smart ED4 17.6kWh battery repair (feat. P1D2785 / P1D2885 / P0DE71C / bricked BMS)

The battery of the smart ED4 very reliable and robust, however, it still can fail some times. In the following I would like to list everything that is available and how to repair or replace it:

  • Desiccant cartridge
  • Bricked from an accident
  • Deep discharge (P0DE71C)
  • Battery cells & module replacement
  • HV contractors

You will also find useful additional information on how to prevent these defects. In my shop you'll also find the offer to reset you BMS from error codes you can'd delete anymore.

smart eMotion would be nothing without its community and supporters:

Most of all, I want to thank the anonymous smart eMotion reader for entrusting us. He waited patiently until we had everything together to repair her battery. His trust was the trigger that we took on the subject.

I would particularly like to mention Thomas Rebele from Tom's Garage. As a motor vehicle master with high voltage certificate, Thomas is a great help. His workshop in Stuttgart (Germany) has helped smart eMotion readers by repairing smaller and larger defects. In his private life, he is fully electric, with a smart ED3 convertible and a Tesla Model 3. In the same way, all workshop employees are now fully electric.

And last but not least, Vanja Katic from the EV Clinic, he contributed the photo of the battery cell so we didn't have to destroy the battery for it.


Nevertheless, electric cars are vehicles with up to 400V on-board networks, some even up to 800V. It is a great advantage to know what that means: Even if there are a lot of safty features in the car, incorrect handling can endanger life. It is always advisable to visit an experienced workshop whose employees have a HV certificate to work on high-voltage components. For private individuals, this is only at your own risk.

Before working on high-voltage components, always pull the service disconnect. This reliably de-energizes the high-voltage system. All high-voltage cables are clearly identifiable as these thanks to the orange plastic sheaths.

Desiccant cartridge

smart ED4 17,6kWh Akku - Accumotive E18-2evo Teardown & Analyse

The desiccant cartridge ensures that any humidity in the battery is bound so that it does not condense anywhere.

It is screwed into the battery from below and can be easily checked and replaced, if necessary. As long as it rattles nicely when you shake it, it is still good. Usually you don't have to change it because the battery is glued, screwed and riveted very well. In case you need a replacement, you can visit your dealer and get one with the part number A 789 340 33 00.

Bricked BMS from an accident or a digital drivers logbook

If your BMS is bricked from an accident, you'll only need to pull the BMS, get it reset and place it back in to the battery. You can order a reset in the smart eMotion Shop. Please verify before, that you'll only need to unbrick it, as the battery isn't deep discharged. You can check that with the DIY battery tool.

It can also happen that the battery, unnecessarily, sets the HV contactor cycle counter down from 300,000 to 0. This occurs in rare cases when using, for example, digital drivers logbooks that are plugged into the OBD port. Unfortunately, it is not known why this is so. (Error code: P1D2771; P1D2777; P1D2785; P1D2871; P1D2877)

How you remove the BMS from the battery is explained in the following section. Together with all informations you'll need in case that battery is also deep discharged:

Deep discharge (P18051C)

If a smart ED4 has been standing around for many months, it can happen, that the 12V battery is empty. If this happens, the smart assumes that the 12V battery has been removed. It then switches the smart into "junkyard mode", since the 12V block is always removed immediately at junkyards. In junkyard mode, it completely discharges the HV battery via the CSE, so that the battery is also internally voltage-free and can no longer cause surprises.

The topic is more widespread in the USA, where the smart is often stored away in winter and taken out of its warehouse after six months. As long as the vacation does not last longer than 4 weeks, this is not an issue. (With which battery level do I park my electric car while I'm on vacation?) Should it take longer, I recommend keeping the 12 V battery fully charged by a classic 12 V charger.

What to do when you have the problem?

Then it becomes time-consuming, there are now two construction sites:

  • The BMS must be reset because a permanent error is stored.*
  • The battery needs to be opened and recharged.

* in case one or more cells have less than 3.0 V. You don't need to reset, if every cell it above 3,0 V.

Reset the BMS:

If the battery is deeply discharged, the BMS has stored a permanent and non-erasable error: "P18051C" This cannot be erased with the usual Mercedes workshop tools. To get rid of it, you have to set up the BMS from scratch.


The BMS has send the command to discharge to the CSE, even if the BMS is removed, the three CSE will discharge all cells again because they are still in junkyard mode. This only stops when the reset BMS has been reinstalled. It is therefore advisable to have the BMS reset first and then to charge the battery.

EUR 749.95

Open and recharge the battery:

After all, the battery can usually still be saved, the cells of the smart ED4 are rather robust and can even withstand temporary total discharge. The first thing to do is to determine which cells are deeply discharged. That is suitable for this DIY battery tool. If it shows voltage values which correspond to "65535", the corresponding cell is deeply discharged. The same applies to all cells with an open circuit voltage of < 3,2 V.

Before removing the battery, you have to pull the Service Disconnect and before you open the battery: You will find an illustrated removal and teardown of the battery here. With its help, you know what to expect and what it looks like in the battery before you open it. If the battery is then opened, the question naturally arises as to where the deeply discharged cells are. This overview will help you:

Schematische Draufsicht E18-2evo EN


If the voltage of a battery cell drops to <2.5 V, it should no longer be charged, according to pure teaching. There is then a risk of fire. Although some users around the world have shown that the cells of the smart ED3 can even be saved with up to 0.5 V and even less, I can of course not recommend such a rescue attempt. Charging and reviving a deeply discharged battery is the last resort and is always at your own risk.

At best, all cells are evenly discharged, then you can charge the battery at its poles with a laboratory power supply. Depending on your options, all modules at the same time, with a 400V laboratory power supply or the modules individually with a 120 V laboratory power supply. Since the lower final voltage is 320 V, it is advisable to charge everything to 330 V to 340 V. When you're done, every cell should be at > 3,2 V, usually I target 3,6 V.

As long as the cells are deeply discharged, you should not load them with more than 0.25 A. We used a 500 V laboratory powersupply and charged all cells together. From 230 V to 320 V this takes roughly 10 hours.

If a single cell does not keep up, it might already hasn't had the same voltage of the other cells when you started recharging the pack. In this case you need to individually recharge it. Therefor you won't have to dismantle the cooling plates. Just use the copper pads of the cells, which are also being used by the CSE to monitor the individual cell voltages. If the cells still won't cooperate, it is unfortunately short-circuited. Then only battery cells or module replacement can help.

Once the battery is charged, you have to reinstall the removed busbar and screw the BMS into the housing. Now you can install the battery in the smart and start it. To reseal the battery, I would use car body adhesive

Due to the deep discharge, it is possible that the battery is not particularly well balanced. You can check this with the DIY battery tool:

If the cell drift is less than 100 mV, everything is fine. If it is more than that, you have to help the battery to balance itself a little: You do this by driving it to < 20 %, then leaving it to stand for 30 minutes, then plugging it in and then charging it to 100%. Leave it for multiple hours plugged in. If it wasn't enough the first time, you have to repeat this process.

Battery cells & module replacement

smart ED4 17,6kWh Akku - Accumotive E18-2evo Teardown & Analyse

Replacing individual modules requires some specialist knowledge and should only be carried out by someone who knows what he is doing. At this point I would recommend Vanja Katic from the EV Clinic in Croatia, for example. I know him personally and know that he has already done this successfully.

It is again somewhat easier to exchange entire modules. These are also not linked to the BMS, so they can be exchanged without further training.

Unfortunately, individual cells or even entire modules are very rare. Here, too, I would recommend you to exchange ideas with Vanja.

HV contactors

smart ED4 17,6kWh Akku - Accumotive E18-2evo Teardown & Analyse

It is rare for the HV contactors to fail, but it has happened. Since they are also mechanical, it can happen that they jam or something breaks internally.

Replacing the two HV contactors is relatively easy. in the Teardownw do you see where they are located. They are classically interchangeable and their function can even be checked using a 12 V power supply. To do this, please remove them from the battery.

After installing the battery in the smart again:

Check if there is still enough cooling fluid in the tank. If you need to replace it, it's just a water glycol mixture. As usual, you have to mix it up in such a way that it cannot freeze at your place.


Working on the HV battery is definitely not for untrained personnel, but also not something that cannot be learned.

Opening the battery and recharging it manually is outside of any regulations and should be the last option. Same goes for just reseting the BMS. Still, it is of course better than having to scrap the smart.

Overall, the battery of the smart ED4 is very nice to work with. Almost everything is modular and interchangeable as it was screwed in.

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