1. A system for monitoring status of a battery in an implantable medical device, the system comprising: a battery analyzer configured to analyze the status of the battery, the battery analyzer including a trending module including: a parameter input configured to receive measured values of a battery functional parameter and calculated values of the battery functional parameter, the battery functional parameter indicative of a state of depletion of the battery, the measured values each indicative of an actual state of depletion of the battery, the calculated values each indicative of an expected state of depletion of the battery; and a processing circuit configured to produce a measured battery status trend being a trend of the measured values of the battery functional parameter and a calculated battery status trend being a trend of the calculated values of the battery functional parameter.