The Nano v3.0 chip power is separate and uses 4 Ni-MH rechargeables connecting to 5V.
I certainly hope you don't mean this literally . Do you mean you are NOT connecting the batteries to Vin but instead are connecting
them DIRECTLY to the +5V pin (which happens to be the OUTPUT of the regulator) ?
Why am asking this question ?
What is the nominal voltage per cell of NiMh ? (answer: 1.2V)
What is the SPEC for TTL (look it up on Wikopedia if you don't know what TTL means) (Answer: 4.75-5.25Vdc)
What is the chance that 4 NiMh batteries (charged) will always be within this range ? (Answer: SLIM to none)