Supercaps last longer

Lifetime of Ultracaps - Capcomp GmbH

What factors determine the service life of an ultracap?

  • Ambient temperature
  • Cell voltage
  • Charge current
  • Charge cycle

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.

The service life of a supercap mainly depends on the ambient temperature and the cell voltage. Other influencing factors include the charge/discharge currents and the charge cycles (charging times and breaks).

Decisive influence of temperature and voltage

Supercap lifetime dependencies: temperature and cell 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 cycle

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.

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.