Read the remaining capacity of your smart electric drive yourself

Here is an overview of all relevant data that you can read out with the tool. A details explnation of every value can be found here.

General data of the car

  • Mileage of your car
  • Manufacturing date of the battery

Details of the battery

  • State of charge of the battery (SoC)
  • Current battery voltage
  • Current voltages of the individual cells
  • Current voltage difference between the weakest and strongest cell
  • Specifying how precise the last remaining capacity measurement was
  • Current temperatures of the 3 modules inside the battery
  • Remaining capacity of the battery (SoH)

Information about the installed charger and, while charging, to the charging station

  • Maximum power (vehicle / charging station / cable)
  • AC voltage
  • DC current when charging

Details of the cooling system and subsystems

  • Current temperature of the coolant of HV components
  • Outside temperature
  • Cooling fan
  • Capacity coolant pump (only ED3)

Operating hours counter (only ED3)

  • Cooling fan
  • Cooling pump
  • Battery heating
  • Brake booster

First of all you need the hardware:

  • Arduino Uno (see also)
  • CAN bus shield with OBD2 connector
  • USB A to USB B cable
  • OBD2 16Pin to DB9 Serial Port
  • PCB header in 2.54 grid (depending on CAN BUS shield)

Pin assignment for the DIY battery tool

You will find everything in the usual online shops. If no PCB headers were needed, soldering will be spared. When assembled, it should look like this:

DIY battery tool

Then the software:

Depending on your smart's year of construction the required software varies:

In case that's your first Arduino project, here you can find the Arduino software. after installing it, start it once, so it can create all the necessary folders. When this is done, it has created an Arduino folder in the documents folder of the logged in user. In these than place the smart specific software. That should look like this:

Arduino folder structure

Now you can connect the Arduino Uno to the computer. Select "Tools > Board > Arduino / Genuino Uno". Besides, in tools still select the port over which it is connected, which is in this case USB.

Now you can open the software for the smart:

Year of construction 2012 to 2016: File > Sketchbook > sketches > ED_BMSdiag > ED_BMSdiag

Year of construction 2017 until today: File > Sketchbook > sketches > ED4scan > ED4scan

If your smart is from 2017 or newer and has a 22 kW charger:

Open the "_OBL_dfs.h" tab and switch "boolean const FASTCHG" from "false" to "true".

Now you want to upload it to the Arduino board, then you can open the serial monitor.

Auswahl Arduino und Board DE

The serial monitor greets you with incomprehensible something. To fix this, set the line break (CR) and baud rate 115200.

Command serial monitor EN

If you now connect the tool with the smart you are ready for use. The required OBD port is located in the drivers footwell.* In the ED3, you'll find it, on the right hand side, in the ED4, it's on the left hand side. It's easy to locate, when you sit next to your car, on your knees. If the smart does not respond, simply press any button on the car key and the CAN bus will wake up again.

*If you're smart is right hand driven, it's in the passenger footwell.

Unlocking the 7,2 kW charger does only work for the smart ED4. As it has got a 7,2 kW charger by factory, which is limited to 4,7 kW in the EU. The smart ED3, on the other hand, has got a 3,7 kW charger, which can't do more than 3,7 kW.


Although the standard 4,6 kW charger is actually capable of charging with 7,2 kW, this was deliberately limited to just 4.6 kW for the European left-hand drive market. The reason for this is the unbalanced load, which is caused by 7.2 kW.

When the 7,2 kW is activated, the vehicle warranty expires and I assume no liability whatsoever. The use of this hidden function is at your own risk!

To enable the function in the tool, the setting window for the current consumption of the charger must be increased from 20A to 32A. You have to do this in the tool's source code before you upload it.

Once this has been done, the charger can be set to 32A using set ac_max 32 -yes. You can set it back to 20A in the same way.


When the command is executed, it is important that the smart is not charging, it must not be plugged in. This explanation for the activation is deliberately so general as it is not for everyone. It is very important that you know what you are doing as nobody will be held responsible for any mistakes!

It's not difficult to assemble and set up the tool yourself. However, if you do not want to handle the soldering iron, you can also buy the tool here and support EQpassion.

For the money you get ..

... the tool send. Already equipped with the hardware, you no longer have to solder anything. In addition, the right software for your smart is already installed and instructions for commissioning are included. There is also noted where you can get the right software for your LapTop and how to set it up.

starting at EUR 139.96

In the following I'm only refering to ED3 and ED4. It's about which generation your smart belongs to. If your smart is from 2012 to 1016, it's an ED3, it's from 2017 or newer, it's an ED4.

When you connect the smart ED3, the tool automatically shows all values. To see all values in ED4, please enter "/all". The lines differ a bit, so I list how each line looks like, so you'll find the value you're looking for. If there is more information in the line than what you are looking for, the extra stuff is grayed out.

Here is an overview of all commands:

For smart ED3

For smart ED4

General data

Mileage of your car

With the smart ED4:

ODO: 62872 km, Amb.T: 29.4 degC

With the smart ED3:

Time [hh: mm]: 5:45 pm, ODO: 45074 km

The mileage is displayed in whole kilometers and refers to the smart, not to the battery alone. The specification is very easy to understand. It should be pretty close to what the on-board computer says. ( +/- 5km)

Manufacturing date of the battery

With the smart ED4:

Y/M/D: 2015/2/17

With the smart ED3:

Battery Production [Y/M/D]: 2015/2/17

The values are sorted by year, month, day. This means that the exemplary battery here was made at the 17th of February 2015.

Details of the battery

State of charge of the battery

With the smart ED4:

SOC: 21.0% = 3.16 kWh = 22 km

rSOC: 22.6, 24.4, 24.7%

With the smart ED3:

SOC: 100.0%, realSOC: 96.0%

The SOC ("state of charge") indicates the level of batteries. That's the value you can also read off the display directly in the car. The rSOC ("real state of charge") refers to the actual charge of the battery. If you have ever driven your smart to 0%, you know that the smart will not stop yet, there are still a few percent available. When the rSOC is at 0%, the smart does not drive until it has been recharged. That is similar when it's fully charged, if the smart shows 100%, it is not actually at 100%.

When discharging, the SoC eventually overtakes the rSoC. But this does not happen linearly. It's also a question of what the programmers of the BMS have decided, what the user should see in which situation.

It's also worth mentioning, that both, rSoC and SoC, are always an estimate. This is the case with every car and every battery. Accordingly, it fluctuates simply because the BMS has to constantly correct its estimates.

Current battery voltage

With the smart ED4:

HV: 347.3 V, 57.88 A, 20.10 kW

With the smart ED3:

HV: 389.0 V, -0.78 A, -0.30 kW

The battery voltage changes depending on how full the battery is currently and is given in volts. If a voltage of <100V is specified, the HV contactor is not energized, thus de-energizing the HV mains outside the battery. The voltage window of the ED4 is between 320V and 400V, the ED3 is between 310V and 390V.

Behind the battery voltage, you can also see the power the battery is currently being charged. While, in this example, only the DCDC converter is recharging the 12V battery at ED3, the ED4 is charging at 22kW.

Current voltages of the individual cells

With the smart ED4:

#; mV; As / 10

01; 3619; 16778

02; 3620; 16836

03; 3616; 16689

04; 3616; 16652

05; 3615; 16601


With the smart ED3:

#; mV; As / 10

01; 4164; 17606

02; 4170; 17855

03; 4169; 17898

04; 4162; 17940

05; 4147; 17983


One cell after another is listed here. At the very front is the number of the cell, then its current voltage in mV and then the, by theBMSpredicted, current energy content in As. These values alone do not say too much. Only if they very a lot you should take a closer look. Up to 100mV are still ok.

It's also perfectly normal for one cell to dodge a bit out of the line, that's the "reference cell". But even this should not deviate more than 100mV from the others. In addition, it is possible that the cell voltages are at "65535". Then the cell is clearly deeply discharged and the tool can no longer display a precise voltage.

Current voltage difference between the weakest and strongest cell

With the smart ED4:

CV mean: 3617 mV, dV = 17 mV, s = 2.51 mV

CV min: 3606 mV, # 78

CV max: 3623 mV, # 33

With the smart ED3:

CV mean: 4167 mV, dV = 38 mV, s = 4.97 mV

CV min: 4147 mV, # 5

CV max: 4185 mV, # 63

Here's listed how much voltage difference between the strongest and the weakest cell is at the moment. By nature, the values are closer together at 100% than at <20%. It's also listed which cell is currently the strongest (CV max) and which is the weakest (CV min).

Specifying how precise the last remaining capacity measurement was

With the smart ED4:

Estimate: 0.514 of 0.781, 15 day (s)

With the smart ED3:

Last measurement: 0 day (s)

Measurement estimation: 0.705

Actual estimation: 0.705

TheBMScalculates, from time to time, independently, a health value for the battery. It than also creates a value wich indicates how precise it thinks the health value should be. The value should be > 0.8 to be able to output reliable data. If it is below, the values are only a rough idea where the battery is. This forecast only relates to the forecastedremaining capacity of the battery.

It also shows when the last measurement has been taken.

Current temperatures of the 3 modules inside the battery

With the smart ED4:

Modules 1: 39, 40, 41, 40, 41, 42, 40, 41, 41

Module 2: 40, 40, 41, 41, 42, 42, 40, 41, 41

module 3: 40, 41, 41, 40, 41, 42, 40, 42, 42

With the smart ED3:

module 1: 33.9, 39.0, 35.8

module 2: 35.2, 39.9, 36.3

module 3: 34.0, 39.0, 34.9

Thebattery packof the smart electric drive consists of 3 modules. The output values indicate the temperature at various points in ° C. Usually the outer values (left and right) are slightly lower than the values in the middle of the module. The temperatures are normally completely irrelevant. If they are too high or too cold, the battery will automatically slow down it's performance.

Remaining capacity of the battery

With the smart ED4:

CAP mean: 16689 As / 10, 46.4 Ah

CAP min: 16470 As / 10, 46.7 Ah, # 16

CAP max: 16973 As / 10, 47.1 Ah, # 64

With the smart ED3:

CAP mean: 17980 As / 10, 49.9 Ah

CAP min: 16747 As / 10, 47.5 Ah, # 63

CAP max: 18200 As / 10, 50.6 Ah, # 48

The weakest cell rules the whole battery, no matter how good the others are. Accordingly, we are only interested in this cell. 100% and thus 17.6kWh correspond to 52Ah. By means of the rule of three you can then calculate which ROUGH state of heath your battery has got. The ED3 has, in this example, > 91.3% remaining capacity. Note, however, that theBMSbasically makes the battery normally worse than it really is.

Please note that this value is never given as precise as you wish. It can sometimes fluctuate around 1-2Ah. Unless you feel the missing range and the value is > 45Ah, everything is fine. In addition, please also note the value of accuracy defined by the BMS.

HV contactor cycle counter

With the smart ED4:

HV control: ON

Cycles : 184316 of 200000

With the smart ED3:

HV contactor state ON, low current: 6 s

Cycle left : 261961

of maximum cycles: 300000

The HV contactor has a theoretical maximum of x cycles, after which it is declared defective and the smart no longer drives. Depending on the circumstances under which the contactor is opened and closed, a different number of cycles are deducted. Ultimately, however, you don't have to worry about the contactor at all. You won't actually experience that the counter reaches zero. For example, my own smart ED still had over 260.000 cycles after 235.000 km.

If the counter is 0, you most likely have a smart fortwo or forfour from 2017. Then the contactor is not defective, but the BMS needs a reset. Read more here.

Information about the installed charger and, while charging, to the charging station

Current temperature of the coolant of HV components

With the smart ED4:

Cool: 39

With the smart ED3:

coolant: 30.2

Both generations of the smart electric drive have a liquid-cooled battery. This value is also in ° C and corresponds to the temperature of the coolant.

Maximum power (vehicle / charging station / cable)

With the smart ED4:

EVSE status: 13 A, State: C

Cable code: 1500 ohms

Charger max: 20 A, State: CHG

With the smart ED3:

User selected: 32 A

Cable maximum: 32A

Charging point: 16 A

Here are the different values that were adjusted before and during the charging process. EVSE / Chargepoint is the charging power that the wallbox allows. Cable maximum / Cable code is the maximum of the cable * and user selected / charger max is the maximum value of the car **

* While the ED3 it's value is in amperes, the ED4 indicates the value in ohms, since charging cables are encoded with a resistor. It is deposited which resistor stands for which current.

** On the smart ED3, the on-board computer can be used to set how fast the smart is allowed to charge at maximum. The ED4 is not adjustable on the on-board computer. Here it's the value deposited in the charger at the factory. (-> Charger ED3; -> Chargers ED4)

AC voltage

With the smart ED4:

AC L1-2-3: 403.0, 402.5, 400.5V

31.1, 31.4, 31.1 A; 21,647 kW

With the smart ED3:

AC L1: 232.7V, 32.1A

AC L2: 233.3V, 31.6A

AC L3: 234.2 V, 31.8 A

Here you can see the values that are displayed when charging. You can see the voltages from the grid. Depending on whether you have the surcharge 22kW charger or not only L1 or L1-2-3. All values refer to phase to neutral. Only the ED4 with 22kW charger refers to the voltage between phase and phase, because its charger, in order to charge 3-phase, actually needs three-phase current.

Behind it you see the current on the respective phase. All values are collected at the time of the measurement, that's not live data.

DC current when charging

With the smart ED4:

DC HV: 346.0 V, 57.70 A

With the smart ED3:

DC HV: 387.4 V, 58.7 A

Here you can see the current in amps, which is currently being charged into the battery. Of course, this varies depending on the battery voltage and charging rate. You can calculate the DC power with the current and the voltage. (-> Calculate DC power) The Efficiency measurements of the smart ED3 and smart ED4 are based on these values.

Details of the cooling system and subsystems

Outside temperature

With the smart ED4:

ODO: 62872 km, Amb.T: 29.4 degC

With the smart ED3:

Temperature: 31.0 degC

Here you can see the temperature value, which you also see on the on-board computer.

Cooling fan

With the smart ED4:

System: 55%

With the smart ED3:

Cooling fan: 0.0%

Cooling pump: 78.0%, 44 degC

This value indicates how much the fan is currently running. 0% means that it does not run at all, 100% means that it runs at maximum speed. With the ED3 you also see how much the pump of the cooling circuit is running.

Operating hours counter (only ED3)

Operating hours counter (only ED3)

With the smart ED3:

Cooling fan: 17 h

Cooling pump: 2137 h

Battery heater: 22 h, OFF

Vacuum pump: 42.864 h

All values are given in hours. From top to bottom you can see here the operating hours counter of the fan, the cooling pump, the battery heater and the pump of the brake booster. If your ED3 does not have a 22kW charger, it will not have any battery heater so it's counter will be at 0h.

I assume no liability for the information given here. These cars are my hobby and I like to share my experiences with others to make it easier for them to opt for or against.


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