Originally Posted by TurnNBurn

Since the cells are rated at twice the capacity of the original cells, given the same charge rate from that charger, yes, it should theoretically take twice as long to charge given that they are rated at twice the capacity. That being said, the commenter above is correct. You might want to invest in a reasonable smart charger. Most LiPo chargers that I’ve seen out there have a NiMh mode, so I’d say shop around for one that suits your need and budget.

As for the heat, you might want to see if it’s the whole pack or any specific cells in the pack you built. There might be either a bad cell in there, or maybe a bad solder joint that is producing resistance, hence heat. A half-decent smart charger might even monitor resistance while charging to let you know if it’s a problem with the pack or not.