That's normal Jeanette. It's a function of NiMh batteries that they heat up as the charge nears completion, and in fact it's that temperature rise that's used to determine when to switch off the charge. The middle connector in the plug connects to a thermistor in the battery, that being a type of resistor which detects the temperature rise and allows through the switch-off signal when the temperature is right.
On the Li-ion charger, the middle connector on the plug isn't used as there is no corresponding temperature rise to detect. Inside the Li-ion battery there's complex electronics which constantly monitor all the cells both individually and as a set, and those in an eZee battery I'm showing in two photos below:
