Plotting the OCV and terminal voltage on the same graph

[MatHunt]

This is probably more of a Python question than a pybamm question. I have computed the OCV curve from the data used using:
x=parameter_values["Positive electrode OCP [V]"] y=parameter_values["Negative electrode OCP [V]"] def OCV(t): return x-y

giving the OCV as a function of t(I think). I can't seem to plot this as I get an "Attribute error" whatever that is. I'm guessing I can't plot the function. How do I plot two things on one graph?

Can I let the OCV as a function of time perhaps? that would be good to compare the terminal voltage with.

[YannickNoelStephanKuhn]

You did mean to say return x(t) - y(t), right?

[MatHunt]

I've now changed it, and I still can't plot the thing.

[rtimms]

I think this does what you want, but the individual OCPs might not actually line up this way to give the OCV (e.g. fully-charged might not be sto=1, 0 in neg., pos.).

import pybamm
import matplotlib.pyplot as plt

parameter_values = pybamm.ParameterValues("Chen2020")

x = pybamm.linspace(0, 1, 1000)  # sto
U_n = parameter_values["Negative electrode OCP [V]"]
U_p = parameter_values["Positive electrode OCP [V]"]

fig, ax = plt.subplots(1, 3, figsize=(12, 4))
ax[0].plot(x.entries, U_n(x).entries)
ax[0].set(xlabel="sto [-]", ylabel="$U_n$ [V]")
ax[1].plot(x.entries, U_p(x).entries)
ax[1].set(xlabel="sto [-]", ylabel="$U_p$ [V]")
ax[2].plot(x.entries, U_p(x).entries - U_n(1-x).entries)
ax[2].set(xlabel="sto [-]", ylabel="$U$ [V]")
plt.tight_layout()

[MatHunt]

Although this works, I have no idea why, and I can't seem to figure out how it works.

[rtimms]

x = pybamm.linspace(0, 1, 1000) creates a pybamm.Array object. Doing U_n = parameter_values["Negative electrode OCP [V]"] loads the function from the parameters. You can call this by doing U_n(x) and the results is a pybamm.Vector. You can call .entries to return the elements of the Vector as an ordinary numpy array for plotting.

If you've already solved the model the easiest way to make the plot is to do

import pybamm

model = pybamm.lithium_ion.DFN()
parameter_values = pybamm.ParameterValues("Chen2020")
sim = pybamm.Simulation(model, parameter_values=parameter_values)
sim.solve([0, 3600])
sim.plot([["Terminal voltage [V]", "Measured open circuit voltage [V]"]])