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Li-ion cell model

brucelee2

Hi all,

Im currently involved in Li-ion cell balancing methods. I have come to a time where i need a model of a li-ion cell (to have a voltage profile with State of charge). Im aware of a schematic to do this(other suggestions are welcome). I now need to make a model in spice. Can someone briefly describe to me the process for doing so? 

Thanks in advance,

 

BRuce 

 

  • Hi Bruce,

     you can try it by using a lookup table. Therefore you only need a graph of the open circuit voltage over SOC. The cell type should match with your "planned" Li-ion cell in capacity and cell chemistry.

    If its allowed to tell, I'm interested in which kind of cell balancing do you use (active or passive)?

     

    Funkensprung

  • Hi, 

    Funkensprung said:

    Hi Bruce,

     you can try it by using a lookup table. Therefore you only need a graph of the open circuit voltage over SOC. The cell type should match with your "planned" Li-ion cell in capacity and cell chemistry.

    If its allowed to tell, I'm interested in which kind of cell balancing do you use (active or passive)?

     

    Funkensprung

    Active cell balancing with a flying capacitor. I have come across a model in Intusoft ICAPS. Im also aware of a model in MATLAB Simulink too. With what you suggested a look up table will do. But the question is how do you change the SOC of the cell with time to use this look up table. Thats the complicated part, so that you may charge and discharge in effect.....ect. Well ive found a model but there are some complications so im tearing it apart at the moment.

     

     

    Regards,

     

    B

  •  A possible hint for the look-up table would be:

     http://www.cadence.com/rl/Resources/application_notes/nonlinear_capacitor_model_appnote.pdf

    How to change the SOC in SPICE is a good question. Maybe you could do it by converting the cells capacity from SOC into Coulomb.
    What I mean: A fully charged cell (with 10Ah capacity for example) has 36000As . For estimation of the "SOC" - and the according cell voltage - you could integrate (coulomb counting) the current going through the cell. => But this is be only an idea. I never did it in SPICE.

    In a former work I modeled a Li-ion cell in MATLAB, together with an idealized model of a transformer for cell balancing. The major focus was on the performance of cell balancing.

     

    I think it mainly depends on what you want to get out. Would it be only the general behaviour/performance of the switched capacitor (then MATLAB), or detailed switching behaviour of the flying capacitor (then SPICE).

     

    Funkensprung

  • Hi,

    I think some thing to do with coulomb counting for SOC and then a look up table for OCV. The model i found in Intusoft ICAPS uses IsSpice4 and has a look up table for the OCV. However im unsure how they get the SOC. I know there are a few RC combinations to get time constants for behaviour but the actual SOC change is a mystery to me. Maybe you can enlighten me :-).

    The model: http://www.intusoft.com/nlhtm/nl78.htm#Modeling_Lithium_Ion_Batteries 

    The model is based on a paper : http://www.reapsystems.co.uk/_publications/Published%20Paper.pdf

    Im mainly involved in investigating this method for balancing; after finding other methods are either inefficient of bulky and expensive. Meaning: carrying our simulations to see the lengths of time to balance and the minimum SOC difference that can be resolved etc; then building a test setup for a specified battery. 

    Im a student :-)

    Kind Regards,

     

    Bruce 

     

     

     

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