Yes, the PLC is nearly always on, as the managing computer for a 
Tri-fuel (gasoline, propane and natural gas (our own well)) 12.5KW 
standby generator. Concern is even if the piggyback battery was replaced 
with a new one figuring 10 year shelf life, I don't want to depend that 
in 10 or 15 years I (or more probably a younger member of the family) 
can obtain the proper part, and in fact even TRI-PLC is still around. It 
seems more reasonable to me to provide a simple way to replace a very 
common battery style on a schedule along with other batteries in the house.

The piggyback only serves to preserve the real time clock, some memory 
and system settings. The is EEprom but a limited number of writes 
available. The PLC acceptable power is 12 - 24 VDC, and I run it on 12 
VDC (beefy lead/acid marine battery) since that's what the engine uses. 
On starting the brief dip to 9 VDC is enough to reset the PLC, which I 
work around with a 35AH sealed lead acid battery that came out of a 
homebuilt airplane we dismantled last spring, and 2 diodes.

Like the idea of the lithium cell, would be simple to mount that 
cabinet. Then do I just break the power to the chip, add 2 1N4001 
diodes, one to the board power, and the other to the lithium? Do I need 
to add something to keep the lithium charged?

Thanks again for responses. :)

Peter P. wrote:
> 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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