a fully charged battery only increases in voltage by about .2v, hence i said between 1.2 and 1.4v depending on the type of battery (duracells are 1.2v and rechargables are often 1.4v)
you say a standard sub c battery gives out 2.7-3v when fully charged.. that would make a standard 6c 7.2v battery 18v when fully charged when in reality they are 8.4v as each cell has only increased by 0.2v.
also check a charger when its charging a battery, set it to 7.2v and it will charge at 8.4-9.6v so its impossible for it to increase the voltage per cell to 2.7-3v