7 rechargeable batteries have a few volts?
7 rechargeable batteries are mainly two kinds: nickel-cadmium rechargeable batteries and nickel-metal hydride rechargeable batteries, the standard voltage is 1.2V, fully charged when the voltage can reach 1.5V, there is a 7 lithium rechargeable battery, but not common, voltage 3.2V, full of electric voltage about 3.6V.
7 rechargeable battery capacity is usually relatively small, mostly hundreds of mAh.
General electrical voltage are allowed to have a certain range of changes, you only difference 1.2V so the problem should be small. Ordinary batteries are all 1.5V and rechargeable batteries (nickel-metal hydride batteries or nickel-cadmium batteries) are 1.2V.