Part 2 of the Mercedes EQB battery repair series. OGS Hybrid & EV Repair borrowed a working EQB from a viewer, copied the data from its battery control unit, and used that data to put a condemned EQB back on the road the same evening. Based on the workshop video by OGS Hybrid & EV Repair.
When this Mercedes EQB was first diagnosed, the official Mercedes test plan reached one conclusion: replace the high-voltage battery. Part 1 of this series showed why that conclusion was wrong. The cells were healthy, the only physical fault was a blown fuse, and the car was blocked by a stored error code in the battery control unit that no diagnostic tool could clear, including Mercedes’ own dealer equipment. The team at OGS Hybrid & EV Repair therefore attempted a Mercedes EQB battery repair that is not described in any service manual. They borrowed a second, working EQB, removed its battery, read the clean data from its control unit, and wrote that data into the faulty car. This article covers the full repair: the customer who drove 100 miles to lend his car, the new fault that appeared after the data transfer, the moment the car finally showed READY, and a complete breakdown of what the job cost.
Table of Contents
- The Short Answer
- The Donor Car Plan
- A £1,000 Ad and a £600 Answer
- Two EQBs, Two Batteries, One Evening
- The Wall After the Clone
- The Moment It Came Alive
- What the Repair Really Cost
- What This Means If You Own an EQB
- Watch the Source Video
- Frequently Asked Questions
- Conclusion
The Short Answer
OGS Hybrid & EV Repair posted an appeal on its social media channels, offering £1,000 for temporary access to a working EQB with the same battery type. A viewer responded and drove his own EQB down from Coventry. The team removed the batteries from both cars in a single evening. Florin, the software specialist on the job, read the data from the donor car’s battery control unit at chip level and wrote it into the control unit of the car that would not run. The transfer created one new problem: the corrected control unit no longer matched the software configuration of the HV safety box, the unit that controls the flow of energy in and out of the battery. A 23-minute software update and a replacement safety box resolved it. The EQB then showed READY for the first time in months, charged to full, and completed a motorway road test with an indicated range of around 250 miles.
That is the outcome. The rest of this article explains how the team got there, and what it cost.
The Donor Car Plan
A donor car is a working vehicle of the same model that supplies parts or data for the repair of another. This repair needed no parts. It needed a clean copy of the data stored inside a healthy battery control unit, and that data cannot be ordered from a parts catalogue.
The reasoning came directly from Part 1. Every component in the battery had tested sound, and the car was blocked by a single stored error code in the battery control unit, of a type that diagnostic equipment is not designed to remove. The team had already tried a modified version of the software, and the results were unreliable. That failure settled the question: only original data from an identical, running EQB would restore the control unit to a known-good state.
The team also considered buying a used battery, and rejected the idea. A second-hand pack can arrive with the same stored fault, or with faults of its own, and there is no way to verify its condition before installation. The donor approach was slower, but it started from the only condition that could be verified in advance: a car that demonstrably runs.
A £1,000 Ad and a £600 Answer
The workshop published the appeal to its YouTube and social media audience: £1,000 for access to a working EQB with the same battery type. One viewer responded. He drove his own EQB down from Coventry, an auction purchase with light cosmetic damage, a matching specification and a healthy battery.
He then declined the £1,000 and asked for £600 instead, enough to cover his travel, because he wanted to see the repair succeed. The team considered it a remarkably fair gesture and said so on camera.
The arrangement carried one constraint: he needed his car back the same day, roadworthy, for the drive home. Work started at 3:21 in the afternoon.
Two EQBs, Two Batteries, One Evening
The donor battery came out first. The team disconnected the cooling lines and the high-voltage connections, lowered the pack, removed the lid and worked down to the battery management system, or BMS. The BMS is the computer inside the pack that supervises the cells and stores the system’s fault memory. An identical BMS in the subject car held the corrupted record that had kept it off the road.
By 5:30pm the donor pack was open on the bench and Florin was reading its data at chip level. The subject EQB came into the workshop at the same time to have its own battery removed, for what turned out to be the fourth time in this repair.
Reading the Good Module
By 5:46pm both control units were on the bench: one from the running car, its data already read, and one from the car that had not moved in months.
Florin transferred the clean data from the donor module into the subject car’s control unit, overwriting the corrupted memory that dealer equipment could not reach. While he worked, the rest of the team rebuilt the donor car, whose owner still had a 100-mile drive home ahead of him.
The donor EQB was reassembled first and immediately showed a battery malfunction warning, with its owner watching. The team expected it. Powering up a battery module while the pack is disconnected writes error codes, because every event inside a traction battery is recorded. They cleared the stored codes, the car started normally, and it went onto a 70-minute coolant bleeding procedure. The bleed cannot be skipped: air trapped in an EV cooling circuit can damage battery modules or leave the car stranded, so the procedure runs to completion whatever the hour.
The subject car was then reassembled, and the theory was tested.
The Wall After the Clone
With the cloned data installed, the EQB should have started. It did not. At 9:30pm the car still refused to run, and some of the original error codes had returned to the scanner.
The failure put the whole diagnosis in doubt. If the BMS software had never been the cause, then the evening’s work, two batteries removed, a donor car driven down from Coventry, hours of chip-level programming, had been spent on the wrong fault.
The team ran one more check before drawing that conclusion. The cloned data carried a slightly different software configuration from the original, and an update was available to align it with the rest of the high-voltage system. The update took 23 minutes, completed successfully, and changed the diagnosis entirely.
The Safety Box That Was Out of Sync
The remaining fault now pointed at a different unit: the HV safety box, also called the cutoff box. It is mounted at the rear of the battery and houses the contactors, the heavy-duty relays that physically connect and disconnect the pack from the car. The error code stated that the contactors were stuck closed.
Testing showed they were not. The relays worked correctly, which meant this was another software fault: a mismatch between the newly corrected control unit software and a safety box still configured for the old, corrupted data. As with the original fault, the error stored in the safety box could not be cleared with diagnostic equipment.
The team had already replaced this box once, earlier in the repair, while the BMS still carried the wrong software. That mismatch had damaged the first replacement unit. The box came off a second time, another used unit went in, and the pack went back onto the car. Everyone understood the stakes: if this attempt failed, the diagnosis had been wrong from the start.
The Moment It Came Alive
With the second safety box installed, the team scanned the car before switching on the ignition. There is no value in starting a vehicle against a list of stored faults. This time the scan was different: the faults cleared, the high-voltage system reported no errors, and the status indicators showed green across the board.
Ignition on, foot on the brake, start button pressed. The instrument cluster showed READY.
For the first time in months the EQB had drive, gears and a working dashboard. Nobody in the workshop had ever seen the READY indicator on this particular car, because nobody had seen it running. The charger delivered the second confirmation: the charge light came on, turned green, and the pack charged to full without a fault.
The road test ran north on the M1 towards Watford. The dashboard stayed clear, battery statistics and temperatures were normal, and a small cell imbalance sat within acceptable limits. Indicated range on a full charge was approximately 250 miles. One low-priority fault remained on the auxiliary 12-volt circuit, to be monitored over further driving, and the car passed a final inspection before returning to its owner with no error codes in any system.
The timeline below summarises the evening.
| Time | Milestone |
|---|---|
| 3:21pm | Donor EQB arrives from Coventry, work starts |
| 5:30pm | Donor battery off, data being read at chip level |
| 5:46pm | Both control units side by side on the bench |
| 6:34pm | Donor battery rebuilt and going back on the car |
| 7:33pm | Donor car on its 70 minute coolant bleed, subject EQB reassembly starts |
| 9:30pm | Old errors reappear, BMS software update found and run |
| Late evening | Second safety box installed, all faults clear, READY light on |
Table 1: Timeline of the two-battery clone, as shown in the workshop video.
What the Repair Really Cost
Most workshops keep these figures private. The team put them on a board, in two columns: what the customer paid, and what the job cost the workshop.
The customer paid exactly what was quoted, around £2,400 plus VAT for the complete job. That covered removing and stripping the pack, replacing the blown fuse from Part 1, manually recharging the cells, and the largest single item, reflashing the BMS with clean data.
The workshop’s costs ran higher.
| Item | Amount |
|---|---|
| Charged to customer (whole job, as quoted) | £2,400 + VAT |
| Two used HV safety boxes | £600 + VAT each |
| Donor car access (owner asked for travel costs only) | £600 |
| Software updates on the donor car | £300 + VAT value, done as thanks |
| Battery removed and refitted four times | Unbilled workshop time |
| Approximate total cost to the workshop | Over £3,000 + VAT |
Table 2: Costs of the EQB repair, as itemised in the workshop video.
On paper the workshop lost roughly £1,500 before counting the additional hours, and it accepted the loss knowingly. Both safety boxes were destroyed by the software mismatch rather than by any hardware defect, and the four battery removals were the price of developing a repair nobody had documented before. In exchange, the workshop now holds a proven, repeatable procedure. The next EQB with this fault can be repaired without the development cost.
The comparison that matters is with the alternative. Mercedes offers one remedy for this fault: a new battery pack, the most expensive single component on the car. The entire repair, absorbed losses included, cost a fraction of that.
What This Means If You Own an EQB
Part 1 established the first principle: a battery fault code does not mean the pack has failed. Part 2 adds a second: even a stored fault that survives dealer equipment, official reset procedures and software updates can be repaired, because the data itself can be rewritten from a healthy source.
Three practical lessons to follow for any EQB owner given a battery replacement verdict.
First, insist on a full diagnosis before approving the expense. This car’s cells were healthy from the first scan to the last. The fault that condemned it was corrupted data, and no owner should pay for a new pack to fix a data problem.
Second, understand that the battery is a system of connected control units. The BMS and the HV safety box must carry matching software, and correcting one without the other creates new faults, which is how two safety boxes were lost on this job. Competent high-voltage battery repair addresses every unit in the chain, not only the one showing the code.
Third, be aware that chip-level repair exists. Cloning control unit data appears in no Mercedes manual and no main dealer offers it. The capability sits with independent control unit programming specialists, which is where this repair was developed. If your EV has a battery fault that nobody can clear, contact OGS Hybrid & EV Repair at 0208 346 6755 before agreeing to a new pack.
A footnote from the road test: the drive to Watford ended at Car Keys Solutions, the vehicle security specialists who handled the software side of this repair. For spare keys, immobilisers, alarms or van security, Car Keys Solutions at 0203 393 5669 offers 10% off to customers who mention OGS Hybrid & EV Repair.
Watch the Source Video
The full repair, including the data transfer, the setback and the moment the car starts, is documented in the video. On the published page this section carries the embedded YouTube player.
Two further repairs arrived at the workshop as this one ended: a Mercedes EQ shipped over from Ireland, and a Porsche Taycan with a 12-volt lithium battery that has its own control unit.
Frequently Asked Questions
Can a Mercedes EQB battery control unit be cloned from a donor car?
How much does it cost to repair a Mercedes EQB battery instead of replacing it?
What is the HV safety box on a Mercedes EQB?
Why did new error codes appear after the correct software was installed?
What is a donor car in EV battery repair?
How long does the Mercedes EQB coolant bleeding procedure take?
Will a Mercedes dealer clone battery data to fix an EQB?
What driving range did the repaired Mercedes EQB show?
Conclusion
This Mercedes EQB battery repair began with an official verdict of a failed pack and ended with a running, fully charging car showing 250 miles of range, without a replacement battery ever being fitted. The fault was never in the hardware. It was in the data, and the fix was a donor EQB from Coventry, a chip-level copy of its battery control unit, one software update, and a safety box configured to match. The workshop paid for the development in parts and unbilled hours, and now holds a repeatable repair that Mercedes’ own manuals do not contain. If your EV has a battery fault that nobody can clear, this car is the evidence that the pack is not always the problem. Watch the full video, then start with a proper diagnosis rather than a replacement.