smart ED3 17.6kWh battery repair (feat. P18051C / bricked BMS)

The battery of the smart ED3 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 (P18051C)
  • Current sensor
  • Battery cells & module replacement
  • HV contractors

You will also find useful additional information on how to prevent these defects. Should something break, you can find suitable used parts in the Shop, there you will also find the offer to reset the P18051C error.

smart EMOTION would be nothing without its community and supporters:

Most of all, I want to thank Anita, a smart EMOTION follower, for entrusting us. She waited patiently until we had everything together to repair her battery. Her trust was the trigger that we took on the subject. In the meantime we have worked on several batteries and can handle them very well by now.

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. He also had the tip to Pauliaus Habdank from PHS-Lab. For many years he has been a professional in software for all smarties and together with smart EMOTION he now also has all sorts of topics relating to the electric smart under control. He's also who's able to reset the BMS, which you can order in the Shop.

A great help has also been Willi. We were able to borrow his 500V laboratory power supply several times and get tips about HV batteries. He and his Company have been full professionals when it comes to Tesla Roadster repairs of all kinds for a long time.

I would also like to thank the ElectrifyBW trio around the Spanish car2go. They brought almost 100 decommissioned car2go smarties back into shape. The exchange with them is beneficial for both sides and has also helped a lot for this post.

Last but not least: Marcus Zacher from the GenerationStrom Blog. During his studies, he had worked at Accumotive and therefor is a big help at understanding how the battery works.


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 ED3 17,6kWh Akku - Accumotive E18-1 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 from an accident

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.

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 ED3 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. In the Shop I offer this with two options:

  • Resetting the BMS
  • Resetting the BMS & the error cannot be saved again in the future


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.

starting at EUR 799.95

Open and recharge the battery:

After all, the battery can usually still be saved, the cells of the smart ED3 are very 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 E18-1 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.

Before you charge the battery, you have to completely disconnect the current sensor. Otherwise it will break. The easiest way is to unscrew the busbar parallel to module 1. Together with the open HV contactor, it is completely separated from the charging process.

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 310 V, it is advisable to charge everything to 320 V to 330 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 power supply 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. It is also advisable to check the temperature regularly during the charging process:

smart ED3 17,6kWh Akku - Accumotive E18-1 Teardown & Analyse

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 ED3 17,6kWh Akku - Accumotive E18-1 Teardown & Analyse

Unfortunately, individual cells are not interchangeable*. They have been welded together using a ultrasonic welder. The CSE would notice if the resistances between the cells were no longer correct. You can only swap individual modules. These are not married to the BMS, so exchanging them is easy to do.

Whole modules are unfortunately very rare, the cells and the modules were built exclusively for the smart ED3. They are no longer made new, you only get those who are in accident smarties.

* I've got readers who told me, it's doable. But I haven't been able to verify it myself. If you want to try it, feel free to contact me via direct message to get support for that from me.


Bevor you close the battery again, check if the cooling loop is tight.

HV contactors

HV Schütz

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. They have been in my personal smart for over 200,000 km and that is also the rule.

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.

Of the P18051C error can be reset. Opening the battery and recharging it manually is outside of any regulations and should be the last option. Still, it is of course better than having to scrap the smart.

Unfortunately, individual battery cells cannot be replaced, only entire modules. But since these are not available for sale, it is difficult to get hold of them. Again, all other components of the HV battery can be found in the smart EMOTION Shop, there you can also have your BMS sent in for it's reset.

Overall, the battery of the smart ED3 is very nice to work with. Almost everything is modular and interchangeable as it was screwed in. Should you have any questions, I am happy to assist you.

EUR 645.00

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