The charging efficiency of a lithium-ion capacitor (LIC) is an important problem. Until now, due to the stepwise charging method, the charging efficiency of 95.5% has been realized. However, the problem is that the issue of what level the charging efficiency can be increased to, is yet to be well investigated. In this article, the problem is investigated under the galvanostatic charging condition. The charging efficiency is measured as a function of the charging current. As a result, it can be more than 99.5% when the charging is quasi-static, in other words, an adiabatic process is realized. Next, the problem of how much energy can be taken out from the energy-stored capacitor is investigated with a load resistor circuit. It is clarified that the discharging energy from the capacitor is equal to the stored energy in the case when a load resistor is used and the discharging is quasi-static. It is confirmed that LICs are suitable for use as energy storage devices.

キャパシタを定電流充電で一定時間充電したときに，蓄積される電気エネルギーとキャパシタ容量の関係を導き，最も電気エネルギーが大きくなるときの容量値を導出した．またこのときの充電効率と容量との関係についても明らかにする．

An adiabatic reversible circuit was designed for charging a Lithium ion capacitor. The duty ratio of the switching transistors is digitally controlled by a microprocessor. A Lithium ion capacitor has the minimum and the maximum operating voltage. Therefore, the charging voltage is set to be in that range. Using this energy storage system, it is clarified from the experiment that the efficiency is 95.5% during charging and also discharging process. Then the total efficiency is 91.2% during one cycle, which is the best value in the proposed stepwise adiabatic charging circuit.