-230 miles under typical UK driving conditions.
When a car shows 94 miles on a full charge, there are two possible explanations. Either the battery has degraded severely, losing more than half of its original capacity, or only part of the battery is being used. The car in question had covered 104,000 miles, which for an EQC is not high mileage. Severe degradation at that level would be unusual.
The second explanation turned out to be correct. Only part of the battery was active.
Here is where it gets interesting.
The EQC does not operate on a single battery. It runs on two. And when one of them is offline, range drops to roughly half of what the car is rated for. A 94-mile range reading on a full charge is telling you, precisely, that one of those two packs is not participating.
The question was why. And the answer required a full scan, a failed dealer procedure, a battery drop, and a specialist called Florin.
What Nobody Tells You About the Mercedes EQC Battery
The Mercedes EQC has an 80kWh battery, 86kWh gross, and 80kWh usable. What is not widely known is that this battery is built from two separate battery packs inside one sealed housing. They are connected in parallel, which means they share voltage output but together provide double the energy capacity of either one alone.
This architecture is not exclusive to the EQC. It appears across the Mercedes EQ range. The EQA, EQB, EQS, the same layout has been seen across multiple models. But the EQC is where this specific fault pattern is showing up most frequently in independent workshops.
Two Packs, One Housing
Inside the battery casing, there are six modules in total. These are split into two groups, two module groups of three, each forming one of the two battery packs. Each group is physically separate, chemically independent, and electrically managed by its own control hardware.
The two packs sit inside the same aluminium housing, sealed together with hard silicone, cooled by a shared cooling circuit, and connected to the rest of the car through the same relay and high-voltage cable run.
From outside the car, they look like one battery. From inside the diagnostic system, they are two.
The Three Controllers
Each battery pack has its own Battery Management System (BMS) controller. In Mercedes diagnostic software (XENTRY), these are labelled N82-3 and N82-4. Both controllers monitor their respective packs, cell voltages, temperatures, state of charge, and report that data upward to a third unit: the master controller, N82-2.
The master is the one that talks to the car. It decides when to allow charging, when to permit discharge, and when to declare a fault. When the master detects a voltage difference between N82-3 and N82-4 that it considers outside acceptable limits, it logs an error, restricts the affected pack, and in some cases writes a permanent fault code that does not clear through normal procedures.
That permanent fault code is the core problem on this car.
What the Scan Revealed
Connecting to the car’s systems produced a clear picture straight away. Battery Module A (N82-3) showed a state of charge of 10%. Battery Module B (N82-4) showed 95%. Two packs connected in parallel should track each other, rise and fall together, charge and discharge in sync. These two were completely out of phase.
The voltage numbers confirmed the picture.
| Measurement | Module A (N82-3) | Module B (N82-4) |
|---|---|---|
| State of charge | 10% | 95% |
| Cell voltage | 3.46V per cell | 4.16V per cell |
| Total pack voltage | 331V | 400V |
| Internal cell balance | 20mV — acceptable | 10mV — acceptable |
| Error code present | Yes — “replace module” | Communication faults only |
Table 1: EQC battery module scan data at workshop arrival. Source: OGS Mechanics diagnostic records, 2026.
Two things stand out in this data. First, the internal cell balance within each module is fine. Module A is balanced within itself. Module B is balanced within itself. The problem is not inside either pack, it is between them. Second, Module A carries the most serious error code: “high voltage battery module must be replaced.” That code points at the hardware, but the hardware is not the problem.
Research from Midtronics confirms that in high-voltage EV packs, cell voltage variations beyond 30 millivolts are considered early signs of imbalance (Midtronics, 2024). The internal balance of each individual module here was well within that range. The inter-module voltage gap was the trigger.
And this is the part most people miss.
The diagnostic records from the master controller (N82-2) confirmed when the fault was originally triggered. At that moment, Module A was at 3.7V per cell. Module B was at 3.42V per cell, a difference of 0.28V. Both were at roughly 10% state of charge. The car was put on charge. Module B started charging. Module A did not. The master had already locked it out.
Why the Official Mercedes Fix Failed
The Mercedes XENTRY dealer tool offers a structured test procedure for this fault code. You navigate to the N82-3 module, select the voltage fault, and the tool launches a rebalancing procedure. The process states it will take 180 minutes. The precondition is that Module B (N82-4) must be at a low charge level before the process begins.
The theory behind it: discharge Module B to match Module A, run the 180-minute equalization process, then put the car on charge, and both packs should charge together, restoring balance.
That procedure was run on this car. Not once. More than twice, with full discharge cycles in between. Every time, the same result: Module B would charge after the process. Module A would not move. The error code remained.
This is consistent with behaviour reported by other EQC owners who have gone through the dealer procedure for similar faults. The XENTRY procedure assumes the fault is a live, recoverable imbalance. It does not account for a permanently stored error code in the BMS controller’s non-volatile memory.
Mercedes’ wider BMS sensitivity is documented. In June 2024, approximately 15,000 EQE and EQS vehicles were recalled for a separate BMS software issue where the management module could be overloaded by diagnostic requests. The EQ range’s BMS architecture appears sensitive to edge-case conditions, and once a fault is written to permanent memory, the standard dealer route cannot clear it.
The decision was made to pull the battery.
Opening the Battery: What We Found Inside
Dropping the battery from an EQC is a significant job. The preparation includes disconnecting the coolant circuit, removing high-voltage power cables, disconnecting the BMS connectors, and physically lowering the unit from the underside of the car. The battery itself is a large, heavy aluminium assembly.
Once on the bench, opening it presented its own challenge. The cover is sealed with hard silicone, a thick, industrial-grade sealant between the aluminium lid and the battery body. A standard approach of forcing the cover off risks damaging the housing. The correct method uses a chemical sealant softener applied to the join, followed by careful, even pressure from multiple pry points at once.
Three pry tools placed at intervals. Pressure applied gradually and held. The sealant released slowly. The cover, when it came off, turned out to be a substantial piece of aluminium on its own.
Inside: six modules visible, the dual-pack layout confirmed. Two module groups. Each group has its own BMS controller, part number matching on both sides, identical hardware. The master controller sits outside the main battery housing, on the pack’s external face.
The physical battery is in good condition. No swelling. No damage. Coolant pathways clear. This was always a software and data problem, not a hardware failure.
At this point, the team connected directly to the battery on the bench and measured the module voltages.
| Module Group | Pack Voltage (measured at relay) |
|---|---|
| Module A (N82-3) — offline | 331V |
| Module B (N82-4) — working | 399V |
Table 2: Pack voltages measured directly at battery relay terminals. Source: OGS Mechanics, 2026.
The gap: 68 volts between the two packs. Both internally balanced. The cell imbalance code in N82-3 reading the discrepancy from the other side, not from within its own module.
The theory solidified: this was a software glitch, not a battery fault. The cells themselves are fine. The fault record in the N82-3 controller is the problem.
The Manual Recharge and the BMS Data Transfer
Step 1: Manual recharge of Module A.
The offline pack (Module A, N82-3) was charged directly using an external bench charger connected to the module’s output terminals. Target: bring Module A up to match Module B, both at approximately 400V total pack voltage.
This is not a procedure in the dealer manual. It requires direct connection to the battery terminals, bypassing the vehicle’s charging circuit entirely. The process was monitored continuously using the battery’s 32 internal temperature sensors, watching for any temperature rise on the connectors that might indicate a poor connection or unsafe condition.
The charge progressed without incident. At completion: Module A at 400V. Module B at 399V. Both internally balanced at 20 millivolt cell spread.
But the error code in N82-3 had not cleared. The module still reported: “High voltage battery module must be replaced.”
Charging alone was not enough. The fault record was locked in the controller’s memory. This confirmed what the XENTRY procedure had failed to achieve: the error is permanent unless the controller’s stored data is modified directly.
Step 2: BMS controller data transfer.
This is where the repair moved beyond conventional workshop procedure.
N82-3 and N82-4 carry the same part number. Same hardware. Same software version. The only difference between them is the fault record stored in N82-3’s non-volatile memory. The healthy data in N82-4, which has no fault record, can theoretically be extracted, modified, and written into N82-3, replacing the corrupted memory block.
ECU programming specialist Florin from Car Keys Solutions was brought in to handle this. The two BMS controllers were removed from the battery. Florin extracted the memory data from N82-4, modified it, and wrote the clean data into N82-3.
The controllers were reinstalled. The battery was reconnected to the diagnostic system.
The “high voltage battery module must be replaced” error code was gone.
Other fault codes remained, communication errors and interlock faults from having components disconnected during the bench process, which are expected and normal in that state. But the primary fault, the one that had resisted every other approach over multiple days and more than six hours of XENTRY procedure run time, had been cleared.
Module A was no longer flagged for replacement.
The battery was reassembled and prepared for reinstallation in the car.
Watch the Source Video
George and the OGS Hybrid & EV Repair team, together with Florin from Car Keys Solutions, document this Mercedes EQC BMS repair in full in the video below.
Frequently Asked Questions
What does it mean when a Mercedes EQC shows 94 miles on a full charge?
Why does the Mercedes EQC have two separate batteries inside one pack?
How does low-charge storage cause a Mercedes EQC battery fault?
Does the Mercedes XENTRY dealer procedure fix the EQC dual battery imbalance fault?
What is the N82-3 and N82-4 controller in a Mercedes EQC?
Can a Mercedes EQC BMS fault be fixed by reprogramming the controller?
Is the EQC dual battery imbalance fault becoming more common?
What charge level should an EQC be at for long-term storage?
Will my EQC need a new battery if it has this fault?
How long does it take to fix a Mercedes EQC dual battery BMS fault?
What Happens Next: Part 2
The battery went back into the car with the primary fault cleared. The “high voltage battery module must be replaced” error code was gone. Both packs were at matched voltage. Both were internally balanced.
But when the system came back online after reassembly, there was still an error code. And the cell voltage spread across the pack was not sitting where it should be, ideally under 20 millivolts. The number showing was higher than expected.
Is this where everything started? Or is this a new problem that emerged from the process?
Part 2 follows the reassembly, the result when the ignition comes back on, and what the team found that still needed attention. Read Part 2
If your EQC is showing reduced range or battery warning lights after a period of storage, do not wait. The fault can worsen over time if the car continues running on one pack. Book a specialist EV battery diagnostic