I'd check your wiring and ballast draw if you've blown 2 cells in a row. Something is welding those contacts in the cell.
If this cell is a "button" type, I'd suggest using an 1800W cell, at least, for a ballasted load. They're about the same price as the 300W, and give you some extra headroom. Intermatics rarely give me problems, but I'm going to take twisters advice and take a look at the Tork products, although I've not yet installed those.
Good Luck!
