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In this paper, a new control strategy of battery-ultracapacitor hybrid energy storage system (HESS) is proposed for hybrid electric drive vehicles (HEVs). Compared to the stand, alone battery system may not be sufficient to satisfy peak demand periods during transients in HEVs, the ultracapacitor pack can supply or recover the peak power and it can be used in high C-rates. However, the problem of battery-ultracapacitor hybrid energy storage system (HESS) is how to interconnect the battery and ultracapacitor and how to control the power distribution. This paper reviewed some battery-ultracapacitor hybrid energy storage system topology and investigated the advantages and disadvantages, then proposed a new control strategy. The proposed control strategy can improve the system performance and ultracapacitor utilization, while also decreasing the battery pack size to avoid the thermal runaway problems and increase the life of the battery. The experiment results showed the proposed control strategy can improve 3% - 4% ultracapacitor utilization.

Hybrid energy storage system (HESS) is very important of hybrid electric vehicles because environmental issue and the increasing energy cost need to be considered [

The problem of battery is that battery not be sufficient to satisfy peak demand periods during transients in HEVs, but the ultracapacitor pack can supply or recover the peak power [

Battery/ultra-capacitor passive topology, as shown in

The battery/ultra-capacitor parallel topology [

Battery/ultra-capacitor series topology [

A new current control strategy is proposed in this paper. Because the ratio of peak power and average power in hybrid electric vehicles is 10:1 [_{bat} is show as (1), and the ultracapacitor current I_{UC} is how as (2), where I_{L} is the load current and M is the size of moving average window.

I b a t [ n ] = 1 M ∑ i = 1 i = M I L [ n − i + 1 ] (1)

I U C [ n ] = I L [ n ] − I b a t [ n ] (2)

In HESS, the ultracapacitor’s voltage needs to be control. In this paper, use changing moving average window size between sampling point and sampling

point to produce nonlinear dc current that can use to charge the ultracapacitor. The nonlinear dc current derivations are shown in (3) and (4).

I b a t [ n + 1 ] = 1 M { I L [ n − M + 2 ] + I L [ n − M + 3 ] + … + I L [ n ] + I L [ n + 1 ] } (3)

I b a t + [ n + 1 ] = 1 M + 1 { I L [ n − M + 1 ] + I L [ n − M + 2 ] + … + I L [ n + 1 ] } (4)

The Equation (3) is the battery current at (n + 1) sampling point without changing the window size, the Equation (4) is battery current which change the window size. Comparing (3) and (4), the differential current d i f f + [ n + 1 ] is shown as (5). In this paper, use the difference current to charge the ultracapacitor, and control the ultracapacitor in a range.

d i f f + [ n + 1 ] = I b a t + [ n + 1 ] − I b a t [ n + 1 ] ≅ 1 M + 1 ⋅ I L [ n − M + 1 ] (5)

The Hybrid Energy storage system is implement in digital circuit so the HESS control software flow chart is shown in

U capacitor = P r m s , c a p P r m s , c a p + P r m s , b a t (6)

A small-signal transfer function for PWM DC-DC converter is needed to understand the converter performance and control. Since we have to control the input current of the converter which is also equal to the inductor current, transfer function between duty cycle and inductor current needs to be determined.

The boost converter can be modeled with a circuit averaging technique [

i S = ( D + d ) ( I L + i l ) (7)

v D = ( D + d ) ( V O + v o ) (8)

For Z 1 = s L + r , Z 2 = 1 / s C + r c , and r = r s + r L + D r D S + ( 1 − D ) R F , we can derive the transfer function of the duty cycle to inductor current for CCM boost converter as (9).

G i d ( s ) = i l ( s ) d ( s ) = r C I O + V O L s + I O C ( r C I O + V O ) s 2 + [ r + r C ( 1 − D ) 2 L ] s + ( 1 − D ) 2 L C (9)

lithium battery module, the part B is the ultracapacitor module, and the part C is the control circuit module. The experimental current waveforms are shown in

In this paper, a new control strategy of battery-ultracapacitor hybrid energy storage system (HESS) is proposed for hybrid electric drive vehicles (HEVs). Compared to the stand alone battery system may not be sufficient to satisfy peak demand periods during transients in HEVs, the ultracapacitor pack can supply or recover the peak power and it can be used in high C-rates. However, the problem of battery-ultracapacitor hybrid energy storage system (HESS) is how to interconnect the battery and ultracapacitor and how to control the power distribution. This thesis reviews some battery-ultracapacitor hybrid energy storage system topology and investigates the advantages and disadvantages, then proposes a

new control strategy. The proposed control strategy can improve the system performance and ultracapacitor utilization, while also decreasing the battery pack size to avoid the thermal runaway problems and increase the life of the battery. The experiment results show the proposed control strategy can improve 14% ultracapacitor utilization.

The authors would like to thank the editor and the referee for their comments. This work was supported in part by the Ministry of Science and Technology, Taiwan, under Grant No. 106-2221-E-009-014.

Dung, L.-R. and Lin, Z.-Y. (2018) Design of High-Utilization Current-Sharing Controller for Battery- Ultracapacitor Hybrid Energy Storage System. Circuits and Systems, 9, 125-132. https://doi.org/10.4236/cs.2018.99013