If I want to run an NJ-series Controller without a battery, can I disable detection of the Low Battery Error, like I can in the Programmable Controller system settings for the CS Series and CJ Series?

No, the NJ-series Controllers are designed to always detect the Low Battery error. When this error occurs, the _RetainFail (Retention Failure Flag) system-defined variable changes to TRUE, and the ERROR indicator on the front panel of the CPU Unit flashes.