# No-load voltage of lithium ion battery (10th order polynomials)

Joined May 17, 2014
60
Hello everybody
I'm going to simulate a Lithium ion battery whose no load voltage depends on its DoD (DoD=1-SoC).
I used the book of Soylu in which he obtained a 10th order polynomials for no-load voltage of the battery.
https://www.intechopen.com/books/electric-vehicles-modelling-and-simulations
I plotted the voltage versus its DoD. However, the obtained figure is not reasonable.
I am so thankful if you let me know what is the problem?
I put my mfile and the obtained figure.

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Joined May 17, 2014
60
It is worth mentioning that the datasheet of the battery gives the voltage according to the C-rate. Indeed, there is no information about the open-circuit voltage of the battery.
Thank you

#### KeithWalker

Joined Jul 10, 2017
1,398
Why do you need such precise data on the open circuit battery? You really can't do anything with it until you connect a load.
Regards,
Keith

Joined May 17, 2014
60
Why do you need such precise data on the open circuit battery? You really can't do anything with it until you connect a load.
Regards,
Keith
Yes, you're right, the battery is connected to the load. But, I should create a lookup table for no-load voltage irrespective of the load. This open-circuit voltage equation is needed for Electric vehicle simulation.
Besides, I figured out how to calculate this open-circuit voltage. I'm going to use the datasheet for two C-rates and then according to the two look-up table, I want to achieve the open-circuit voltage.
I hope this work.

#### KeithWalker

Joined Jul 10, 2017
1,398
Calculating the internal resistance of a cell is much more useful for most applictions.
Regards,
Keith

Joined May 17, 2014
60
Calculating the internal resistance of a cell is much more useful for most applications.
Regards,
Keith
Thank you for your reply. I calculate the discharge resistance, and now, I am going to calculate the charge resistance.
I put the picture of discharge resistance and the open-circuit voltage in this post. The horizontal axis is the DoD of the battery (DoD=1-SoC)

Joined May 17, 2014
60
In addition, I plotted the terminal voltage of the battery according to open circuit and discharge resistance.
According to the datasheet I obtained a lookup table and then, I calcullated the polynomials that fits to the very lookup table. I inserted terminal voltage versus DoD. There is a little difference between them.
I hope my method is correct.

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#### KeithWalker

Joined Jul 10, 2017
1,398
It looks as though you are getting some very usable results.

Joined May 17, 2014
60

#### KeithWalker

Joined Jul 10, 2017
1,398
What units are you using on the X axis? From practical experience, you discharge curve is much more realistic but the final 10% puzzles me. What is the discharge cut-off criteria?

Joined May 17, 2014
60
The X-axis is DoD, which is 1-SoC. when the battery is full the DoD is 0 and when the battery is empty its DoD is 1.
Sorry, I don't understand your latter question.

#### KeithWalker

Joined Jul 10, 2017
1,398
If you discharge a lithium ion battery until it is empty, it is no longer a battery because it can no longer be re-charged. The discharge point of no return is about 2.7V per cell. Usually the discharge is terminated at 3V to be safe.

Joined May 17, 2014
60
Thank you for your point. The cut-off voltage is 2.5V.

#### KeithWalker

Joined Jul 10, 2017
1,398
Thank you for your point. The cut-off voltage is 2.5V.
OK. Now it all makes sense.