The electrolytes based propylene carbonate (PC) on the background of lithium tetrafluoroborate for lithium-fluorocarbon (Li/CFx) power sources for implantable pacemakers are studied. Electrolytes with a salt content of 0.7-0.9 mol/l have a conductivity of (4.44–4.46)∙103 S/cm at 37°C, which is provides a full work of pacemaker at low rates of discharge. Conducted accelerated tests on the keeping also confirmed the stability of this electrochemical system. And the proposed electrolyte can be recommended to replace the traditional butyrolactone in Li/CFx primary power source for pacemakers.