Ultracap knowledge
Graphene Supercaps - Revolutionary Energy Storage
Supercaps last longer
Ultracap-Knowledge | Rainer Hake | reading time: 5 minutes
What factors determine the service life of an ultracap?
The question of the service life of ULTRACAPs is frequently asked. Usually, the lifetime of EDLCs is given as 1 million charge cycles at 25 °C and in compliance with the load specification. A cycle usually extends from a full charge to half the charge (Un - Un/2).
In the course of a supercap life, the capacitor loses capacity (C) and the internal resistance (ESR) increases. By definition, the end of life is reached when the capacitance has dropped to 70% of its original value or the internal resistance has doubled.
Electronics designers are particularly interested in the question of how the service life behaves when only 25% or 50% of the full number of cycles are used, i.e. when a discharge is made to 75% or 50% of the original discharge voltage. What are the exact effects of different frequencies of charging and discharging? How is the life affected by sporadic discharge in the presence of an electrostatic field and frequent discharge at high currents?
During the service life of a supercap, the capacitor loses capacitance (C) and the equivalent serial resistance (ESR) increases. The service life has reached its end when the capacitance has fallen to 70% of its original value, or the equivalent serial resistance has doubled.
Many factors influencing the ageing process
It is a fact that the ageing process of capacitors depends on various factors. Temperature, voltage, electrical currents and time play a significant role. Another influencing factor is the "diffusion capacitance" and the "diffusion resistance", which are explained in more detail in this Wikipedia article [diffusion capacitance].
What influences the lifetime of ultracaps the most?
- Ambient temperature
- Cell voltage
- Charge / discharge current
- Charge cycle (charging time and breaks)
We refer to two publicly available scientific publications (see below).
Decisive influence of temperature and voltage
The service life of a SUPERCAP is mainly influenced by two parameters: the temperature and the cell voltage. Both values should be taken into account when considering the service life and the layout of the system. The lower temperature range of 40°C is almost ideal for the UCs and the UCs are not significantly influenced by low temperatures. However, the ESR increases due to the lower ionic mobility at lower temperatures, which is generally compensated for by the self-heating during operation. The service life of the cells is severely impaired at high temperatures, as is the case for normal electrolytic capacitors. However, it is possible to counteract the influence of the temperature by reducing the cell voltage.
The following rule of thumb applies: 0.1V cell voltage equals approximately 10°C. This should, however, be considered on an individual basis and taken into account when designing the system. Please see the Technical Guide of the manufacturer for more information. The usual service life specification of 10 years given in the technical guides generally applies to room temperature and individual voltage specifications.
Example of a test for high charge cycles load cycles
The service life of large energy-storage systems, such as those used in railways, buses and cranes, plays a crucial role when considering the overall system costs. It is therefore important to monitor the parameters that determine reliability (and thus service life) and to adjust them as necessary. The main parameters that determine the service life include:
- Cell voltage
- Cell temperature
- Ambient temperature
- Current load of the cells
- Number of cycles
The rest period plays a particularly important role for high cycle counts and large currents. As a result, the heat distribution in the cells and the cooling of such systems is particularly important and requires precise, individual, application-based consideration.
Accelerated service-life tests
For this reason, industrial users have developed their own test methods and load cycles to best meet their requirements. These tests are designed to simulate the calculated service life across a short period of time. These accelerated service-life tests use values that occur in actual operation, but the values are generally configured at the upper or lower limits. This allows the load cycles to be applied more quickly and with greater capacities. Potential temperature cycles are incorporated at the same time. The illustration shows an example of such a load cycle.
Current scientific publications on the
subject of "Life-time of supercapacitors
- The article of the EPCI - European Passive Component Institute "Supercapacitor Degradation and Life-time" August 2022 describes in detail the effects of voltage and current through experiments with many cycles (>100,000) as well as effects of temperature in the range between -35 °C to 65 °C determined over 9,000 hours. It also deals with diffuse capacitance and diffuse resistance.
- The dissertation by Moritz Teuber from RWTH Aachen University from June 2019 "Lifetime Assessment and Degradation Mechanisms in Electric Double-Layer Capacitors analyses chemical processes that affect the lifetime. The supposedly simple rules of thumb, according to which small changes in voltage double or halve the life, are questioned here. The work shows the complex interrelationships between voltage, temperature and time. Different materials and properties are illuminated, also with regard to the life span. The dissertation gives designers a valuable guide to improving performance and lifetime.
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