Generally speaking, heat destroys batteries fairly quickly. RC's demand alot out of NiMH. I never formed my first set of batteries. I just charged them at 3 amps on a peak charger and I would run them immediately after I charged them, so they were hot when they started discharging. After a couple months they were charging above 9.6 volts and they got so hot when I ran them that they melted the shrink rap. That is why I am being gentle with my batteries now. I run them until they start to fade, then stop and let the cars rest, which is still too long for the cars' motors/ESC in our summer heat. I leave the batteries is that state of charge until the next week, when I'll start charging them again.

If yours ever get that bad, contact Tenergy and ask for them to be replaced. They were very good to me on supporting their product.

Keeping a fan on them helps, but that only cools the outside of the cells. As the internal resistance of the cell increase over time, the batteries will drop voltage (cut-out) under a high amp discharge demand. You really notice this if the batteries are cool, and then they get better as they warm up.

Joe