*3.1. Determination of Parameters T*<sup>0</sup> *and α*<sup>0</sup>

The first step is to determine the onset temperature. A new definition is proposed. Rather than defining the temperature onset as the temperature at the instant when the heat rate is over 0.02 ◦C/min, we will say that it is the temperature at the instant when the heat rate starts to increase.

The heat rate is defined as:

$$
\beta(t) = \frac{\partial T(t)}{\partial t} \tag{8}
$$

ans so the time onset *t*<sup>0</sup> is defined as:

$$t \,\forall t \le t\_0, \frac{\partial \beta(t)}{\partial t} \ge 0 \tag{9}$$

This instant is denoted *t*0. Different parameters can be evaluated at this instant:


In Figure 1, an example of heat rate measured during an ARC test on a new cell is given. On the left side (Figure 1a), the different heating levels (black peaks) can be observed. On the right side (Figure 1b), a zoom on the plot is chosen as the onset temperature. In orange is the trend.
