Sizing Scheme of Hybrid Energy Storage System for Electric Vehicle

Abstract

Energy storage system (batteries) plays a vital role in the adoption of electric vehicles (EVs). Li-ion batteries have high energy storage-to-volume ratio, but still, it should not be charged/discharged for short periods frequently as it results in degradation of their state of health (SoH). To resolve this issue, a conventional energy storage system (ESS) is being replaced by hybrid ESS (HESS). The requirement of high-voltage energy sources is increasing with the increasing number of performance based EVs. High-voltage storages are usually difficult to design due to the involvement of higher rating devices; hence, there is a need to create a method to modularize the storage. Modularization can be implemented using lower rating converters to decouple the ultra-capacitors (UCs) and batteries from the load, reducing the cost of storage. This article proposes a fully active series–parallel HESS topology which uses a set of UCs deployed in conjunction with the batteries. UCs provide the advantage of quick and frequent charging/discharging without degrading the battery SoH and are also used to absorb most of the energy generated due to regenerative braking. The major source of energy is Li-ion cells which provide the energy required to run the vehicle, whereas the UCs will provide above-average energy required by the motor. The proposed topology is managed by rule-based energy management systems (EMS), which considers pre-decided threshold parameters of various storage devices. Firstly, a power-based method to find the specifications of UCs and batteries is described which provides specifications for ESS hybridization. The proposed method, which is based on the prescribed set of limiting values of current and voltages, tries to maintain the UC voltage and battery current within range. This method reduces the above-average peaks of the required current from the batteries. Similarly, while recharging due to the regenerative braking, the proposed method removes the above-average peaks of the charging current of UCs. The proposed topology along with the EMS provides better state of charge (SoC) levels, giving a 38.6% increase in SoH levels of the batteries.

Appendices

Appendix 1

See Table 3

Table 3 Vehicle parameters

The SoC of battery is defined as the normalized range between the 20% and 80% nominal voltage level of the battery. Let the nominal voltage of battery be $V_{{{\text{nom}}}}$.

Therefore, minimum voltage becomes:

$$V_{\min } = 0.2 \times V_{nom}$$

(15)

And the maximum voltage becomes:

$$V_{\max } = 0.8 \times V_{nom}$$

(16)

Hence, the SoC is formulated as:

$$SoC = \frac{{V - V_{\min } }}{{V_{\max } - V_{\min } }}$$

(17)

where V is real-time voltage of the battery.

Similarly, the SoC of UCs is defined as normalized range between 0 and 100% nominal voltage level of UCs.