To perform battery-less operation in the FX3U/FX3UC series, set parameters using GX Works2(or GX Developer).
Please refer to the following manual for further details;
FX3U SERIES PROGRAMMABLE CONTROLLERS USER'S MANUAL(Hardware Edition)
9.3.2 Setting the battery mode
If you wish to set this operation by using a sequence program, please refer to the following manual for further details;
FX3S/FX3G/FX3GC/FX3U/FX3UC SERIES PROGRAMMING MANUAL - Basic & Applied Instructions Edition
37.2.10 Battery [BATT (BAT)] LED and [ALM] LED OFF command [M8030]