RE: can the SS45d ESC run on 5 cell lipo?
21V hot off the charger, but quickly down to around 18.5V at rest, and then down to 14V under load.
Even though the starting voltage of the 5S LiPoly is about the same, it stays there, and you can expect 17-18V under load unless you undersized your pack (an expensive mistake, as it won't last long).
I'm betting you're borderline now, and the additional voltage the 5S will bring to the party will push the ESC over the edge. BTDT trying to run a 4S pack on an ESC rated for 6-12 NiCD. Doesn't look all that far out of spec on paper, but it doesn't take long for the ESC to go into thermal shutdown. Thankfully the ESC was smart enough to protect itself.
Experience has taught me that a 4S LiPoly is a good equivalent to 14 NiCd cells. 5S is more appropriate in a 16-18 cell application.