[Answered]Matlab Example: Lithium Battery Cell - 1RC ECM model mistake? Why is Voltage at almost 4.5V, despite the battery voltage source using a table with a maximum of 4.2V?

So I was looking at the model again "Lithium Battery Cell - One RC-Branch Equivalent Circuit" see https://de.mathworks.com/help/simscape/ug/lithium-battery-cell-one-rc-branch-equivalent-circuit.html

And the table for the voltage source only goes to 4.2 V,

but running the example shows terminal voltages of the batty up to 4.5V.

I could not figure out why that is an would appreciate if someone has the time to swiftly explain or offer pointers of what this could be due.

The Em_table.ssc uses

v == tablelookup(SOC_Table,Temp_Table,Em_Table,SOC,T,...
    interpolation=linear,extrapolation=nearest)

and the initlaisation file uses

Em_LUT = [
    3.4966    3.5057    3.5148
    3.5519    3.5660    3.5653
    3.6183    3.6337    3.6402
    3.7066    3.7127    3.7213
    3.9131    3.9259    3.9376
    4.0748    4.0777    4.0821
    4.1923    4.1928    4.1930]; %Volts

Answer 1

[Xiangchun]

Hi Christian,

Em is the table for Open Circuit Voltage (OCV). You are right that OCV should never go above 4.193 V because the tablelookup uses nearest for extrapolation. However, the simulated terminal voltage is different from OCV. During discharge, we would expect terminal voltage to be lower than OCV due to internal IR loss. During charge, we would expect terminal voltage to be higher than OCV due to additional voltage across internal resistance. The 4.5 V observed is during charge, noting that SOC was going up all the way to 100%. To summarize, the internal dynamics of batteries cause difference between OCV and terminal voltage. The sign of this difference (whether terminal voltage is higher or lower than OCV) depends on whether a discharge and charge is performed.

On a separate note, a question can be asked whether we should allow the terminal voltage of such a battery to go above 4.2 V during charge without risking overcharging. To address this, a constant current and constant voltage charging protocol can be used where the charge is finished with a constant voltage hold, for example at 4.2 V. For more details, please see Charger and Charge Battery Module with Charger Block.

If you are interested in battery modeling, there are much newer blocks for this provided by Simscape Battery, for example, Battery Equivalent Circuit. It is the latest block for modeling batteries using equivalent circuit model (ECM). It provides some advanced features such as current directionality, hysteresis voltage model, reversible (entropic) heat model, aging model, and faults model.

Best wishes,

Xiangchun

[Christian]

Dear Xiangchun,

Thank you very much for your very swift and detailed response.

You answered my confusion about the Problem I had and put it into perspective for me.

I was mainly confused, ebcause I applied both constant-current and puls charging to the modell and it easily hit beyond 4.2 V,

even at lower SOC, like in the default example around 0.55 SOC, if I recall correctly.

Best Regards,

Christian