I just brewed an imperial stout partial mash kit and racked to secondary. The kit calls for 3 1/2 months in the secondary fermenter. Will there be enough viable yeast left at the end of secondary for bottle conditioning or will I have to add some fresh yeast? If I have to add yeast before bottling, any suggestions? I was thinking of using Danstar cbc-1-cask-and-bottle-conditioned-beer-yeast. I don't want to over-carbonate.
I would definitely repitch fresh yeast when priming an Imp. stout. I've never used the Danstar stuff, but it seems like it has the chops to handle a high-alcohol brew so I'd say give it a shot.
If your Imp. Stout is in the 9%+ category and it's been aging for ~4 months, most of the yeast in there won't be too happy. Some will definitely survive, but it won't be in good shape physiologically. This will make accurate carbonation difficult.
If your beer's really strong, you might get an extra bit of carbonation if you pitch fresh yeast, because there could be residual fermentables from primary that the first yeast crapped out on due to alcohol inhibition. Fresh yeast may ferment this down before giving up again. This very much depends, of course, on %ABV and primary yeast strain, so not sure you'll run into this issue. You could easily take a small sample and pitch some fresh yeast on it though, and see if it does any extra fermenting.