From the Ingress Help site:
Resonators must be periodically Recharged to maintain their power and control a Portal. If Resonators lose too much XM, then they will be at risk of easy destruction during enemy attacks. If all the Resonators on a Portal drop in XM health to the Critical level, then Links from the Resonators’ Portal will fail. Recharging a Resonator involves transferring XM from your Player reserve to the Resonator(s) on a Portal.
So, at what level of charge does a resonator go to critical level?
Best Answer
I believe it is 10% or less.
It is difficult to say for sure (as an attacker you don't see the exact percentage). However one of my resonators decayed naturally and lost all links exactly one day before decaying to 0% level.