The adjacent image shows the equivalent circuit diagram of a capacitor. Parallel to the actual capacitance C, there is the parallel resistance Rp, which is responsible for self-discharge. The ESR (equivalent serial resistance) is shown in series to this. It is responsible for the charge and discharge losses that cause the ultracap to heat up and that create a certain level of induction, which is negligible in practice.
The extent of self-discharge is mainly dependent on the ambient temperature and the cell voltage. The leakage current specified in the data sheets is the maximum leakage current that occurs over 72 hours.
In the first hours the leakage current can be significantly higher. The figure shows the course of the first 72 hours.
The adjacent figure shows how the voltage of an unloaded cell changes in the first 72 hours. After the cell has been charged to its maximum voltage of 2.7V and then disconnected from the power supply, the voltage decreases asymptotically. The time which needs to be bridged from charging to the use of the electrical energy must be taken into account when the energy storage is being designed.