A common problem with the MT Honda Insight, and to a lesser degree the Civic hybrids is a condition called recalibration. The most common version of this condition shows up as a rapid drop in the battery SOC gauge from some intermediate value to empty. The car goes into a forced charge to bring the SOC back up, which if it happens frequently, can take its toll on the MPG.
One of the possible reasons for this condition is an unbalanced SOC between the different cells in the pack.
The warning label from a civic battery pack warns to run the car for 30 minutes a month. It is likely that the self discharge characteristics of the cells is not predictable, so some subpacks will discharge to a lower level than others, and there does not seem to be a rebalancing mechanism in place to correct the imbalance once developed?
The civic hybrid battery packs made 2005 or before are a good source for replacement 6 cell subpacks with the PTC temperature strips attached.