You discharge the pack at a high rate (20-40amps) to build heat in the pack (i go down to about 3.5v per cell), then start the charge cycle at a high rate to keep the heat in the pack (30-40amps). Charging a pack that you just ran (no significant discharge) at 5amps on a heating pad will do next to nothing other than put some heat in the case...not the actual cells. I have not temped my pack but i will try and remember next time (it was definitely warm though). By doing the above you can discharge the pack and re-charge it in about 12-15 minutes...if not less.

disclaimer...all this means next to nothing if you have a hard time keeping shiny side up.