An implantable medical device (IMD) may include a battery dedicated to providing cardiac stimulation therapy and a separate power source that provides power for electrical stimulation therapy. Such a configuration preserves the battery dedicated for providing cardiac stimulation therapy even if the second power source is depleted. As an example, the IMD may comprise a cardiac stimulation module configured to deliver at least one stimulation therapy selected from a group consisting of pacing, cardioversion and defibrillation. The IMD further comprises a electrical stimulation module configured to deliver electrical stimulation therapy, a first power source including a battery, wherein the first power source is configured to supply power to the cardiac stimulation module and not to the electrical stimulation module, and a second power source. The second power source is configured to supply power to at least the electrical stimulation module.