The solver in BatteryDict strives to perform a physically accurate simulation for any given set of material parameters (like conductivities), material structure parameters (morphological), and simulation parameters (like the applied current). This is done by calculating the time evolution of the battery simulation step-by-step.

First, the solver works to establish the initial state or equilibrium state for every simulation in BatteryDict. Then, the solver undertakes performing time steps. In every time step, the solver first attempts a time step with the “maximum time step” given by the user. If this time step does not result in a physically meaningful state of the battery cell, the solver reduces the time step and tries again. This time-step reduction is repeated until the solver finds a physically meaningful state. If it cannot find one, the solver produces a warning message indicating that “Time step has not converged”.

In some cases, the cell-potential might rise well above any reasonable end-of-charging voltage, like for example if a large charge rate is applied. The solver might reach 10 V or even 100 V, but at some point, it will stop. In these kinds of situations, the cell-potential diverges, but the solver produces a meaningful and helpful result.

However, there are other situations in which the cell-potential does not diverge, but the solver stops regardless of that and as mentioned above, the solver has not converged.

In BatteryDict, the possible reasons why the solver does not converge are revealed in warnings, shown before the solver starts and written into the report. The warnings help figuring out why the solver stopped and include:

1. Having very large particles with very low ionic diffusion at a very fast charge rate: If grains of the active material are large or the diffusion is slow, the lithium takes a long time to get from the active surface to the core of the active material grain or from the core of the grain to the surface. In this case it can happen that the lithium concentration at the surface of the active material gets larger than the maximum lithium concentration defined for the material or even smaller than zero. This can also happen, if the active surface is small, the charge rate is high, or if there are at least two active materials and one of them is nearly filled or nearly empty.
2. Having chosen untested material parameters: different tests have been carried out during the development of BatteryDict to identify material parameters that should lead to converging simulations.
3. Sometimes an error message “error -9” appears. This happens, if the operating system ended the process, for example if not enough RAM is available. To prevent this, use Estimate Memory before running a simulation.