In many energy harvesting systems an energy storage component needs to exist to store electrical energy for use when needed. Commonly these are capacitors, or super capacitors, or some other buzzword. While capacitance and voltage specifications are relatively easy to define something very important to low energy systems is often not defined, the capacitors leakage current and leakage settling time.
Today’s technology allows active components to sleep and consume only several hundred nano amps of current. The EnOcean STM300 for example consumes 220nA while sleeping. Passives don’t have this state and all capacitors have leakage current. The physical chemistry generally is the determining factor for the magnitude of leakage. In powered applications this is almost never considered. However, what if a capacitor is leaking uA of current? If the system is asleep for 99% of the time you just increased your energy consumption 10x!
Besides the leakage current itself a second item to consider is how long a capacitor takes to settle to its steady state leakage value. A capacitor could have a leakage current of nA but it might take 2 days of settling to reach that point. This is not ideal either.
Leakage currents need to be measured, but more importantly a 3.6V 470uF tantalum capacitor will not have the same leakage properties from manufacturer to manufacturer so it is important to measure this and pick quality parts!