A battery is not required when a memory cassette is used. This is because the sequence programs stored in the memory cassette, except for RAM memory cassette, are not lost.
However, to latch clock data or device data, such as auxiliary relay and data register, a battery needs to be installed.
In addition, a battery may be required when the special data register and the special auxiliary relay are used.
Whether a battery is required or not, differs depending on the programmable controller used. For details, refer to the following.

Section 2.6.3 in the FX3S/FX3G/FX3GC/FX3U/FX3UC Series Programmable Controllers Programming Manual [Basic & Applied Instruction Edition] (Manual number: JY997D16601)