*4.3. Temperature Control Strategy*

Based on the analysis on Sections 4.1 and 4.2, the storage temperature of the battery needs to be kept above 0 ◦C to prevent the battery from low-temperature loss of the battery capacity. The heating system in the battery storage compartment ensures that the temperature of the battery can be constant within the ideal operating temperature range and reduces energy consumption by means of intermittent starting. The proportion-integral-differential (PID) algorithm was chosen to design the temperature control strategy.

The chosen heating system is a complex system with larger time lag and inertia. The mathematical model of the temperature control system in this study is described by a first-order inertia lag link. The transfer function of the heater can be expressed as follows.

$$G(s) = k e^{-\tau s} / (Ts + 1) \tag{14}$$

where *k* is static gain; *T* is time constant; τ is pure lag time.

This study uses an incremental PID control algorithm. A step input signal is applied to the controlled object to measure the step response of the controlled object, and the approximate transfer function of the controlled object can be obtained by the flying up curve method. Parameters of the transfer function can be seen in Table 7.


**Table 7.** Parameters of the transfer function.

The flowchart of the temperature control strategy is given in Figure 7.

After the PID was initialized, the target temperature for battery storage *Ta* was set first and then the real-time temperature of battery storage *Tr* was obtained. Generally, *Ta* is set to be higher than 0 ◦C. If *Ta* < *Tr*, the battery storage temperature could be considered suitable. If *Ta* > *Tr*, the heating system will be started and the difference between the target temperature for battery storage *Ta* and the real-time temperature of battery storage *Tr* will calculated, which is marked as *e*. If *e* < 3, the incremental PID control algorithm will be used to heat the battery storage room. If *e* > 3, the full power heating will be activated to quickly reach the target temperature for battery storage. After heating, the difference between *Ta* and *Tr* will be evaluated again. If *Ta* ≤ *Tr*, the heating system will end the heating of the battery storage compartment.

**Figure 7.** The flowchart of the temperature control strategy.
