What are esr ratings?

Some articles on esr, rating, esr ratings:

Capacitor - Non-ideal Behaviour - Ripple Current
... The equivalent series resistance (ESR) is the amount of internal series resistance one would add to a perfect capacitor to model this ... as well as some film capacitors have a specified rating value for maximum ripple current ... ripple current and generally have the highest ESR ratings in the capacitor family ...
Capacitator - Non-ideal Behavior - Ripple Current
... The equivalent series resistance (ESR) is the amount of internal series resistance one would add to a perfect capacitor to model this ... capacitors, as well as some film capacitors have a specified rating value for maximum ripple current ... are limited by ripple current and generally have the highest ESR ratings in the capacitor family ...