I understand the batteries in the HC are two AAA? that would be about 1.2v x 2 in normal state, or 2.4v
an individual li-ion rechargeable is about 3.6 volts... as stated, a completely different charging circuit, and voltage, etc.
Also I believe the batteries are Nickel Metal Hydride, not Nickel Cadmium... (but voltages are same)