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Mercedes EQB Battery Fault – Part 1: They Said New Pack. It Was One Fuse and a Software Glitch

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A Mercedes EQB arrived at OGS Hybrid & EV Repair from auction, refusing to charge and refusing to drive. This is the full story of what was really inside the battery, shown in the workshop video by OGS Hybrid & EV Repair.

Table of Contents

The official route for a fault inside a Mercedes EQB battery points one way: a new pack, because the internal modules are not sold separately. The owner of this EQB had also been told that dealer diagnosis alone runs at £200 an hour. What the workshop actually found inside was different. One blown fuse. A set of healthy cells sitting at a very low charge. And a software error that insisted two relays were stuck when both tested fine. This is a Mercedes EQB battery fault case study from the workshop floor, and it shows why “replace the battery” should never be the first answer.

The Short Answer

The EQB came from auction with no charge, no drive, and almost no history. The dashboard showed “Drive will be deactivated, charge high voltage battery” with the turtle symbol, which means limp mode, plus zero miles of range. The team dropped the battery, found and replaced one blown maxi fuse, manually charged the deeply discharged cells, and cleared a stubborn contactor error through chip-level data work on the battery control unit. The battery itself was never the problem. At the end of this episode the car was still fighting back with a second software fault outside the pack, and the job continues in the next video.

What the First Scan Showed

The car would not take a charge. Plugging it in turned the charge indicator red and the whole dashboard flashed red with it. The scan brought back only a handful of error codes, but two of them mattered.

The first was a BMS fault reading “electrical fuse B malfunction”. The BMS is the battery management system, the computer that watches over the high-voltage pack. The second error pointed at the air conditioning compressor, which raised the possibility of an isolation problem, a leak of high voltage to the car’s body.

That left a chicken-and-egg question. Was the car refusing to charge because the battery had discharged below its safe limit? Or was a high-voltage system error blocking the charge, and the flat battery was just the result? The state of charge reading gave a clue: 2%. The overall cell voltage told the fuller story at close to 3.4 volts per cell. Low, but not disastrously low.

Here is where it gets interesting.

A Blown Maxi Fuse Inside the Pack

The percentage alone does not tell a technician much. The cell voltage does. So the decision was made to take the battery out, both to check the fuse the BMS was complaining about and to charge the cells manually if needed.

The EQB pack turned out to be a serviceable design. The cover is sealed with a gasket and a light protective coating rather than hard silicone, which makes opening it a cleaner job than on many packs. Inside sat a pyrofuse, which is a small explosive-triggered safety fuse, and several maxi fuses, the heavy-duty fuses that protect the main circuits.

One of those maxi fuses was blown. There is no way to reach it without removing the battery and opening the pack, so the teardown had already paid for itself. The pyrofuse tested fine. A second fuse tested fine. The relays in the safety box, the unit that controls energy flowing in and out of the pack, were stripped, checked, and confirmed working.

But a blown maxi fuse leaves a question hanging. Fuses do not blow for fun. Something made it blow, and that something had not yet shown itself.

Charging the Cells by Hand

With the pack open, the team connected to the battery and set up a monitored manual charge. The cells sat at 3.40 volts, and the target was 3.70 to 3.80 volts, enough for roughly 10% charge or more. Live data ran throughout, watching cell performance and the critical temperature readings. Charging a lithium pack outside the car is a waiting game, and the monitoring is not optional.

New fuse in, cells charged, relays confirmed good. Battery back on the car. Half the battery showing, 20 miles of range. Progress.

Then the scanner delivered the twist.

The Contactor Error That Would Not Clear

The car now reported that both battery contactors, the positive and the negative, were jammed closed. Contactors are the heavy relays that connect the pack to the car. If they truly weld shut, that is serious.

Except the team had physically checked those relays days before. Both operated correctly. And both jamming at the same time is close to impossible. With the battery later stripped again on the bench, the error stayed stored even when the relays were not energised at all. The error was lying.

That pointed to software, not hardware. The team applied an available update covering the drivetrain and the DC-DC converter, followed the full reset procedure with ignition off, vehicle locked and bonnet closed, and opened a technical ticket to Mercedes headquarters. None of it moved the error. The workshop has seen this pattern on several cars: an error gets triggered, cannot be cleared through any official channel, and the official remedy becomes a new pack because the modules are not available separately.

So the team stepped off the official path.

A Chip-Level Fix on the Battery Control Unit

The battery came down again. The control unit went to Florin at Car Keys Solutions, who works at chip level on automotive electronics. The plan: get into the unit’s stored data and remove the false contactor error at its source.

On the bench, it worked. The error that had been permanent, the one no scanner could clear, was gone. Back on the car, after a 25-minute software update that reprogrammed several control units, the battery control unit reported clean. No red on the charge port. The pack, its cells, its fuses, and its brain were finally in order.

And this is the part most people miss. The battery was never dead. It was discharged, missing one fuse, and slandered by its own software.

The Second Box With the Same Fault

The celebration lasted about a minute. A new fault appeared, this time from the rear distribution box, a safety box outside the battery that integrates relays and controls energy flow towards the rear of the car. Its complaint was familiar: contactors jammed closed, both positive and negative.

The team opened the unit. The relays inside were open, exactly as they should be. The error was wrong, again. The dealer-tool verdict for this unit was blunt: replace it, no repair route offered. Reading and clearing the chip directly was not yet possible for this type of processor, so the team sourced a complete used distribution box and fitted it.

That cleared part of the picture, but the positive contactor error later resurfaced, and an attempt to transplant modified data from a control unit of a different configuration did not take. Ingenious, close, but not the fix. The lesson was already clear though: the team knew exactly what was needed. Good data from a matching EQB.

The £1,000 Donor Car

Finding a used EQB battery proved almost impossible, and buying one blind carried an obvious risk: the fault here was in control electronics, not cells, so a used pack could arrive with the same disease. What the job really needed was a matching donor car for a day or two.

The workshop paid £1,000 to have a similar-specification EQB, slightly damaged, brought in for two days. Its battery came apart on the bench so the data could be extracted directly from the chips of its control unit.

By the end of this episode the donor data was in hand, but the EQB on the ramp was still showing malfunctions, including some errors the team had already fought once. The car was not yet running. The battle continues in the next video, and the workshop is not walking away from it.

What This Means If You Own an EQB

Strip away the drama and this case leaves EQB and EV owners with four practical points.

A “dead” EV battery is often not dead. This pack needed a fuse and a charge. The cells were healthy. A full high-voltage battery repair assessment costs a fraction of a replacement pack.

Error codes describe symptoms, not causes. This car reported jammed contactors twice. Both times the relays were physically fine and the fault lived in stored data. Reading codes is the start of diagnosis, not the end.

Software is now half the job. Updates, resets, gateway procedures and control unit programming decided this repair far more than spanners did.

The official route can dead-end at “new pack”. When internal modules are not sold separately and a stored error cannot be cleared with official tools, replacement becomes the default answer. An independent specialist with chip-level capability has more moves on the board.

Watch the Source Video

The full Mercedes EQB battery fault investigation is shown in the OGS Hybrid & EV Repair workshop video below, from the first red-flashing charge attempt to the donor car teardown.

The story does not end here. The follow-up video picks up with the donor car data and the final push to get this EQB back on the road.

Frequently Asked Questions

Why is my Mercedes EQB not charging?
In this case the EQB refused to charge because of a blown maxi fuse inside the battery, cells discharged to around 3.4 volts, and stored software errors on the high-voltage system. A red charge indicator with BMS fault codes needs proper diagnosis, because the cause can be electrical, software, or both at once.
Does a Mercedes EQB battery fault mean the whole pack must be replaced?
Not automatically. In this repair the cells were healthy and the physical faults came down to one blown maxi fuse. The remaining problems were software errors in control units. The official route pointed to replacement because internal modules are not sold separately, but the pack itself did not need to go.
What does a “contactor jammed closed” error mean on a Mercedes EV?
It means the car believes the heavy relays connecting the battery to the vehicle are stuck shut. On this EQB the error appeared twice, once in the battery and once in the rear distribution box, and both times the relays were physically fine. The fault lived in stored control unit data, not in the hardware.
What is a maxi fuse in an EV battery?
A maxi fuse is a heavy-duty fuse inside the high-voltage pack that protects a main circuit. On this EQB one maxi fuse had blown, and it could only be reached by removing the battery from the car and opening the sealed cover. A blown maxi fuse always raises a second question: what made it blow.
How low was the EQB battery discharged?
The state of charge read 2%, with the cells at close to 3.4 volts. That is low, but not below the point of no return. The team charged the pack manually on the bench, targeting 3.70 to 3.80 volts per cell for roughly 10% charge or more, with live temperature monitoring throughout.
Can a stored EV error code be removed if the dealer tools cannot clear it?
Sometimes, yes. On this EQB a permanent contactor error in the battery control unit survived software updates, official reset procedures and a support ticket to Mercedes. It was finally removed by working directly on the control unit’s stored data at chip level, and the error stayed gone on the bench.
What is the turtle symbol on a Mercedes EQB dashboard?
The turtle indicates limp mode, a protection state where the car limits or deactivates drive power. On this EQB it appeared with the message “Drive will be deactivated, charge high voltage battery” and zero miles of range, because the pack was deeply discharged and the high-voltage system had stored faults.
Is it worth repairing an EV battery instead of replacing it?
This case shows why it is worth investigating first. The physical repair came down to one fuse and a manual charge. The expensive route would have replaced the entire pack without ever finding that the cells were healthy. A diagnosis-first approach protects owners from paying pack prices for fuse problems.

Conclusion

This Mercedes EQB battery fault looked like a worst-case story from the outside: an auction car, no charge, no drive, and an official path that ends at a new pack. Inside, the workshop found one blown maxi fuse, healthy cells that needed a careful manual charge, and software errors that accused working relays of being jammed. The chip-level fix cleared the battery’s own fault, a used distribution box dealt with the second, and a £1,000 donor EQB is now supplying the data for the final round.

The car is not finished yet, and the next video carries the job forward. If your EQB or any EV is showing battery faults, refusing to charge, or has been quoted a full replacement pack, contact OGS Hybrid & EV Repair in North London before signing off on a new battery pack.