Select the Simulation Type Fully Resolved Simulation or Homogenized Simulation.

The fully resolved simulation runs directly on the voxels of the battery structure. It is therefore very accurate, and, in addition to charge curves, also provides solution files with results resolved on the voxel scale and slice mean plots.

The homogenized simulation is much faster than the fully resolved simulation. It uses a simplified model (an advanced 1D-Newman-model) that is created from the given battery structure. The effective parameters for this simulation are computed in GeoDict. It returns a charge curve as well, but no solution fields resolved on the voxel scale.

For a Fully Resolved Simulation, the Solver Type LIR or BESTmicro can be selected.

For the Homogenized Simulation, the BESTmeso solver is available.

For the LIR solver, additional parameters for the computation need to be defined.

With the LIR solver, the partial differential equations of each time step are solved iteratively with an adaptive tree structure. Several solver options can be chosen in the panel on the right.

Stopping criteria Error Bound, Maximal Iterations, and Maximal Run Time can be selected. The stopping criteria Error Bound stops the solver if the relative difference between computed and predicted solution is smaller than the error bound defined.

Note that all stopping criteria selected are applied for each time step separately. I.e., the maximal run time defined is not the maximal run time for the whole charging simulation, but for each time step.

Unfold the Advanced Options to gain access to the following options:

The Grid Type decides what kind of tree structure is used for the simulation.

The default option is LIR-Tree and should always be used. The solver uses an adaptive tree structure called LIR-tree and needs up to 10 times less runtime and memory compared to the Regular Grid option.

The solver can analyze the result field during the computation and improves the adaptive grid in places where more accuracy is needed. The LIR solver splits cells where a high gradient occurs.

The balance between stable and fast simulation is managed for the LIR solver through the Relaxation value, that must be positive and should not be larger than 2: For a relaxation value smaller than 1, the simulation is more stable. For a value larger than 1, it is faster.

The LIR solver can Optimize for speed or memory.

• If Speed is chosen, the solver constructs additional optimization structures. The runtime is decreased but it requires more memory compared to the Memory option.
• If Memory is chosen, the runtime is increased but the solver requires less memory.

Control how many threads are used for the computation. Parallelization is possible if your license and hardware allow it. The Parallelization Options dialog opens when clicking the Edit button, to choose between Sequential, Parallel (Shared Memory) or Automatic Number of Threads.

Selecting Sequential will not apply parallelization.

When Parallel (Shared Memory) is selected, the Number of Threads can be entered. Below, the maximum number of available threads and the maximum number of licensed parallel processes is shown in the dialog.

If Automatic Number of Threads is selected, the number of parallel processes is automatically selected for optimal speed, based on the CPU cores and licensed parallel processes.

For up to eight Available Threads, all of them will be used. If more than eight threads are available, two cases might occur.

• The number of Available Threads is larger (or equal) than the Number of CPU Cores:
  • Then the maximum of eight and Number of CPU Cores divided by 2 is used.

• The number of Available Threads is smaller than the Number of CPU Cores:
  • Then the maximum of eight and number of Available Threads divided by 2 is used.

Note! As an example for the parallelization, a full battery structure of size 409 x 400 x 400 voxels with 200 nm voxel length is used to simulate the charging from the cell state of charge 20% till 70%. The computation is run on a server with 2 x Intel E5-2697A v4 processors with 16 cores each, running with a maximum of 3.60 GHz, and 1 TB RAM. The simulations are computed with GeoDict 2024, r71884. When the binder is considered as solid (first plot), the LIR solver needs only 1/3 of the runtime compared to BESTmicro (legacy) with GeoDict 2024. In GeoDict 2024, with the LIR solver, the runtime is almost the same with in GeoDict 2023. In GeoDict 2024, the BESTmicro solver takes only 54% of runtime of BESTmicro (legacy). The LIR solver needs only 7.8 GB of RAM which is 3% of the memory required by BESTmicro (legacy) and 5.5% of the memory required by BESTmicro. The speedup for LIR solver is nearly optimal. When the binder is considered as porous (second plot), since GeoDict 2022, LIR can also solve the porous case. The LIR solver needs 9 GB of RAM, only 2.6% of the memory required by BESTmicro (legacy), 347 GB and 6.2% of the memory required by BESTmicro. The runtimes of LIR, BESTmicro and BESTmicro (legacy) with 32 threads are 8 h, 12 h, and 25 h.

Finally, in the Time Step panel an option for the Time-Step Input Mode may be selected.

Any of the battery solvers calculates the time evolution of the given Li-ion battery. It starts at the initial state, which has time = 0 in the result plots. It takes the time step defined as maximum time step size and tries to find the battery state at time = last time + time step by searching an equilibrium solution fulfilling the given system of differential equations. If no solution can be found for this time step size, it reduces the time step size, and the solver starts again. This procedure is repeated until an appropriate time step size is found. This time step size is used for the next steps in the simulation as well. After several successful steps with a certain time step size, the solver tries to increase this time step size again, to reduce overall computation time. With the Time-Step Input Mode, the maximum time-step size can be influenced. For more information on time steps see State Of Charge (SOC).

• Choosing Automatic, the solver selects the time-step size without restrictions.
• If a Minimum Number of Time Steps is defined, at least this number of points are computed for the solution and the maximum time-step size is selected accordingly. If your simulation does not produce enough points in the plots of the result file, then you might want to increase the Minimum Number of Time Steps.
• Maximum Time Step Size defines the maximum time step in seconds used in the simulation. If the size of the time step is oscillating a lot, the recommendation is to reduce this maximum value.

The time steps that were finally realized in the simulation are saved in the Result Map of the GeoDict result file (*.gdr file), created for the charging simulation, under the key TimeDependentValues:TimeStep.

If a new battery state is found in a simulation with a charge rate boundary condition, the solver computes the Cell-SOC step of the just found new battery state i via:

with the time step and the charge rate. So, it is not clear from the start which is the next computed Cell-SOC in a simulation, this depends on the time step.

Example:

• Charge Rate
• Time Step Size
• Cell-SOC step

If you choose to set the current density instead of the charge rate, then the calculation of the Cell-SOC step size depends on the given current density and the cell capacity. Thereby, the cell capacity depends on the cell-geometry and the maximum lithium concentration of the active materials.

In the GeoDict result file (*.gdr file) created for the charging simulation, you can find the time-evolution of the Cell-SOCs in percent as well as the corresponding times in seconds in the Result Map under the keys TimeDependentValues: CellStateOfCharge and TimeDependentValues:Time, respectively.

For simple simulations (charge rate <= 1, physical domain size < = 5 µm, only one active material per electrode, diffusivities >= 1e-12 m²/s, conductivities >= 1 S/m and no warnings during the simulation), the time step size is usually the maximum time step size set. Then, also the Cell-SOC step is a constant value.

However, for more complex simulations (charge rate >= 10, physical domain size >= 50 µm, two active materials in an electrode with different parameters, low diffusivities in the active materials and small conductivities), it might happen that the time step size may be smaller than the maximum time step size defined and not constant over the whole simulation.