If the PLC is nearly always on then the battery life should approach its shelf
life, likely near 10 years. In the past, I have used 'piggybacked' backup
arrangements involving a Lithium coin cell and diodes. A really long life
Lithium backup solution can be achieved using a 4Ah cylindrical cell as used in
some computers.

But the piggyback element probably does more than just backup, it may contain a
clock and some circuitry to protect /WE while the power is not stable.

Peter P.


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