ESR and LIC when use as a battery substitute

[GH Crash]

I need some electronic expertise on the value (importance) of ESR for lithium-ion capacitor when the capacitor will be used as battery replacement for small brushed DC motors.

In model aviation, there is a class of aircraft that are referred to as free flights. One group of free flight aircraft use a small DC motor that is powered from a small, usually Lipo, battery. Part of the electric free flight group have replaced the battery with a 10 to 100F supercapacitor (EDLC). The higher voltage, higher capacitance and low self-discharge of the LIC seem to offer advantages over the EDLC. Several individuals feel that the relatively high ESR of a LIC make them unsuitable as an EDLC replacement. Other individuals feel that the ESR may not be a significant factor since the capacitor will be operating at virtually zero frequency.

I would be interested to hear your ideas concerning ESR when using a LIC as if it were a battery. Remember that we are talking about capacitors with a capacitance greater than ten Farads.

Please, don't bother to tell me that a battery would be a better choice. For the sake of this thread (posting) assume a battery cannot be used for reasons that are outside this discussion.

 

[Diver300]

For applications where high current pulses have to be handled, supercapacitors are often used and then their low ESR is really important.

However the ESR is only important where the voltage drop due to ESR matters, so where a model aircraft is flying for several minutes, the ESR will make next to no difference.

If the brushed motors are being controlled using PWM, it will be the motor current, which will equal the peak current taken from the capacitors, that has to be considered, and that can be a lot more than the the average current.

The other consideration is ESL, or equivalent series inductance. The current will not be steady, either due to PWM or due to the switching each time a brush connects with or disconnects from a commutator segment. The ripple current can cause significant voltage spikes. I have no idea how the ESL of a supercapacitor compares with that of an LIC, but it's always a good idea to put generous amount of low ESR capacitors on the input to any PWM controller, so that inductance of the supply, including any wires, has less effect.

 

[ChrisP58]

ESR, or Equivalent Series Resistance is not dependent on frequency. It will be the same for DC or AC.

ESL, or Equivalent Series Inductance, is frequency dependent, and will be lowest for DC.

Any resistance in the total power loop will have a current dependent voltage drop across it, and will reduce the total power efficiency of the system.